Thread

1. no universally correct setting for fsync

   Kevin Grittner <kevin.grittner@wicourts.gov> — 2010-05-07T13:47:15Z

   Someone just posted to the -admin list with a database corrupted
   while running with fsync=off.  I was all set to refer him to the
   documentation explaining why he should stop doing this, but to my
   surprise the documentation waffles on the issue way past what I
   think is reasonable.
    
   http://www.postgresql.org/docs/8.4/interactive/runtime-config-wal.html#GUC-FSYNC
    
   There are dire-sounding statements interspersed with:
    
   | using fsync results in a performance penalty
    
   | Due to the risks involved, there is no universally correct setting
   | for fsync.
    
   | If you trust your operating system, your hardware, and your
   | utility company (or your battery backup), you can consider
   | disabling fsync.
    
   Isn't this a little too rosy a picture to paint?
    
   -Kevin
   
   

2. Re: no universally correct setting for fsync

   Robert Haas <robertmhaas@gmail.com> — 2010-05-07T14:00:50Z

   On Fri, May 7, 2010 at 9:47 AM, Kevin Grittner
   <Kevin.Grittner@wicourts.gov> wrote:
   > Someone just posted to the -admin list with a database corrupted
   > while running with fsync=off.  I was all set to refer him to the
   > documentation explaining why he should stop doing this, but to my
   > surprise the documentation waffles on the issue way past what I
   > think is reasonable.
   >
   > http://www.postgresql.org/docs/8.4/interactive/runtime-config-wal.html#GUC-FSYNC
   >
   > There are dire-sounding statements interspersed with:
   >
   > | using fsync results in a performance penalty
   >
   > | Due to the risks involved, there is no universally correct setting
   > | for fsync.
   >
   > | If you trust your operating system, your hardware, and your
   > | utility company (or your battery backup), you can consider
   > | disabling fsync.
   >
   > Isn't this a little too rosy a picture to paint?
   
   I agree.  I've always thought this part of the documentation made
   setting fsync=off much more reasonable than I feel it to be.
   
   -- 
   Robert Haas
   EnterpriseDB: http://www.enterprisedb.com
   The Enterprise Postgres Company
   
   

3. Re: no universally correct setting for fsync

   Magnus Hagander <magnus@hagander.net> — 2010-05-07T14:03:55Z

   On Fri, May 7, 2010 at 16:00, Robert Haas <robertmhaas@gmail.com> wrote:
   > On Fri, May 7, 2010 at 9:47 AM, Kevin Grittner
   > <Kevin.Grittner@wicourts.gov> wrote:
   >> Someone just posted to the -admin list with a database corrupted
   >> while running with fsync=off.  I was all set to refer him to the
   >> documentation explaining why he should stop doing this, but to my
   >> surprise the documentation waffles on the issue way past what I
   >> think is reasonable.
   >>
   >> http://www.postgresql.org/docs/8.4/interactive/runtime-config-wal.html#GUC-FSYNC
   >>
   >> There are dire-sounding statements interspersed with:
   >>
   >> | using fsync results in a performance penalty
   >>
   >> | Due to the risks involved, there is no universally correct setting
   >> | for fsync.
   >>
   >> | If you trust your operating system, your hardware, and your
   >> | utility company (or your battery backup), you can consider
   >> | disabling fsync.
   >>
   >> Isn't this a little too rosy a picture to paint?
   >
   > I agree.  I've always thought this part of the documentation made
   > setting fsync=off much more reasonable than I feel it to be.
   
   +1, definitely. fsync=off should only be done if you *really*
   understand what it means, and that requires a lot more explanation
   than that...
   
   
   -- 
    Magnus Hagander
    Me: http://www.hagander.net/
    Work: http://www.redpill-linpro.com/
   
   

4. Re: no universally correct setting for fsync

   Tom Lane <tgl@sss.pgh.pa.us> — 2010-05-07T14:24:47Z

   "Kevin Grittner" <Kevin.Grittner@wicourts.gov> writes:
   > | If you trust your operating system, your hardware, and your
   > | utility company (or your battery backup), you can consider
   > | disabling fsync.
    
   > Isn't this a little too rosy a picture to paint?
   
   I think that statement is true as far as it goes, but I agree with
   rejiggering the surrounding text.  The whole thing was written back
   when Postgres was by far the least reliable component of the stack.
   It isn't anymore.  We should make it clear that fsync=off is not ever
   recommended for production.
   
   			regards, tom lane
   
   

5. Re: no universally correct setting for fsync

   Andrew Dunstan <andrew@dunslane.net> — 2010-05-07T14:38:47Z

   
   Kevin Grittner wrote:
   >  
   > There are dire-sounding statements interspersed with:
   >  
   > | using fsync results in a performance penalty
   >  
   > | Due to the risks involved, there is no universally correct setting
   > | for fsync.
   >  
   > | If you trust your operating system, your hardware, and your
   > | utility company (or your battery backup), you can consider
   > | disabling fsync.
   >  
   > Isn't this a little too rosy a picture to paint?
   >  
   >
   >   
   
   I think the critical question is really whether you are prepared to lose 
   your database.
   
   I have a customer who rotates databases in and out of line, and 
   processes major updates on the out of line database. If they lose the 
   database occasionally they are prepared to wear that risk for the 
   performance gain they get from running with fsync off. It just means 
   that they have to recover and so the inline database will get a bit 
   staler than usual while they do.
   
   So I think its true that there is no universally right answer. Maybe the 
   criteria mentioned in the last para need tweaking some, though. It's not 
   just a matter of trusting hardware etc. I have seen mishaps when idiots 
   knock out power cords and the like. The unexpected does sometime happen, 
   despite the best planning.
   
   cheers
   
   andrew
   
   
   
   

6. Re: no universally correct setting for fsync

   Kevin Grittner <kevin.grittner@wicourts.gov> — 2010-05-07T14:48:53Z

   Andrew Dunstan <andrew@dunslane.net> wrote:
    
   > I think the critical question is really whether you are prepared
   > to lose your database.
    
   Precisely; and the docs don't make that at all clear.  They mention
   the possibility of database corruption, but downplay it:
    
   | When fsync is disabled, the operating system is allowed to do its
   | best in buffering, ordering, and delaying writes. This can result
   | in significantly improved performance. However, if the system
   | crashes, the results of the last few committed transactions might
   | be lost in part or whole. In the worst case, unrecoverable data
   | corruption might occur.
   
   > [valid use case for fsync=off]
   > 
   > So I think its true that there is no universally right answer.
   > Maybe the criteria mentioned in the last para need tweaking some,
   > though.
    
   I think it goes beyond "tweaking" -- I think we should have a bald
   statement like "don't turn this off unless you're OK with losing the
   entire contents of the database cluster."  A brief listing of some
   cases where that is OK might be illustrative.
    
   I never meant to suggest any statement in that section is factually
   wrong; it's just all too rosy, leading people to believe it's no big
   deal to turn it off.
    
   -Kevin
   
   

7. Re: no universally correct setting for fsync

   Josh Berkus <josh@agliodbs.com> — 2010-05-07T17:56:13Z

   > I never meant to suggest any statement in that section is factually
   > wrong; it's just all too rosy, leading people to believe it's no big
   > deal to turn it off.
   
   Yeah, that section is overdue for an update.  I'll write some new text 
   and post it to pgsql-docs.
   
   -- 
                                      -- Josh Berkus
                                         PostgreSQL Experts Inc.
                                         http://www.pgexperts.com
   
   

8. Re: no universally correct setting for fsync

   Bernd Helmle <mailings@oopsware.de> — 2010-05-07T23:32:59Z

   
   --On 7. Mai 2010 09:48:53 -0500 Kevin Grittner 
   <Kevin.Grittner@wicourts.gov> wrote:
   
   > I think it goes beyond "tweaking" -- I think we should have a bald
   > statement like "don't turn this off unless you're OK with losing the
   > entire contents of the database cluster."  A brief listing of some
   > cases where that is OK might be illustrative.
   >
   
   +1
   
   > I never meant to suggest any statement in that section is factually
   > wrong; it's just all too rosy, leading people to believe it's no big
   > deal to turn it off.
   
   I think one mistake in this paragraph is the passing mention of 
   "performance". I've seen installations in the past with fsync=off only 
   because the admin was pressured to get instantly "more speed" out of the 
   database (think of "fast_mode=on"). In my opinion, phrases like 
   "performance penalty" are misleading, if you need that setting in 99% of 
   all use cases for reliable operation.
   
   I've recently even started to wonder if the performance gain with fsync=off 
   is still that large on modern hardware. While testing large migration 
   procedures to a new version some time ago (on an admitedly fast storage) i 
   forgot here and then to turn it off, without a significant degradation in 
   performance.
   
   
   -- 
   Thanks
   
   	Bernd
   
   

9. Re: no universally correct setting for fsync

   Tom Lane <tgl@sss.pgh.pa.us> — 2010-05-07T23:49:15Z

   Bernd Helmle <mailings@oopsware.de> writes:
   > I've recently even started to wonder if the performance gain with fsync=off 
   > is still that large on modern hardware. While testing large migration 
   > procedures to a new version some time ago (on an admitedly fast storage) i 
   > forgot here and then to turn it off, without a significant degradation in 
   > performance.
   
   That says to me either that you're using a battery-backed write cache,
   or your fsyncs don't really work (no write barriers or something like
   that).
   
   			regards, tom lane
   
   

10. Re: [HACKERS] no universally correct setting for fsync

    Josh Berkus <josh@agliodbs.com> — 2010-05-08T00:02:59Z

    Folks,
    
    This is what I have to replace the current fsync entry in config.sgml.
    
    I believe that the note about needing fsync for Warm Standby to work
    correctly is true, but could someone verify it?
    
    =========================
    
         <varlistentry id="guc-fsync" xreflabel="fsync">
          <indexterm>
           <primary><varname>fsync</> configuration parameter</primary>
          </indexterm>
          <term><varname>fsync</varname> (<type>boolean</type>)</term>
          <listitem>
           <para>
            If this parameter is on, the <productname>PostgreSQL</> server
            will try to make sure that updates are physically written to
            disk, by issuing <function>fsync()</> system calls or various
            equivalent methods (see <xref linkend="guc-wal-sync-method">).
            This ensures that the database cluster can recover to a
            consistent state after an operating system or hardware crash.
           </para>
    
           <para>
            While turning off <varname>fsync</varname> is often a performance
            benefit, this can result in unrecoverable data corruption in the
    event
            of an unexpected shutdown. Thus it is only advisable to turn off
            <varname>fsync</varname> if you can easily recreate
            your entire database from external data.  <varname>fsync</varname>
            must be on for WAL archiving to work correctly
            (see <xref linkend="continuous-archiving">).
           <para>
    
           <para>
            In many situations, turning off <xref
    linkend="guc-synchronous-commit">
            for noncritical transactions can provide much of the potential
            performance benefit of turning off <varname>fsync</varname>, without
            the attendant risks of data corruption.
           </para>
    
           <para>
            <varname>fsync</varname> can only be set in the
    <filename>postgresql.conf</>
            file or on the server command line.
            If you turn this parameter off, also consider turning off
            <xref linkend="guc-full-page-writes">.
           </para>
          </listitem>
         </varlistentry>
    
    -- 
                                      -- Josh Berkus
                                         PostgreSQL Experts Inc.
                                         http://www.pgexperts.com
    
    

11. Re: [HACKERS] no universally correct setting for fsync

    Tom Lane <tgl@sss.pgh.pa.us> — 2010-05-08T00:13:32Z

    Josh Berkus <josh@agliodbs.com> writes:
    > This is what I have to replace the current fsync entry in config.sgml.
    
    s/unexpected shutdown/system crash/, perhaps.  The wording you have
    suggests that a forced Postgres stoppage produces a problem, which it
    doesn't.  It takes a failure at the OS level or below to cause a
    problem.
    
    > I believe that the note about needing fsync for Warm Standby to work
    > correctly is true, but could someone verify it?
    
    AFAIK that's nonsense.  The filesystem state that pg_standby could see
    will be updated in any case; pg_standby has no direct access to the bits
    on the platters, any more than Postgres does.
    
    			regards, tom lane
    
    

12. Re: no universally correct setting for fsync

    Bernd Helmle <mailings@oopsware.de> — 2010-05-08T00:16:23Z

    
    --On 7. Mai 2010 19:49:15 -0400 Tom Lane <tgl@sss.pgh.pa.us> wrote:
    
    > Bernd Helmle <mailings@oopsware.de> writes:
    >> I've recently even started to wonder if the performance gain with
    >> fsync=off  is still that large on modern hardware. While testing large
    >> migration  procedures to a new version some time ago (on an admitedly
    >> fast storage) i  forgot here and then to turn it off, without a
    >> significant degradation in  performance.
    >
    > That says to me either that you're using a battery-backed write cache,
    > or your fsyncs don't really work (no write barriers or something like
    > that).
    >
    
    Well, yes, BBU present and proven storage. Maybe i'm wrong, but it seems 
    battery backed write caches aren't that seldom even in low end systems 
    nowadays.
    
    -- 
    Thanks
    
    	Bernd
    
    

13. Re: [HACKERS] no universally correct setting for fsync

    Josh Berkus <josh@agliodbs.com> — 2010-05-08T00:17:53Z

    On 5/7/10 5:13 PM, Tom Lane wrote:
    > Josh Berkus <josh@agliodbs.com> writes:
    >> This is what I have to replace the current fsync entry in config.sgml.
    > 
    > s/unexpected shutdown/system crash/, perhaps.  The wording you have
    > suggests that a forced Postgres stoppage produces a problem, which it
    > doesn't.  It takes a failure at the OS level or below to cause a
    > problem.
    
    I actually meant "unexpected *system* shutdown", i.e. power-out.  A lot
    of people think "crash" just means kernel dump, whereas a UPS failure or
    tripped power cord is a lot more likely (except maybe on Windows).
    
    Revised:
    
    ==================
    
    
         <varlistentry id="guc-fsync" xreflabel="fsync">
          <indexterm>
           <primary><varname>fsync</> configuration parameter</primary>
          </indexterm>
          <term><varname>fsync</varname> (<type>boolean</type>)</term>
          <listitem>
           <para>
            If this parameter is on, the <productname>PostgreSQL</> server
            will try to make sure that updates are physically written to
            disk, by issuing <function>fsync()</> system calls or various
            equivalent methods (see <xref linkend="guc-wal-sync-method">).
            This ensures that the database cluster can recover to a
            consistent state after an operating system or hardware crash.
           </para>
    
           <para>
            While turning off <varname>fsync</varname> is often a performance
            benefit, this can result in unrecoverable data corruption in the
    event
            of an unexpected system shutdown or crash.  Thus it is only
    advisable
            to turn off  <varname>fsync</varname> if you can easily recreate
            your entire database from external data.
           <para>
    
           <para>
            In many situations, turning off <xref
    linkend="guc-synchronous-commit">
            for noncritical transactions can provide much of the potential
            performance benefit of turning off <varname>fsync</varname>, without
            the attendant risks of data corruption.
           </para>
    
           <para>
            <varname>fsync</varname> can only be set in the
    <filename>postgresql.conf</>
            file or on the server command line.
            If you turn this parameter off, also consider turning off
            <xref linkend="guc-full-page-writes">.
           </para>
          </listitem>
         </varlistentry>
    
    
    
    -- 
                                      -- Josh Berkus
                                         PostgreSQL Experts Inc.
                                         http://www.pgexperts.com
    
    

14. Re: no universally correct setting for fsync

    Craig Ringer <craig@postnewspapers.com.au> — 2010-05-08T08:07:01Z

    On 8/05/2010 1:56 AM, Josh Berkus wrote:
    >
    >> I never meant to suggest any statement in that section is factually
    >> wrong; it's just all too rosy, leading people to believe it's no big
    >> deal to turn it off.
    >
    > Yeah, that section is overdue for an update. I'll write some new text
    > and post it to pgsql-docs.
    
    It's probably worth mentioning that people who want to turn off fsync to 
    gain a performance boost should instead look at a RAID controller with a 
    BBU so they can safely enable write-back caching, getting most of the 
    benefits of fsync=off safely.
    
    --
    Craig Ringer
    
    

15. Re: no universally correct setting for fsync

    Michael Tharp <gxti@partiallystapled.com> — 2010-05-10T14:48:01Z

    On 05/08/2010 04:07 AM, Craig Ringer wrote:
    > It's probably worth mentioning that people who want to turn off fsync to
    > gain a performance boost should instead look at a RAID controller with a
    > BBU so they can safely enable write-back caching, getting most of the
    > benefits of fsync=off safely.
    
    Which options specifically should be set if a BBU is in use? Obviously 
    fsync should be on always, but can full_page_writes be disabled? Are 
    there other tweaks that can be done?
    
    It would be great to see some practical hints in the documentation while 
    the fsync part is getting changed.
    
    -- m. tharp
    
    

16. Re: no universally correct setting for fsync

    Bruce Momjian <bruce@momjian.us> — 2010-05-10T15:12:35Z

    Michael Tharp wrote:
    > On 05/08/2010 04:07 AM, Craig Ringer wrote:
    > > It's probably worth mentioning that people who want to turn off fsync to
    > > gain a performance boost should instead look at a RAID controller with a
    > > BBU so they can safely enable write-back caching, getting most of the
    > > benefits of fsync=off safely.
    > 
    > Which options specifically should be set if a BBU is in use? Obviously 
    > fsync should be on always, but can full_page_writes be disabled? Are 
    > there other tweaks that can be done?
    > 
    > It would be great to see some practical hints in the documentation while 
    > the fsync part is getting changed.
    
    Uh, our docs have:
    
    	Turning this parameter off speeds normal operation, but might
    	lead to a corrupt database after an operating system crash or
    	power failure. The risks are similar to turning off
    	<varname>fsync</>, though smaller.  It might be safe to turn
    	off this parameter if you have hardware (such as a battery-backed
    	disk controller) or file-system software that reduces the risk
    	of partial page writes to an acceptably low level (e.g., ZFS).
    
    -- 
      Bruce Momjian  <bruce@momjian.us>        http://momjian.us
      EnterpriseDB                             http://enterprisedb.com
    
    

17. Re: no universally correct setting for fsync

    Robert Haas <robertmhaas@gmail.com> — 2010-05-10T15:41:22Z

    On Mon, May 10, 2010 at 11:12 AM, Bruce Momjian <bruce@momjian.us> wrote:
    > Michael Tharp wrote:
    >> On 05/08/2010 04:07 AM, Craig Ringer wrote:
    >> > It's probably worth mentioning that people who want to turn off fsync to
    >> > gain a performance boost should instead look at a RAID controller with a
    >> > BBU so they can safely enable write-back caching, getting most of the
    >> > benefits of fsync=off safely.
    >>
    >> Which options specifically should be set if a BBU is in use? Obviously
    >> fsync should be on always, but can full_page_writes be disabled? Are
    >> there other tweaks that can be done?
    >>
    >> It would be great to see some practical hints in the documentation while
    >> the fsync part is getting changed.
    >
    > Uh, our docs have:
    >
    >        Turning this parameter off speeds normal operation, but might
    >        lead to a corrupt database after an operating system crash or
    >        power failure. The risks are similar to turning off
    >        <varname>fsync</>, though smaller.  It might be safe to turn
    >        off this parameter if you have hardware (such as a battery-backed
    >        disk controller) or file-system software that reduces the risk
    >        of partial page writes to an acceptably low level (e.g., ZFS).
    
    "It might be safe" is a bit of a waffle.  It would be nice if we could
    provide some more clear guidance as to whether it is or is not, or how
    someone could go about testing their hardware to find out.
    
    -- 
    Robert Haas
    EnterpriseDB: http://www.enterprisedb.com
    The Enterprise Postgres Company
    
    

18. Re: no universally correct setting for fsync

    Bruce Momjian <bruce@momjian.us> — 2010-05-10T15:49:15Z

    Robert Haas wrote:
    > On Mon, May 10, 2010 at 11:12 AM, Bruce Momjian <bruce@momjian.us> wrote:
    > > Michael Tharp wrote:
    > >> On 05/08/2010 04:07 AM, Craig Ringer wrote:
    > >> > It's probably worth mentioning that people who want to turn off fsync to
    > >> > gain a performance boost should instead look at a RAID controller with a
    > >> > BBU so they can safely enable write-back caching, getting most of the
    > >> > benefits of fsync=off safely.
    > >>
    > >> Which options specifically should be set if a BBU is in use? Obviously
    > >> fsync should be on always, but can full_page_writes be disabled? Are
    > >> there other tweaks that can be done?
    > >>
    > >> It would be great to see some practical hints in the documentation while
    > >> the fsync part is getting changed.
    > >
    > > Uh, our docs have:
    > >
    > > ? ? ? ?Turning this parameter off speeds normal operation, but might
    > > ? ? ? ?lead to a corrupt database after an operating system crash or
    > > ? ? ? ?power failure. The risks are similar to turning off
    > > ? ? ? ?<varname>fsync</>, though smaller. ?It might be safe to turn
    > > ? ? ? ?off this parameter if you have hardware (such as a battery-backed
    > > ? ? ? ?disk controller) or file-system software that reduces the risk
    > > ? ? ? ?of partial page writes to an acceptably low level (e.g., ZFS).
    > 
    > "It might be safe" is a bit of a waffle.  It would be nice if we could
    > provide some more clear guidance as to whether it is or is not, or how
    > someone could go about testing their hardware to find out.
    
    Agreed.  It is "safe" for us to be definitive here?
    
    -- 
      Bruce Momjian  <bruce@momjian.us>        http://momjian.us
      EnterpriseDB                             http://enterprisedb.com
    
    

19. Re: no universally correct setting for fsync

    Kevin Grittner <kevin.grittner@wicourts.gov> — 2010-05-10T15:55:40Z

    Robert Haas <robertmhaas@gmail.com> wrote:
     
    > "It might be safe" is a bit of a waffle.  It would be nice if we
    > could provide some more clear guidance as to whether it is or is
    > not, or how someone could go about testing their hardware to find
    > out.
     
    I think that the issue is that you could have corruption if some,
    but not all, disk sectors from a page were written from OS cache to
    controller cache when a failure occurred.  The window would be small
    for a RAM-to-RAM write, but it wouldn't be entirely *safe* unless
    there's some OS/driver environment where you could count on all the
    sectors making it or none of them making it for every single page. 
    Does such an environment exist?
     
    -Kevin
    
    

20. Re: no universally correct setting for fsync

    Greg Stark <gsstark@mit.edu> — 2010-05-10T17:46:53Z

    On Mon, May 10, 2010 at 4:55 PM, Kevin Grittner
    <Kevin.Grittner@wicourts.gov> wrote:
    > Robert Haas <robertmhaas@gmail.com> wrote:
    >
    >> "It might be safe" is a bit of a waffle.  It would be nice if we
    >> could provide some more clear guidance as to whether it is or is
    >> not, or how someone could go about testing their hardware to find
    >> out.
    >
    > I think that the issue is that you could have corruption if some,
    > but not all, disk sectors from a page were written from OS cache to
    > controller cache when a failure occurred.  The window would be small
    > for a RAM-to-RAM write, but it wouldn't be entirely *safe* unless
    > there's some OS/driver environment where you could count on all the
    > sectors making it or none of them making it for every single page.
    > Does such an environment exist?
    
    The reason for the waffle is that the following sentence describes a
    whole set of environments based the following description:
    
    > > ? ? ? ?if you have hardware (such as a battery-backed
    > > ? ? ? ?disk controller) or file-system software that reduces the risk
    > > ? ? ? ?of partial page writes to an acceptably low level
    
    Depending on which set of hardware and how low the risk is it might be safe.
    
    I think with WAFL or ZFS it's entirely safe. There may be other
    filesystems with similar guarantees. With a BBU the risk might be very
    low -- but it might not, it would be hard to determine without a
    detailed analysis of the entire stack from the buffer cache,
    filesystem, lvm, hardware drivers, BBU design, etc.
    
    -- 
    greg
    
    

21. Re: no universally correct setting for fsync

    Joshua D. Drake <jd@commandprompt.com> — 2010-05-10T18:42:57Z

    On Mon, 2010-05-10 at 18:46 +0100, Greg Stark wrote:
    > On Mon, May 10, 2010 at 4:55 PM, Kevin Grittner
    > <Kevin.Grittner@wicourts.gov> wrote:
    > > Robert Haas <robertmhaas@gmail.com> wrote:
    > >
    > >> "It might be safe" is a bit of a waffle.  It would be nice if we
    > >> could provide some more clear guidance as to whether it is or is
    > >> not, or how someone could go about testing their hardware to find
    > >> out.
    > >
    > > I think that the issue is that you could have corruption if some,
    > > but not all, disk sectors from a page were written from OS cache to
    > > controller cache when a failure occurred.  The window would be small
    > > for a RAM-to-RAM write, but it wouldn't be entirely *safe* unless
    > > there's some OS/driver environment where you could count on all the
    > > sectors making it or none of them making it for every single page.
    > > Does such an environment exist?
    > 
    > The reason for the waffle is that the following sentence describes a
    > whole set of environments based the following description:
    > 
    > > > ? ? ? ?if you have hardware (such as a battery-backed
    > > > ? ? ? ?disk controller) or file-system software that reduces the risk
    > > > ? ? ? ?of partial page writes to an acceptably low level
    > 
    > Depending on which set of hardware and how low the risk is it might be safe.
    > 
    > I think with WAFL or ZFS it's entirely safe. There may be other
    > filesystems with similar guarantees. With a BBU the risk might be very
    > low -- but it might not, it would be hard to determine without a
    > detailed analysis of the entire stack from the buffer cache,
    > filesystem, lvm, hardware drivers, BBU design, etc.
    > 
    
    The answer to this is:
    
    PostgreSQL.org recommends that this setting be left on at all times.
    Turning it off, may lead to data corruption.
    
    Anything else is circumstantial and based on knowledge and facts we
    don't have about environmental factors. 
    
    Joshua D. Drake
    
    
    > -- 
    > greg
    > 
    
    
    -- 
    PostgreSQL.org Major Contributor
    Command Prompt, Inc: http://www.commandprompt.com/ - 503.667.4564
    Consulting, Training, Support, Custom Development, Engineering
    
    
    
    

22. Re: no universally correct setting for fsync

    Kevin Grittner <kevin.grittner@wicourts.gov> — 2010-05-10T19:00:37Z

    "Joshua D. Drake" <jd@commandprompt.com> wrote:
     
    > The answer to this is:
    > 
    > PostgreSQL.org recommends that this setting be left on at all
    > times.  Turning it off, may lead to data corruption.
    > 
    > Anything else is circumstantial and based on knowledge and facts
    > we don't have about environmental factors. 
     
    Perhaps Josh's language for fsync could be modified to work here
    (we're now talking about full_page_writes, for anyone who's lost
    track):
     
    | it is only advisable to turn off fsync if you can easily recreate
    | your entire database from external data.
     
    That covers bulk loads to an empty or just-backed-up database and
    entirely redundant databases.  Saying it should never be turned off
    would tend to make one wonder why we have the setting at all.
     
    -Kevin
    
    

23. Re: no universally correct setting for fsync

    Tom Lane <tgl@sss.pgh.pa.us> — 2010-05-10T19:57:42Z

    "Kevin Grittner" <Kevin.Grittner@wicourts.gov> writes:
    > Perhaps Josh's language for fsync could be modified to work here
    > (we're now talking about full_page_writes, for anyone who's lost
    > track):
     
    > | it is only advisable to turn off fsync if you can easily recreate
    > | your entire database from external data.
    
    > That covers bulk loads to an empty or just-backed-up database and
    > entirely redundant databases.  Saying it should never be turned off
    > would tend to make one wonder why we have the setting at all.
    
    +1.  Perhaps for both of them, we should specify that the intended
    use-case is for improving performance during initial database load
    and similar cases.
    
    			regards, tom lane
    
    

24. Re: no universally correct setting for fsync

    Cédric Villemain <cedric.villemain.debian@gmail.com> — 2010-05-10T20:22:05Z

    2010/5/8 Bernd Helmle <mailings@oopsware.de>:
    >
    >
    > --On 7. Mai 2010 09:48:53 -0500 Kevin Grittner <Kevin.Grittner@wicourts.gov>
    > wrote:
    >
    >> I think it goes beyond "tweaking" -- I think we should have a bald
    >> statement like "don't turn this off unless you're OK with losing the
    >> entire contents of the database cluster."  A brief listing of some
    >> cases where that is OK might be illustrative.
    >>
    >
    > +1
    >
    >> I never meant to suggest any statement in that section is factually
    >> wrong; it's just all too rosy, leading people to believe it's no big
    >> deal to turn it off.
    >
    > I think one mistake in this paragraph is the passing mention of
    > "performance". I've seen installations in the past with fsync=off only
    > because the admin was pressured to get instantly "more speed" out of the
    > database (think of "fast_mode=on"). In my opinion, phrases like "performance
    > penalty" are misleading, if you need that setting in 99% of all use cases
    > for reliable operation.
    >
    > I've recently even started to wonder if the performance gain with fsync=off
    > is still that large on modern hardware. While testing large migration
    > procedures to a new version some time ago (on an admitedly fast storage) i
    > forgot here and then to turn it off, without a significant degradation in
    > performance.
    
    On a recent pg_restore -j 32, with perc 6i with BBU, RAID10 8 hd,
    results were not so bas with fsync turn on. (XFS with nobarrier su and
    sw)
    -- deactivate fsync
    time pg_restore -U postgres -d foodb -j 32 foo.psql
    real    170m0.527s
    user    43m12.914s
    sys     1m56.499s
    -- activate fsync
    time pg_restore -U postgres -d foodb -j 32 foo.psql
    real    177m0.121s
    user    42m54.581s
    sys     2m0.452s
    
    >
    >
    > --
    > Thanks
    >
    >        Bernd
    >
    > --
    > Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
    > To make changes to your subscription:
    > http://www.postgresql.org/mailpref/pgsql-hackers
    >
    
    
    
    -- 
    Cédric Villemain
    
    

25. Re: no universally correct setting for fsync

    Josh Berkus <josh@agliodbs.com> — 2010-05-10T20:35:32Z

    All,
    
    Updated docs based on tracking this discussion.  fsync through full page
    writes recorded below.
    
    ============
    
    
         <varlistentry id="guc-fsync" xreflabel="fsync">
          <indexterm>
           <primary><varname>fsync</> configuration parameter</primary>
          </indexterm>
          <term><varname>fsync</varname> (<type>boolean</type>)</term>
          <listitem>
           <para>
            If this parameter is on, the <productname>PostgreSQL</> server
            will try to make sure that updates are physically written to
            disk, by issuing <function>fsync()</> system calls or various
            equivalent methods (see <xref linkend="guc-wal-sync-method">).
            This ensures that the database cluster can recover to a
            consistent state after an operating system or hardware crash.
           </para>
    
           <para>
            While turning off <varname>fsync</varname> is often a performance
            benefit, this can result in unrecoverable data corruption in the
    event
            of an unexpected system shutdown or crash.  Thus it is only
    advisable
            to turn off  <varname>fsync</varname> if you can easily recreate
            your entire database from external data.
           </para>
    
           <para>
    	Examples of safe times to turn off <varname>fsync</varname> would be
            when initially loading a new database from a backup file, on a
    database which is
    	only used for processing statistics on an hourly basis and is then
    deleted,
            or on a reporting read-only clone of your database which gets
    recreated very
            night and is not used for failover.  High quality hardware alone
    is not a
            sufficient justification for turning off <varname>fsync</varname>.
           </para>
    
           <para>
            In many situations, turning off <xref
    linkend="guc-synchronous-commit">
            for noncritical transactions can provide much of the potential
            performance benefit of turning off <varname>fsync</varname>, without
            the attendant risks of data corruption.
           </para>
    
           <para>
            <varname>fsync</varname> can only be set in the
    <filename>postgresql.conf</>
            file or on the server command line.
            If you turn this parameter off, also consider turning off
            <xref linkend="guc-full-page-writes">.
           </para>
          </listitem>
         </varlistentry>
    
         <varlistentry id="guc-synchronous-commit"
    xreflabel="synchronous_commit">
          <term><varname>synchronous_commit</varname>
    (<type>boolean</type>)</term>
          <indexterm>
           <primary><varname>synchronous_commit</> configuration
    parameter</primary>
          </indexterm>
          <listitem>
           <para>
            Specifies whether transaction commit will wait for WAL records
            to be written to disk before the command returns a <quote>success</>
            indication to the client.  The default, and safe, setting is
            <literal>on</>.  When <literal>off</>, there can be a delay between
            when success is reported to the client and when the transaction is
            really guaranteed to be safe against a server crash.  (The maximum
            delay is three times <xref linkend="guc-wal-writer-delay">.)  Unlike
            <xref linkend="guc-fsync">, setting this parameter to
    <literal>off</>
            does not create any risk of database inconsistency: a crash might
            result in some recent allegedly-committed transactions being
    lost, but
            the database state will be just the same as if those
    transactions had
            been aborted cleanly.  So, turning
    <varname>synchronous_commit</> off
            can be a useful alternative when performance is more important than
            exact certainty about the durability of a transaction.  For more
            discussion see <xref linkend="wal-async-commit">.
           </para>
           <para>
            This parameter can be changed at any time; the behavior for any
            one transaction is determined by the setting in effect when it
            commits.  It is therefore possible, and useful, to have some
            transactions commit synchronously and others asynchronously.
            For example, to make a single multi-statement transaction commit
            asynchronously when the default is the opposite, issue <command>SET
            LOCAL synchronous_commit TO OFF</> within the transaction.
           </para>
          </listitem>
         </varlistentry>
    
         <varlistentry id="guc-wal-sync-method" xreflabel="wal_sync_method">
          <term><varname>wal_sync_method</varname> (<type>enum</type>)</term>
          <indexterm>
           <primary><varname>wal_sync_method</> configuration
    parameter</primary>
          </indexterm>
          <listitem>
           <para>
            Method used for forcing WAL updates out to disk.
            If <varname>fsync</varname> is off then this setting is irrelevant,
            since WAL file updates will not be forced out at all.
            Possible values are:
           </para>
           <itemizedlist>
            <listitem>
            <para>
             <literal>open_datasync</> (write WAL files with
    <function>open()</> option <symbol>O_DSYNC</>)
            </para>
            </listitem>
            <listitem>
            <para>
             <literal>fdatasync</> (call <function>fdatasync()</> at each
    commit)
            </para>
            </listitem>
            <listitem>
            <para>
             <literal>fsync_writethrough</> (call <function>fsync()</> at
    each commit, forcing write-through of any disk write cache)
            </para>
            </listitem>
            <listitem>
            <para>
             <literal>fsync</> (call <function>fsync()</> at each commit)
            </para>
            </listitem>
            <listitem>
            <para>
             <literal>open_sync</> (write WAL files with <function>open()</>
    option <symbol>O_SYNC</>)
            </para>
            </listitem>
           </itemizedlist>
           <para>
            Not all of these choices are available on all platforms.
            The default is the first method in the above list that is supported
            by the platform.
            The <literal>open_</>* options also use <literal>O_DIRECT</> if
    available.
            The utility <filename>src/tools/fsync</> in the PostgreSQL
    source tree
            can do performance testing of various fsync methods.
            This parameter can only be set in the <filename>postgresql.conf</>
            file or on the server command line.
           </para>
          </listitem>
         </varlistentry>
    
         <varlistentry id="guc-full-page-writes" xreflabel="full_page_writes">
          <indexterm>
           <primary><varname>full_page_writes</> configuration
    parameter</primary>
          </indexterm>
          <term><varname>full_page_writes</varname>
    (<type>boolean</type>)</term>
          <listitem>
           <para>
            When this parameter is on, the <productname>PostgreSQL</> server
            writes the entire content of each disk page to WAL during the
            first modification of that page after a checkpoint.
            This is needed because
            a page write that is in process during an operating system crash
    might
            be only partially completed, leading to an on-disk page
            that contains a mix of old and new data.  The row-level change data
            normally stored in WAL will not be enough to completely restore
            such a page during post-crash recovery.  Storing the full page image
            guarantees that the page can be correctly restored, but at the price
            of increasing the amount of data that must be written to WAL.
            (Because WAL replay always starts from a checkpoint, it is
    sufficient
            to do this during the first change of each page after a checkpoint.
            Therefore, one way to reduce the cost of full-page writes is to
            increase the checkpoint interval parameters.)
           </para>
    
           <para>
            Turning this parameter off speeds normal operation, but
            might lead to either unrecoverable data corruption, or silent
            data corruption, after a system failure. The risks are similar
    to turning off
            <varname>fsync</varname>, though smaller, and it should be
    turned off
            only based on the same circumstances recommended for that parameter.
           </para>
    
           <para>
            Turning off this parameter does not affect use of
            WAL archiving for point-in-time recovery (PITR)
            (see <xref linkend="continuous-archiving">).
           </para>
    
           <para>
            This parameter can only be set in the <filename>postgresql.conf</>
            file or on the server command line.
            The default is <literal>on</>.
           </para>
          </listitem>
         </varlistentry>
    
    
    
    
    -- 
                                      -- Josh Berkus
                                         PostgreSQL Experts Inc.
                                         http://www.pgexperts.com
    
    

26. Re: no universally correct setting for fsync

    Kevin Grittner <kevin.grittner@wicourts.gov> — 2010-05-10T20:59:34Z

    Cédric Villemain<cedric.villemain.debian@gmail.com> wrote:
     
    > On a recent pg_restore -j 32, with perc 6i with BBU, RAID10 8 hd,
    > results were not so bas with fsync turn on. (XFS with nobarrier su
    > and sw)
    > -- deactivate fsync
    > time pg_restore -U postgres -d foodb -j 32 foo.psql
    > real    170m0.527s
    > user    43m12.914s
    > sys     1m56.499s
    > -- activate fsync
    > time pg_restore -U postgres -d foodb -j 32 foo.psql
    > real    177m0.121s
    > user    42m54.581s
    > sys     2m0.452s
     
    Wow.  In a situation where you save seven minutes (4%), it's hardly
    worth turning off.
     
    -Kevin
    
    

27. Re: no universally correct setting for fsync

    Josh Berkus <josh@agliodbs.com> — 2010-05-10T21:20:18Z

    > Wow.  In a situation where you save seven minutes (4%), it's hardly
    > worth turning off.
    
    I've had it be much higher, especially for really large databases.
    
    -- 
                                      -- Josh Berkus
                                         PostgreSQL Experts Inc.
                                         http://www.pgexperts.com
    
    

28. Re: no universally correct setting for fsync

    Ross J. Reedstrom <reedstrm@rice.edu> — 2010-05-10T21:21:35Z

    On Mon, May 10, 2010 at 01:35:32PM -0700, Josh Berkus wrote:
    > deleted,
    >         or on a reporting read-only clone of your database which gets
    > recreated very
    >         night and is not used for failover.  High quality hardware alone
    
    s/very/every/
    or 
    s/very night/periodically/
    
    Ross
    -- 
    Ross Reedstrom, Ph.D.                                 reedstrm@rice.edu
    Systems Engineer & Admin, Research Scientist        phone: 713-348-6166
    The Connexions Project      http://cnx.org            fax: 713-348-3665
    Rice University MS-375, Houston, TX 77005
    GPG Key fingerprint = F023 82C8 9B0E 2CC6 0D8E  F888 D3AE 810E 88F0 BEDE
    
    

29. Re: no universally correct setting for fsync

    Greg Smith <greg@2ndquadrant.com> — 2010-05-10T23:03:34Z

    Josh Berkus wrote:
    >> Wow.  In a situation where you save seven minutes (4%), it's hardly
    >> worth turning off.
    >>     
    >
    > I've had it be much higher, especially for really large databases.
    >   
    
    Cedric's system had a non-volatile write cache in it.  In that case, a 
    few percentage points of improvement is normal--the overhead of fsync is 
    very low.  In the case where you don't have one of those, and the write 
    cache on the drives are turned off for safety too, I've seen turning 
    fsync off be a 40X speedup--100 inserts/second jumping to 4000TPS.  
    (This was before synchronous_commit).
    
    The real question is how much of a speed-up fsync provides compared to 
    the same workload with synchronous_commit disabled.  The only case for 
    fsync=off is one where that number is much faster.  That's the case on 
    some low-level operations (I seem to recall there is no async commit 
    speedup for CREATE DATABASE for example).  But for most of what people 
    want to speed, just killing sync commit while keeping fsync is on is 
    good enough.  I suspect there are still some bulk-load workloads where 
    fsync=off helps beyond just going for async commit, but they're tougher 
    to find and the difference isn't huge relative to total load times.
    
    -- 
    Greg Smith  2ndQuadrant US  Baltimore, MD
    PostgreSQL Training, Services and Support
    greg@2ndQuadrant.com   www.2ndQuadrant.us
    
    
    

30. Re: no universally correct setting for fsync

    Josh Berkus <josh@agliodbs.com> — 2010-05-11T00:23:10Z

    > The real question is how much of a speed-up fsync provides compared to
    > the same workload with synchronous_commit disabled.  The only case for
    > fsync=off is one where that number is much faster.  
    
    I can't say I've tested this.  Most of my head-to-heads on fsync were
    before asych existed.
    
    -- 
                                      -- Josh Berkus
                                         PostgreSQL Experts Inc.
                                         http://www.pgexperts.com
    
    

31. Re: no universally correct setting for fsync

    Josh Berkus <josh@agliodbs.com> — 2010-05-11T00:26:54Z

    On 5/10/10 2:21 PM, Ross J. Reedstrom wrote:
    > On Mon, May 10, 2010 at 01:35:32PM -0700, Josh Berkus wrote:
    >> deleted,
    >>         or on a reporting read-only clone of your database which gets
    >> recreated very
    >>         night and is not used for failover.  High quality hardware alone
    > 
    > s/very/every/
    > or 
    > s/very night/periodically/
    
    "frequently" I think.  Periodically could mean once a year.
    
    
    -- 
                                      -- Josh Berkus
                                         PostgreSQL Experts Inc.
                                         http://www.pgexperts.com
    
    

32. Re: no universally correct setting for fsync

    Greg Smith <greg@2ndquadrant.com> — 2010-05-11T01:04:29Z

    Josh Berkus wrote:
    >> The real question is how much of a speed-up fsync provides compared to
    >> the same workload with synchronous_commit disabled.  The only case for
    >> fsync=off is one where that number is much faster.  
    >>     
    > I can't say I've tested this.  Most of my head-to-heads on fsync were
    > before asych existed.
    >   
    
    Ditto for me.  Curious about that, and I'd like to help work on 
    improving this chunk of the docs too.  I don't know about you guys, but 
    I'm swamped until after PGCon though.
    
    I have some hardware testing stuff planned anyway later this month, can 
    check exactly where this situation truly stands on a couple of common 
    pieces of hardware (next system has one of the LSI controllers Dell 
    rebrands too).  I'll have the systems setup for something similar 
    anyway--can certainly see fsync differences with pgbench--easy to throw 
    this test into the mix too.
    
    With that report, we should have the info needed to really nail this 
    down accurately.  I can make my own proofreading pass of what Josh has 
    already been doing that also reflects the new data, and then we can 
    commit something that's good and well reviewed for 9.0 here.
    
    -- 
    Greg Smith  2ndQuadrant US  Baltimore, MD
    PostgreSQL Training, Services and Support
    greg@2ndQuadrant.com   www.2ndQuadrant.us
    
    
    

33. Re: no universally correct setting for fsync

    Yeb Havinga <yebhavinga@gmail.com> — 2010-05-11T08:00:21Z

    Kevin Grittner wrote:
    > "Joshua D. Drake" <jd@commandprompt.com> wrote:
    >  
    >   
    >> The answer to this is:
    >>
    >> PostgreSQL.org recommends that this setting be left on at all
    >> times.  Turning it off, may lead to data corruption.
    >>
    >> Anything else is circumstantial and based on knowledge and facts
    >> we don't have about environmental factors. 
    >>     
    >  
    > Perhaps Josh's language for fsync could be modified to work here
    > (we're now talking about full_page_writes, for anyone who's lost
    > track):
    >  
    > | it is only advisable to turn off fsync if you can easily recreate
    > | your entire database from external data.
    >  
    > That covers bulk loads to an empty or just-backed-up database and
    > entirely redundant databases.  Saying it should never be turned off
    > would tend to make one wonder why we have the setting at all.
    >   
    Would the term "entirely redundant databases" include (synchronously) 
    replicated databases? (ps: I did indeed lose track about whether this is 
    about fsync or full_page_writes and did not get on the track again)
    
    regards,
    Yeb Havinga
    
    
    

34. Re: no universally correct setting for fsync

    Bruce Momjian <bruce@momjian.us> — 2010-05-31T15:52:31Z

    Josh Berkus wrote:
    > All,
    > 
    > Updated docs based on tracking this discussion.  fsync through full page
    > writes recorded below.
    
    I have applied this doc update with the attached patch.
    
    I added the change from "every night" to "frequently", and reworded it
    slightly so it was clear it affects the entire cluster, not just a
    single database.
    
    -- 
      Bruce Momjian  <bruce@momjian.us>        http://momjian.us
      EnterpriseDB                             http://enterprisedb.com