Thread

1. H/W RAID 5 on slower disks versus no raid on faster HDDs

   Rajesh Kumar Mallah <mallah@trade-india.com> — 2002-11-21T16:45:02Z

   Hi folks,
   
   I have two options:
   3*18 GB 10,000 RPM Ultra160 Dual Channel SCSI  controller + H/W Raid 5
   and 
   2*36 GB 15,000 RPM Ultra320 Dual Channel SCSI and no RAID
   
   Does anyone opinions *performance wise*  the pros and cons of above 
   two options.
   
   please take in consideration in latter case its higher RPM and better
   SCSI interface. 
   
   
   
   Regds
   Mallah.
   
   
   
   
   -- 
   Rajesh Kumar Mallah,
   Project Manager (Development)
   Infocom Network Limited, New Delhi
   phone: +91(11)6152172 (221) (L) ,9811255597 (M)
   
   Visit http://www.trade-india.com ,
   India's Leading B2B eMarketplace.
   
   
   
   

2. Re: H/W RAID 5 on slower disks versus no raid on faster

   Charles H. Woloszynski <chw@clearmetrix.com> — 2002-11-21T17:06:03Z

   How are you going to make use of the three faster drives under 
   postgresql?   Were you intending to put the WAL, system/swap, and the 
   actual data files on separate drives/partitions?  Unless you do 
   something like that (or s/w RAID to distribute the processing across the 
   disks), you really have ONE SCSI 15K Ultra320 drive against 3 slower 
   drives with the RAID overhead (and spreading of performance because of 
   the multiple heads).
   
   I don't have specifics here, but I'd expect that the RAID5 on slower 
   drives would work better for apps with lots of selects or lots of 
   concurrent users.  I suspect that the Ultra320 would be better for batch 
   jobs and mostly transactions with less selects.
   
   Charlie
   
   Rajesh Kumar Mallah. wrote:
   
   >Hi folks,
   >
   >I have two options:
   >3*18 GB 10,000 RPM Ultra160 Dual Channel SCSI  controller + H/W Raid 5
   >and 
   >2*36 GB 15,000 RPM Ultra320 Dual Channel SCSI and no RAID
   >
   >Does anyone opinions *performance wise*  the pros and cons of above 
   >two options.
   >
   >please take in consideration in latter case its higher RPM and better
   >SCSI interface. 
   >
   >
   >
   >Regds
   >Mallah.
   >
   >
   >
   >
   >  
   >
   
   -- 
   
   
   Charles H. Woloszynski
   
   ClearMetrix, Inc.
   115 Research Drive
   Bethlehem, PA 18015
   
   tel: 610-419-2210 x400
   fax: 240-371-3256
   web: www.clearmetrix.com
   
   
   
   
   
   

3. Re: H/W RAID 5 on slower disks versus no raid on faster HDDs

   Chris Ruprecht <chris@ruprecht.org> — 2002-11-21T17:19:35Z

   raid 0 (striping) spreads the load over multiple spindels, the same way raid 5 
   does. but raid 5 always needs to calculate parity and write that to it's 
   parity drive.
   
   RPM isn't that critical, a lot depends on the machine, the processor and the 
   memory (and the spped with which the processor can get to the memory). I have 
   recently tested a lot of systems with some database benchmarks we wrote here 
   at work. We're not running Postgres here at work, sorry, these benchmarks are 
   of no use to Postgres ...
   We we found is that a lot depends on motherboard design, not so much on drive 
   speed. We got to stages where we allocated 1.8 GB of RAM to shared memory for 
   the database server process, resulting in the entire database being sucked 
   into memory. When doing reads, 100% of the data is coming out the that 
   menory, and drive speed becomes irrelevant.
   
   From tests I did with Postgres on my boxes at home, I can say: The more shared 
   memory you can throw at the server process, the better. Under MacOS X I 
   wasn't able to allocate more than 3 MB, Under Linux, I can allocate anything 
   I want to, so I usually start up the server with 256 MB. The difference? A 
   process which takes 4 minutes under Linux, takes 6 hours under MacOS - same 
   hardware, same drives, different memory settings.
   
   Best regards,
   Chris
   
   On Thursday 21 November 2002 12:02, you wrote:
   > Thanks Chris,
   >
   > does raid0 enhances both read/write both?
   >
   > does rpms not matter that much?
   >
   > regds
   > mallah.
   >
   > On Thursday 21 November 2002 22:27, you wrote:
   > > RAID 5 gives you pretty bad performance, a slowdown of about 50%. For
   > > pure performance, I'd use the 3 18 GB drives with RAID 0.
   > >
   > > If you need fault tolerance, you could use RAID 0+1 or 1+0 but you'd need
   > > an even number of drives for that, of which half would become 'usable
   > > space'.
   > >
   > > Best regards,
   > > Chris
   > >
   > > On Thursday 21 November 2002 11:45, you wrote:
   > > > Hi folks,
   > > >
   > > > I have two options:
   > > > 3*18 GB 10,000 RPM Ultra160 Dual Channel SCSI  controller + H/W Raid 5
   > > > and
   > > > 2*36 GB 15,000 RPM Ultra320 Dual Channel SCSI and no RAID
   > > >
   > > > Does anyone opinions *performance wise*  the pros and cons of above
   > > > two options.
   > > >
   > > > please take in consideration in latter case its higher RPM and better
   > > > SCSI interface.
   > > >
   > > >
   > > >
   > > > Regds
   > > > Mallah.
   
   -- 
   Network Grunt and Bit Pusher extraordinaire
   
   
   

4. Re: H/W RAID 5 on slower disks versus no raid on faster

   scott.marlowe <scott.marlowe@ihs.com> — 2002-11-21T17:32:05Z

   On Thu, 21 Nov 2002, Rajesh Kumar Mallah. wrote:
   
   > 
   > Hi folks,
   > 
   > I have two options:
   > 3*18 GB 10,000 RPM Ultra160 Dual Channel SCSI  controller + H/W Raid 5
   > and 
   > 2*36 GB 15,000 RPM Ultra320 Dual Channel SCSI and no RAID
   > 
   > Does anyone opinions *performance wise*  the pros and cons of above 
   > two options.
   > 
   > please take in consideration in latter case its higher RPM and better
   > SCSI interface. 
   
   Does the OS you're running on support software RAID?  If so the dual 36 
   gigs in a RAID0 software would be fastest, and in a RAID1 would still be 
   pretty fast plus they would be redundant.
   
   Depending on your queries, there may not be a lot of difference between 
   running the 3*18 hw RAID or the 2*36 setup, especially if most of your 
   data can fit into memory on the server.
   
   Generally, the 2*36 should be faster for writing, and the 3*18 should be 
   about even for reads, maybe a little faster.
   
   
   

5. Re: H/W RAID 5 on slower disks versus no raid on faster HDDs

   Rajesh Kumar Mallah <mallah@trade-india.com> — 2002-11-21T17:46:55Z

   
   Oh i did not mention,
   its linux, it does.
   
   RAM: 2.0 GB
   CPU: Dual 2.0 Ghz Intel Xeon DP Processors.
   
   
   On Thursday 21 November 2002 23:02, scott.marlowe wrote:
   > On Thu, 21 Nov 2002, Rajesh Kumar Mallah. wrote:
   > > Hi folks,
   > >
   > > I have two options:
   > > 3*18 GB 10,000 RPM Ultra160 Dual Channel SCSI  controller + H/W Raid 5
   > > and
   > > 2*36 GB 15,000 RPM Ultra320 Dual Channel SCSI and no RAID
   > >
   > > Does anyone opinions *performance wise*  the pros and cons of above
   > > two options.
   > >
   > > please take in consideration in latter case its higher RPM and better
   > > SCSI interface.
   >
   > Does the OS you're running on support software RAID?  If so the dual 36
   > gigs in a RAID0 software would be fastest, and in a RAID1 would still be
   > pretty fast plus they would be redundant.
   
   >
   > Depending on your queries, there may not be a lot of difference between
   > running the 3*18 hw RAID or the 2*36 setup, especially if most of your
   > data can fit into memory on the server.
   > Generally, the 2*36 should be faster for writing, and the 3*18 should be
   > about even for reads, maybe a little faster.
   
   Since i got lots of RAM and my Data Size (on disk ) is 2 GB i feel  frequent reads
   can happen from the memory.
   
   
   I have heard putting pg_xlog in a drive of its own helps in boosting updates to 
   DB server.
   in that case shud i forget abt the h/w and use one disk exclusively for the WAL?
   
   
   Regds
   mallah.
   
   
   
   
   
   
   
   
   
   -- 
   Rajesh Kumar Mallah,
   Project Manager (Development)
   Infocom Network Limited, New Delhi
   phone: +91(11)6152172 (221) (L) ,9811255597 (M)
   
   Visit http://www.trade-india.com ,
   India's Leading B2B eMarketplace.
   
   
   
   

6. Re: H/W RAID 5 on slower disks versus no raid on faster HDDs

   Rajesh Kumar Mallah <mallah@trade-india.com> — 2002-11-21T17:58:43Z

   
   OK now i am reading Momjian's "PostgreSQL Hardware Performance Tuning" 
   once again ;-)
   
   mallah.
   
   
   On Thursday 21 November 2002 23:02, scott.marlowe wrote:
   > On Thu, 21 Nov 2002, Rajesh Kumar Mallah. wrote:
   > > Hi folks,
   > >
   > > I have two options:
   > > 3*18 GB 10,000 RPM Ultra160 Dual Channel SCSI  controller + H/W Raid 5
   > > and
   > > 2*36 GB 15,000 RPM Ultra320 Dual Channel SCSI and no RAID
   > >
   > > Does anyone opinions *performance wise*  the pros and cons of above
   > > two options.
   > >
   > > please take in consideration in latter case its higher RPM and better
   > > SCSI interface.
   >
   > Does the OS you're running on support software RAID?  If so the dual 36
   > gigs in a RAID0 software would be fastest, and in a RAID1 would still be
   > pretty fast plus they would be redundant.
   >
   > Depending on your queries, there may not be a lot of difference between
   > running the 3*18 hw RAID or the 2*36 setup, especially if most of your
   > data can fit into memory on the server.
   >
   > Generally, the 2*36 should be faster for writing, and the 3*18 should be
   > about even for reads, maybe a little faster.
   
   -- 
   Rajesh Kumar Mallah,
   Project Manager (Development)
   Infocom Network Limited, New Delhi
   phone: +91(11)6152172 (221) (L) ,9811255597 (M)
   
   Visit http://www.trade-india.com ,
   India's Leading B2B eMarketplace.
   
   
   
   

7. Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on faster HDDs

   Steve Crawford <scrawford@pinpointresearch.com> — 2002-11-21T18:56:29Z

   I had long labored under the impression that RAID 5 should give me better 
   performance but I have since encountered many reports that this is not the 
   case. Do some searching on Google and you will probably find numerous 
   articles.
   
   Note 3x18 w/RAID5 will give 36GB usable while 2x36 w/o RAID is 72GB. 
   You could use mirroring on the 2x36 and have the same usable space.
   
   A mirrored 2x36 setup will probably yield a marginal hit on writes (vs a 
   single disk) and an improvement on reads due to having two drives to read 
   from and will (based on the Scientific Wild Ass Guess method and knowing 
   nothing about your overall system) probably be faster than the RAID5 
   configuration while giving you identical usable space and data safety.
   
   You also may see improvements due to the 15,000RPM drives (of course RPM is 
   sort of an arbitrary measure - you really want to know about track access 
   times, latency, transfer rate, etc. and RPM is just one influencing factor 
   for the above).
   
   The quality of your RAID cards will also be important (how fast do they 
   perform their calculations, how much buffer do they have) as will the overall 
   specs of you system. If you have a bottleneck somewhere other than your raw 
   disk I/O then you can throw all the money you want at faster drives and see 
   no improvement.
   
   Cheers,
   Steve
   
   
   On Thursday 21 November 2002 8:45 am, you wrote:
   > Hi folks,
   >
   > I have two options:
   > 3*18 GB 10,000 RPM Ultra160 Dual Channel SCSI  controller + H/W Raid 5
   > and
   > 2*36 GB 15,000 RPM Ultra320 Dual Channel SCSI and no RAID
   >
   > Does anyone opinions *performance wise*  the pros and cons of above
   > two options.
   >
   > please take in consideration in latter case its higher RPM and better
   > SCSI interface.
   >
   >
   >
   > Regds
   > Mallah.
   
   

8. Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on faster HDDs

   Rajesh Kumar Mallah <mallah@trade-india.com> — 2002-11-21T19:08:43Z

   
   Thanks Steve,
   
   recently i have come to know that i can only get 3*18 GB ultra160 10K
   hraddrives,
   
   my OS is lunux , other parameters are
   RAM:2GB , CPU:2*2Ghz Xeon,
   
   i feel i will do away with raid use one disk for the OS 
   and pg_dumps
   
   , one for tables and last one for WAL , does this sound good?
   
   regds
   mallah.
   
   
   On Friday 22 November 2002 00:26, Steve Crawford wrote:
   > I had long labored under the impression that RAID 5 should give me better
   > performance but I have since encountered many reports that this is not the
   > case. Do some searching on Google and you will probably find numerous
   > articles.
   >
   > Note 3x18 w/RAID5 will give 36GB usable while 2x36 w/o RAID is 72GB.
   > You could use mirroring on the 2x36 and have the same usable space.
   >
   > A mirrored 2x36 setup will probably yield a marginal hit on writes (vs a
   > single disk) and an improvement on reads due to having two drives to read
   > from and will (based on the Scientific Wild Ass Guess method and knowing
   > nothing about your overall system) probably be faster than the RAID5
   > configuration while giving you identical usable space and data safety.
   >
   > You also may see improvements due to the 15,000RPM drives (of course RPM is
   > sort of an arbitrary measure - you really want to know about track access
   > times, latency, transfer rate, etc. and RPM is just one influencing factor
   > for the above).
   >
   > The quality of your RAID cards will also be important (how fast do they
   > perform their calculations, how much buffer do they have) as will the
   > overall specs of you system. If you have a bottleneck somewhere other than
   > your raw disk I/O then you can throw all the money you want at faster
   > drives and see no improvement.
   >
   > Cheers,
   > Steve
   >
   > On Thursday 21 November 2002 8:45 am, you wrote:
   > > Hi folks,
   > >
   > > I have two options:
   > > 3*18 GB 10,000 RPM Ultra160 Dual Channel SCSI  controller + H/W Raid 5
   > > and
   > > 2*36 GB 15,000 RPM Ultra320 Dual Channel SCSI and no RAID
   > >
   > > Does anyone opinions *performance wise*  the pros and cons of above
   > > two options.
   > >
   > > please take in consideration in latter case its higher RPM and better
   > > SCSI interface.
   > >
   > >
   > >
   > > Regds
   > > Mallah.
   >
   > ---------------------------(end of broadcast)---------------------------
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   -- 
   Rajesh Kumar Mallah,
   Project Manager (Development)
   Infocom Network Limited, New Delhi
   phone: +91(11)6152172 (221) (L) ,9811255597 (M)
   
   Visit http://www.trade-india.com ,
   India's Leading B2B eMarketplace.
   
   
   
   

9. Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on faster HDDs

   Björn Metzdorf <bm@turtle-entertainment.de> — 2002-11-21T19:24:19Z

   > A mirrored 2x36 setup will probably yield a marginal hit on writes (vs a
   > single disk) and an improvement on reads due to having two drives to read
   > from and will (based on the Scientific Wild Ass Guess method and knowing
   
   slightly offtopic:
   
   Does anyone one if linux software raid 1 supports this method (reading from
   both disks, thus doubling performance)?
   
   Regards,
   Bjoern
   
   
   

10. Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on faster HDDs

    Eric Soroos <eric-psql@soroos.net> — 2002-11-21T19:30:44Z

    > Does anyone one if linux software raid 1 supports this method (reading from
    > both disks, thus doubling performance)?
    > 
    
    From memory of reading slightly old (1999) howtos, I believe that the answer is yes, at least for the md system.  Not sure about LVM, or even if mirroring is supported under LVM.
    
    I would guess that it shouldn't be too hard to test:
    
    1) set up dataset on mirred system.
    2) run pg_bench or one of the tpc benches.
    3) fail one of the drives in the mirror.
    4) run the test again. 
    
    If the read latency goes down, it should be reflected in the benchmark. 
    
    eric
    
    
    
    
    

11. Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on

    scott.marlowe <scott.marlowe@ihs.com> — 2002-11-21T19:39:09Z

    On Fri, 22 Nov 2002, Rajesh Kumar Mallah. wrote:
    
    > 
    > 
    > Thanks Steve,
    > 
    > recently i have come to know that i can only get 3*18 GB ultra160 10K
    > hraddrives,
    > 
    > my OS is lunux , other parameters are
    > RAM:2GB , CPU:2*2Ghz Xeon,
    > 
    > i feel i will do away with raid use one disk for the OS 
    > and pg_dumps
    > 
    > , one for tables and last one for WAL , does this sound good?
    
    That depends.  Are you going to be mostly reading, mostly updating, or an 
    even mix of both?
    
    If you are going to be 95% reading, then don't bother moving WAL to 
    another drive, install the OS on the first 2 or 3 gigs of each drive, then 
    make a RAID5 out of what's left over and put everything on that.  
    
    If you're going to be mostly updating, then yes, your setup is a pretty 
    good choice.  
    
    If it will be mostly mixed, look at using a software RAID1.
    
    More important will be tuning your database once it's up, i.e. increasing 
    shared buffers, setting random page costs to reflect what percentage of 
    your dataset is likely to be cached (the closer you come to caching your 
    whole dataset, the closer random page cost approaches 1)
    
    
    
    
    

12. Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on

    Mike Nielsen <miken@bigpond.net.au> — 2002-11-21T20:03:46Z

    Bjoern,
    
    You may find that hoping for a doubling of performance by using RAID 1
    is a little on the optimistic side.
    
    Except on very long sequential reads, media transfer rates are unlikely
    to be the limiting factor in disk throughput.  Seek and rotational
    latencies are the cost factor in random I/O, and with RAID 1, the
    performance gain comes from reducing the mean latency --  on a single
    request, one disk will be closer to the data than the other.  If the
    software that's handling the RAID 1 will schedule concurrent requests,
    you lose the advantage of reducing mean latency in this fashion, but you
    can get some improvement in throughput by overlapping some latency
    periods.
    
    While not wanting to argue against intelligent I/O design, memory is
    cheap these days, and usually gives big bang-for-buck in improving
    response times.
    
    As to the specifics of how one level or another of Linux implements RAID
    1, I'm afraid I can't shed much light at the moment.
    
    Regards,
    
    Mike
    On Fri, 2002-11-22 at 06:24, Bjoern Metzdorf wrote:
    
    > > A mirrored 2x36 setup will probably yield a marginal hit on writes (vs a
    > > single disk) and an improvement on reads due to having two drives to read
    > > from and will (based on the Scientific Wild Ass Guess method and knowing
    > 
    > slightly offtopic:
    > 
    > Does anyone one if linux software raid 1 supports this method (reading from
    > both disks, thus doubling performance)?
    > 
    > Regards,
    > Bjoern
    > 
    > 
    > ---------------------------(end of broadcast)---------------------------
    > TIP 4: Don't 'kill -9' the postmaster
    
    Michael Nielsen
    
    ph: 0411-097-023 email: miken@bigpond.net.au
    
    
    Mike Nielsen
    
    ________________________________________________________________________
    

13. Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on

    scott.marlowe <scott.marlowe@ihs.com> — 2002-11-21T20:17:05Z

    On Thu, 21 Nov 2002, Bjoern Metzdorf wrote:
    
    > > A mirrored 2x36 setup will probably yield a marginal hit on writes (vs a
    > > single disk) and an improvement on reads due to having two drives to read
    > > from and will (based on the Scientific Wild Ass Guess method and knowing
    > 
    > slightly offtopic:
    > 
    > Does anyone one if linux software raid 1 supports this method (reading from
    > both disks, thus doubling performance)?
    
    Yes, it does.  Generally speaking, it increases raw throughput by a factor 
    of 2 if you're grabbing enough data to justify reading it from both 
    drives.  But for most database apps, you don't read enough at a time to 
    get a gain from this.  I.e. if your stripe size is 8k and you're reading 
    1k at a time, no gain.
    
    However, under parallel load, the extra drives really help.
    
    In fact, the linux kernel supports >2 drives in a mirror.  Useful for a 
    mostly read database that needs to handle lots of concurrent users.
    
    
    

14. Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on

    Björn Metzdorf <bm@turtle-entertainment.de> — 2002-11-21T20:53:02Z

    > In fact, the linux kernel supports >2 drives in a mirror.  Useful for a 
    > mostly read database that needs to handle lots of concurrent users.
    
    Good to know.
    
    What do you think is faster: 3 drives in raid 1 or 3 drives in raid 5?
    
    Regards,
    Bjoern
    
    
    

15. Re: [ADMIN] H/W RAID 5 on slower disks versus no

    Josh Berkus <josh@agliodbs.com> — 2002-11-21T21:20:56Z

    Bjoern,
    
    > Good to know.
    > 
    > What do you think is faster: 3 drives in raid 1 or 3 drives in raid
    > 5?
    
    My experience?  Raid 1.  But that depends on other factors as well;
    your controller (software controllers use system RAM and thus lower
    performance), what kind of reads you're getting and how often.  IMHO,
    RAID 5 is faster for sequential reads (lareg numbers of records on
    clustered tables), RAID 1 for random reads.
    
    And keep in mind: RAID 5 is *bad* for data writes.  In my experience,
    database data-write performance on RAID 5 UW SCSI is as slow as IDE
    drives, particulary for updating large numbers of records, *unless* the
    updated records are sequentially updated and clustered.
    
    But in a multi-user write-often setup, RAID 5 will slow you down and
    RAID 1 is better.
    
    Did that help?
    
    -Josh Berkus
    
    

16. Re: [ADMIN] H/W RAID 5 on slower disks versus no

    Josh Berkus <josh@agliodbs.com> — 2002-11-21T21:21:16Z

    Bjoern,
    
    > Good to know.
    > 
    > What do you think is faster: 3 drives in raid 1 or 3 drives in raid
    > 5?
    
    My experience?  Raid 1.  But that depends on other factors as well;
    your controller (software controllers use system RAM and thus lower
    performance), what kind of reads you're getting and how often.  IMHO,
    RAID 5 is faster for sequential reads (lareg numbers of records on
    clustered indexes), RAID 1 for random reads.
    
    And keep in mind: RAID 5 is *bad* for data writes.  In my experience,
    database data-write performance on RAID 5 UW SCSI is as slow as IDE
    drives, particulary for updating large numbers of records, *unless* the
    updated records are sequentially updated and clustered.
    
    But in a multi-user write-often setup, RAID 5 will slow you down and
    RAID 1 is better.
    
    Did that help?
    
    -Josh Berkus
    
    

17. Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on

    scott.marlowe <scott.marlowe@ihs.com> — 2002-11-21T21:24:00Z

    On Thu, 21 Nov 2002, Bjoern Metzdorf wrote:
    
    > > In fact, the linux kernel supports >2 drives in a mirror.  Useful for a 
    > > mostly read database that needs to handle lots of concurrent users.
    > 
    > Good to know.
    > 
    > What do you think is faster: 3 drives in raid 1 or 3 drives in raid 5?
    
    Generally RAID 5.  RAID 1 is only faster if you are doing a lot of 
    parellel reads.  I.e. you have something like 10 agents reading at the 
    same time.  RAID 5 also works better under parallel load than a single 
    drive.
    
    The fastest of course, is multidrive RAID0.  But there's no redundancy.
    
    Oddly, my testing doesn't show any appreciable performance increase in 
    linux by layering RAID5 or 1 over RAID0 or vice versa, something that 
    is usually faster under most setups.
    
    
    

18. Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on

    Björn Metzdorf <bm@turtle-entertainment.de> — 2002-11-21T21:57:59Z

    > Generally RAID 5.  RAID 1 is only faster if you are doing a lot of
    > parellel reads.  I.e. you have something like 10 agents reading at the
    > same time.  RAID 5 also works better under parallel load than a single
    > drive.
    
    yep, but write performance sucks.
    
    > The fastest of course, is multidrive RAID0.  But there's no redundancy.
    
    With 4 drives I'd always go for raid 10, fast and secure
    
    > Oddly, my testing doesn't show any appreciable performance increase in
    > linux by layering RAID5 or 1 over RAID0 or vice versa, something that
    > is usually faster under most setups.
    
    Is this with linux software raid? raid10 is not significantly faster? cant
    believe that...
    
    Regards,
    Bjoern
    
    
    

19. Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on

    scott.marlowe <scott.marlowe@ihs.com> — 2002-11-21T22:37:47Z

    On Thu, 21 Nov 2002, Bjoern Metzdorf wrote:
    
    > > Generally RAID 5.  RAID 1 is only faster if you are doing a lot of
    > > parellel reads.  I.e. you have something like 10 agents reading at the
    > > same time.  RAID 5 also works better under parallel load than a single
    > > drive.
    > 
    > yep, but write performance sucks.
    
    Well, it's not all that bad.  After all, you only have to read the parity 
    stripe and data stripe (two reads) update the data stripe, xor the new 
    data stripe against the old parity stripe, and write both.  In RAID 1 you 
    have to read the old data stripe, update it, and then write it to two 
    drives.  So, generally speaking, it's not that much more work on RAID 5 
    than 1.  My experience has been that RAID5 is only about 10 to 20% percent 
    slower than RAID1 in writing, if that.
    
    > > The fastest of course, is multidrive RAID0.  But there's no redundancy.
    > 
    > With 4 drives I'd always go for raid 10, fast and secure
    > 
    > > Oddly, my testing doesn't show any appreciable performance increase in
    > > linux by layering RAID5 or 1 over RAID0 or vice versa, something that
    > > is usually faster under most setups.
    > 
    > Is this with linux software raid? raid10 is not significantly faster? cant
    > believe that...
    
    Yep, Linux software raid.  It seems like it doesn't parallelize well.  
    That's with several different setups.  I've tested it on a machine a dual 
    Ultra 40/80 controller and 6 Ultra wide 10krpm SCSI drives, and no matter 
    how I arrange the drives, 50, 10, 01, 05, the old 1 or 5 setups are just 
    about as fast.
    
    
    

20. Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on

    Mario Weilguni <mweilguni@sime.com> — 2002-11-22T07:31:11Z

    Am Donnerstag, 21. November 2002 21:53 schrieb Bjoern Metzdorf:
    > > In fact, the linux kernel supports >2 drives in a mirror.  Useful for a
    > > mostly read database that needs to handle lots of concurrent users.
    >
    > Good to know.
    >
    > What do you think is faster: 3 drives in raid 1 or 3 drives in raid 5?
    >
    > Regards,
    > Bjoern
    >
    
    If 4 drives are an option, I suggest 2 x RAID1, one for data, and one for WAL and temporary DB space (pg_temp).
    
    Regards,
    	Mario Weilguni
    
    
    
    

21. Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on

    Tom Lane <tgl@sss.pgh.pa.us> — 2002-11-22T13:52:48Z

    Mario Weilguni <mweilguni@sime.com> writes:
    > If 4 drives are an option, I suggest 2 x RAID1, one for data, and one for WAL and temporary DB space (pg_temp).
    
    Ideally there should be *nothing* on the WAL drive except WAL; you don't
    ever want that disk head seeking away from the WAL.  Put the temp files
    on the data disk.
    
    			regards, tom lane
    
    

22. Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on

    philip johnson <philip.johnson@atempo.com> — 2002-11-22T14:17:26Z

    pgsql-performance-owner@postgresql.org wrote:
    > Objet : Re: [PERFORM] [ADMIN] H/W RAID 5 on slower disks versus no
    > raid on
    > 
    > 
    > Mario Weilguni <mweilguni@sime.com> writes:
    >> If 4 drives are an option, I suggest 2 x RAID1, one for data, and
    >> one for WAL and temporary DB space (pg_temp). 
    > 
    > Ideally there should be *nothing* on the WAL drive except WAL; you
    > don't ever want that disk head seeking away from the WAL.  Put the
    > temp files on the data disk.
    > 
    > 			regards, tom lane
    > 
    > ---------------------------(end of
    > broadcast)--------------------------- TIP 4: Don't 'kill -9' the
    > postmaster 
    
    which temp files ?
    
    

23. Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on

    Andrew Sullivan <andrew@libertyrms.info> — 2002-11-22T15:01:52Z

    On Fri, Nov 22, 2002 at 08:52:48AM -0500, Tom Lane wrote:
    > Mario Weilguni <mweilguni@sime.com> writes:
    > > If 4 drives are an option, I suggest 2 x RAID1, one for data, and one for WAL and temporary DB space (pg_temp).
    > 
    > Ideally there should be *nothing* on the WAL drive except WAL; you don't
    > ever want that disk head seeking away from the WAL.  Put the temp files
    > on the data disk.
    
    Unless the interface and disks are so fast that it makes no
    difference.
    
    Try as I might, I can't make WAL go any faster on its own controller
    and disks than if I leave it on the same filesystem as everything
    else, on our production arrays.  We use Sun A5200s, and I have tried
    it set up with the WAL on separate disks on the box, and on separate
    disks in the array, and even on separate disks on a separate
    controller in the array (I've never tried it with two arrays, but I
    don't have infinite money, either).  I have never managed to
    demonstrate a throughput difference outside the margin of error of my
    tests.  One arrangement -- putting the WAL on a separate pair of UFS
    disks using RAID 1, but not on the fibre channel -- was demonstrably
    slower than leaving the WAL in the data area.
    
    Nothing is proved by this, of course, except that if you're going to
    use high-performance hardware, you have to tune and test over and
    over again.  I was truly surprised that a separate pair of VxFS
    RAID-1 disks in the array were no faster, but I guess it makes sense:
    the array is just as busy in either case, and the disks are really
    fast.  I still don't really believe it, though.
    
    A
    
    -- 
    ----
    Andrew Sullivan                         204-4141 Yonge Street
    Liberty RMS                           Toronto, Ontario Canada
    <andrew@libertyrms.info>                              M2P 2A8
                                             +1 416 646 3304 x110
    
    
    

24. Re: H/W RAID 5 on slower disks versus no raid on faster HDDs

    Merlin Moncure <merlin@rcsonline.com> — 2002-11-27T18:16:35Z

    On Mon, 2002-11-25 at 23:03, David Gilbert wrote:
    >
    > I'm on a bit of a mission to stamp out this misconception.  In my
    > testing, all but the most expensive hardware raid controllers are
    > actually slower than FreeBSD's software RAID.  I've done my tests with
    > a variety of controllers with the same data load and the same disks.
    >
    
    I agree 100%: hardware raid sucks.
    We had a raid 5 Postgres server on midgrade hardware with 5 60gig 7200rpm
    IDE disks (240 gig total) and the thouroughput was just as high (maybe 10%
    less) than a single disk.  Same for the seek times.  CPU around 1Ghz never
    hit more than 10% for the raid service.  Since very few databases are CPU
    limited, this is a small price to pay.
    
    We confirmed the performance results with heavy testing.  There is virtually
    no disadvatage to software raid, just spend 10$ and get a 10% faster cpu.
    
    The linux software raid drivers (and others I assume) are very optimized.
    Not to sure about m$ but win2k comes with raid services, its pretty
    reasonalbe to believe they work ok.
    
    You can double the performance of a raid system by going 0+x or x+0 (eg 05
    or 50).  just by adding drives.   This really doubles it, and an optmized
    software driver improves the seek times too by placing the idle heads it
    different places on the disks.
    
    p.s. scsi is a huge waste of money and is no faster than IDE.  IMHO, scsi's
    only advantage is longer cables.  Both interfaces will soon be obsolete with
    the coming serial ATA.  High rpm disks are very expensive and add latent
    heat to your system.  Western digitial's IDE disks outperform even top end
    scsi disks at a fraction of a cost.  You can install a 4 drive 10 raid setup
    for the cost of a single 15k rpm scsi disk that will absolutely blow it away
    in terms of performance (reliability too).
    
    Just remember, don't add more than one IDE disk on a raid system to a single
    IDE channel!  Also, do not attempt to buy IDE cables longer than 18"..they
    will not be reliable.
    
    Merlin
    
    
    
    

25. Re: H/W RAID 5 on slower disks versus no raid on faster HDDs

    David Jericho <david.jericho@bytecomm.com.au> — 2002-11-29T02:58:31Z

    On Wed, Nov 27, 2002 at 01:16:35PM -0500, Merlin Moncure wrote:
    > I agree 100%: hardware raid sucks.
    
    I've been mostly ignoring this thread, but I'm going to jump in at
    this point. 
    
    > We confirmed the performance results with heavy testing.  There is virtually
    > no disadvatage to software raid, just spend 10$ and get a 10% faster cpu.
    
    Define heavy testing. 
    
    I can do sequential selects on a low end PC with one client and have it 
    perform as well as an E10K. I could also fire up 600 clients doing 
    seemingly random queries and updates and reduce the same low end PC to 
    a smoldering pile of rubble. 
    
    It'd be easy to fudge the results of the "heavy testing" to match what I 
    wanted to believe.
    
    > The linux software raid drivers (and others I assume) are very optimized.
    
    As are the Solaris drivers, and many others. But there is more to a
    RAID array than drivers. There's the stability of the controller
    chipsets and the latency involved in getting data to and from the
    devices.
    
    Would you feel comfortable if you knew the state data for the aircraft
    you're travelling on was stored on IDE software RAID?
    
    Part of the point of hardware raid is that it does do a small set
    of operations, and therefore far easier to specify and validate the
    correct operation of the software and hardware.
    
    > p.s. scsi is a huge waste of money and is no faster than IDE.  IMHO, scsi's
    > only advantage is longer cables.  Both interfaces will soon be obsolete with
    > the coming serial ATA.
    
    Don't get me wrong, I'm a huge fan of IDE RAID in the right locations,
    but comments like this reflect a total lack of understanding what
    makes SCSI a better protocol over IDE.
    
    Disconnected operation is one _HUGE_ benefit of SCSI, simply being the
    ability for the CPU and controller to send a command, and then both head 
    off to do another task while waiting for data to be returned from the 
    device. Most (that is most, not all) IDE controllers are incapable of 
    this. Another is command reordering (which I believe SATA does have),
    being the reordering of requests to better utilise each head sweep.
    
    This feature comes into play far more obviously when you have many
    clients performing operations across a large dataset where the
    elements have no immediate relationship to each other.
    
    It is amplified when your database of such a size, and used in a way
    that you have multiple controllers with multiple spools.
    
    SCSI is not about speed to and from the device, although this does end
    up being a side effect of the design. It's about latency, and removal of 
    contention from the shared bus. 
    
    Ultra/320 devices in reality are no faster than Ultra/160 devices.
    What is faster, is that you can now have 4 devices instead of 2 on the
    same bus, with lower request latency and no reduction in
    throughput performance.
    
    SCSI also allows some more advanced features too. Remote storage 
    over fibre, iSCSI, shared spools just to name a few.
    
    > High rpm disks are very expensive and add latent heat to your system. 
    
    If you have a real justification for SCSI in your database server, you
    probably do have both the cooling and the budget to accomodate it.
    
    > Western digitial's IDE disks outperform even top end scsi disks at a 
    > fraction of a cost. 
    
    Complete and utter rubbish. That's like saying your 1.1 litre small
    city commuter hatch can outperform a 600hp Mack truck.
    
    Yes, in the general case it's quite probable. Once you start
    shuffling real loads IDE will grind the machine to a halt. Real
    database iron does not use normal IDE.
    
    > You can install a 4 drive 10 raid setup for the cost of a single 15k 
    > rpm scsi disk that will absolutely blow it away in terms of performance 
    
    See above. Raw disk speed does not equal performance. Database spool
    performance is a combination of a large number of factors, one being
    seek time, and another being bus contention.
    
    > (reliability too).
    
    Now you're smoking crack. Having run some rather large server farms
    for some very large companies, I can tell you with both anecdotal, and
    recorded historical evidence that the failure rate for IDE was at
    least double, if not four times that of the SCSI hardware.
    
    And the IDE hardware was under much lower loads than the SCSI
    hardware.
    
    > Just remember, don't add more than one IDE disk on a raid system to a single
    > IDE channel!  Also, do not attempt to buy IDE cables longer than 18"..they
    > will not be reliable.
    
    So now you're pointing out that you share PCI bus interrupts over a large 
    number of devices, introducing another layer of contention and that 
    you'll have to cable your 20 spool machine with 20 cables each no longer 
    than 45cm. Throw in some very high transaction rates, and a large 
    data set that won't fit in your many GB of ram.
    
    I believe the game show sound effect would be similar to "Bzzzt".
    
    IDE for the general case is acceptable. SCSI is for everything else.
    
    -- 
    David Jericho
    Senior Systems Administrator, Bytecomm Pty Ltd
    
    
    --
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