linux-ext4 - Re: Query about DIO/AIO WRITE throttling and ext4 serialization

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Date:	Thu, 2 Jun 2011 11:56:10 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: Query about DIO/AIO WRITE throttling and ext4 serialization

On Thu, Jun 02, 2011 at 10:36:33AM -0400, Vivek Goyal wrote:
> On Thu, Jun 02, 2011 at 10:17:16AM -0400, Vivek Goyal wrote:
> > On Thu, Jun 02, 2011 at 11:22:09AM +1000, Dave Chinner wrote:
> > > On Wed, Jun 01, 2011 at 05:50:49PM -0400, Vivek Goyal wrote:
> > > > Hi,
> > > > 
> > > > If I throttle a DIO/AIO WRITE bio at block device in a cgroup, will it
> > > > lead to any kind of serialization of ext4 file system. IOW, is there any
> > > > filesystem operation which will wait for that DIO/AIO WRITE to finish
> > > > before other filesystem can make progress (fsync, journalling etc?)
> > > 
> > > Truncate?
> > > 
> > > (XFS explicitly serialises truncate against in flight DIO,
> > > regardless of whether ext4 does.)
> > > 
> > 
> > Dave,
> > 
> > Does this serialization happens against that particular inode on which
> > truncate has been called? If yes, then I think I will still be fine
> > as in common use case I am not expecting much sharing of inodes across
> > cgroups.
> 
> Dave,
> 
> I did a quick test of throttling a direct IO on one file and then
> doing "truncate -s 40 testfile" on a different file in different
> cgroup and it seems to work fine.
> 
> But I seem to be having issues with "sync". Looks like in ext4, if
> I throttle a DIO, sync does not hang but in XFS it does. I am 
> wondering if XFS is waiting for all inflight DIO to finish before
> sync completes.

"sync" on XFS seems to be livelocking as long as DIO write operation
is going on and same does not happen on ext4.

I ran "aio-stress -O aiofile1 -s 4G" and in other window I did "sync"
and it does not finish untile and unless aio-stress has finished.
On the other hand ext4 seems to be fine and it does finish earlier.

Thanks
Vivek
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