Re: Random file I/O regressions in 2.6

[Nick Piggin <nickpiggin@yahoo.com.au>]

Andrew Morton wrote:
> Peter Zaitsev <peter@mysql.com> wrote:
> 
>>On Thu, 2004-05-06 at 01:43, Andrew Morton wrote:
>>
>>
>>>One thing I note about this test is that it generates a huge number of
>>>inode writes.  atime updates from the reads and mtime updates from the
>>>writes.  Suppressing them doesn't actually make a lot of performance
>>>difference, but that is with writeback caching enabled.  I expect that with
>>>a writethrough cache these will really hurt.
>>
>>Perhaps.  By the way is there a way to disable update time modification
>>as well ?
> 
> 
> No, there is not.
> 
> 
>>It would make quite a good sense for partition used for
>>Database needs - you do not need last modification time in most cases.
> 
> 
> First up, one needs to remove the inode_update_time() call from
> generic_file_aio_write_nolock() and run the tests.  If this (and noatime)
> indeed makes a significant difference (probably on writethrough-caching
> disks) then yup, we should do something.
> 
> `nomtime' would be simple enough.  But another option would be to arrange
> for a/m/ctime dirtiness to not cause an inode writeout in fsync(). 
> Instead, only sync the a/m/ctime-dirty inodes via sync, umount and pdflush.
> 
> That way, the inodes get written every thirty seconds rather than once per
> second.
> 
> It's probably not standards-compliant, but shoot me.  Who cares if the
> mtimes come up 30 seconds out of date after a system crash?
> 
> `nomtime' would be simpler and safer to implement, but not as nice.
> 
> But we need those numbers first.  I'll take a look.
> 

Can they use fdatasync? Does it do the right thing on Linux?