Thread

1. Why O_SYNC is faster than fsync on ext3

   Yusuf Goolamabbas <yusufg@outblaze.com> — 2004-03-21T04:37:36Z

   I sent this to Bruce but forgot to cc pgsql-hackers, The patches are
   likely to go into 2.6.6. People interested in extremely safe fsync
   writes should also follow the IDE barrier thread and the true fsync() in
   Linux on IDE thread
   
   ----- Forwarded message from Yusuf Goolamabbas <yusufg@outblaze.com> -----
   
   Date: Sat, 20 Mar 2004 20:52:34 +0800
   From: Yusuf Goolamabbas <yusufg@outblaze.com>
   To: Bruce Momjian <pgman@candle.pha.pa.us>
   Subject: Your fsync thread on hackers
   Message-ID: <20040320125234.GA11221@outblaze.com>
   
   Bruce, haven't followed the thread completely. Accessing the web archive
   is slow from Hong Kong but I just wanted to point you to this lkml post
   which shows why O_SYNC is much faster than fsync (at least on ext3)
   
   http://marc.theaimsgroup.com/?l=linux-kernel&m=107959907410443&w=2
   
   There are some pending fsync speedups on XFS also. You might want to
   consider pointing Tom to do this so he can get the Redhat/Fedora guys to
   look at the patches
   
   Hope this helps, Regards, Yusuf
   
   ----- End forwarded message -----
   
   -- 
   If you're not using Firefox, you're not surfing the web 
      you're suffering it
   http://www.mozilla.org/products/firefox/why/
   
   

2. Re: Why O_SYNC is faster than fsync on ext3

   Manfred Spraul <manfred@colorfullife.com> — 2004-03-21T10:45:18Z

   Yusuf Goolamabbas wrote:
   
   >I sent this to Bruce but forgot to cc pgsql-hackers, The patches are
   >likely to go into 2.6.6. People interested in extremely safe fsync
   >writes should also follow the IDE barrier thread and the true fsync() in
   >Linux on IDE thread
   >  
   >
   Actually the most interesting part of the thread was the initial post 
   from Peter Zaitsev on a fcntl(fd, F_FULLSYNC, NULL): He wrote that this 
   is necessary for Mac OS X to force a flush of the write caches in the 
   disks. Unfortunately I can't find anything about this flag with google.
   
   Another interesting point is that right now, ide write caches must be 
   disabled for reliable fsync operations  with Linux. Recent suse kernels 
   contain partial support. If the existing patches are completed and 
   merged, it will be safe to enable write caching.
   
   Perhaps Bruce's cache flush test could be modified slightly to check 
   that the OS isn't lying about fsync: if fsync is faster than the 
   rotational delay of the disks, then the setup is not suitable for 
   postgres. This could be recommended as a setup test in the install document.
   
   --
       Manfred
   
   
   

3. Re: Why O_SYNC is faster than fsync on ext3

   Tom Lane <tgl@sss.pgh.pa.us> — 2004-03-21T16:10:32Z

   Yusuf Goolamabbas <yusufg@outblaze.com> writes:
   > Bruce, haven't followed the thread completely. Accessing the web archive
   > is slow from Hong Kong but I just wanted to point you to this lkml post
   > which shows why O_SYNC is much faster than fsync (at least on ext3)
   > http://marc.theaimsgroup.com/?l=linux-kernel&m=107959907410443&w=2
   
   That patch is broken on its face.  If O_SYNC doesn't take longer than
   O_DSYNC, and likewise fsync longer than fdatasync, then the Unix
   filesystem semantics are not being honored because the file mod time
   isn't being updated.
   
   			regards, tom lane