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

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Message-ID: <20110602143633.GE18712@redhat.com>
Date:	Thu, 2 Jun 2011 10:36:33 -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 10:17:16AM -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.

Thanks
Vivek
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