Re: [RFC] ext3: per-process soft-syncing data=ordered mode

[Andreas Dilger]

On Jan 26, 2008  08:27 +0300, Al Boldi wrote:
> Jan Kara wrote:
> > > data=ordered mode has proven reliable over the years, and it does this
> > > by ordering filedata flushes before metadata flushes.  But this
> > > sometimes causes contention in the order of a 10x slowdown for certain
> > > apps, either due to the misuse of fsync or due to inherent behaviour
> > > like db's, as well as inherent starvation issues exposed by the
> > > data=ordered mode.
> > >
> > > data=writeback mode alleviates data=order mode slowdowns, but only works
> > > per-mount and is too dangerous to run as a default mode.
> > >
> > > This RFC proposes to introduce a tunable which allows to disable fsync
> > > and changes ordered into writeback writeout on a per-process basis like
> > > this:
> > >
> > >       echo 1 > /proc/`pidof process`/softsync
> >
> >   I guess disabling fsync() was already commented on enough. Regarding
> > switching to writeback mode on per-process basis - not easily possible
> > because sometimes data is not written out by the process which stored
> > them (think of mmaped file).
> 
> Do you mean there is a locking problem?
> 
> > And in case of DB, they use direct-io
> > anyway most of the time so they don't care about journaling mode anyway.
> 
> Testing with sqlite3 and mysql4 shows that performance drastically improves 
> with writeback writeout.
> 
> >  But as Diego wrote, there is definitely some room for improvement in
> > current data=ordered mode so the difference shouldn't be as big in the
> > end.
> 
> Yes, it would be nice to get to the bottom of this starvation problem, but 
> even then, the proposed tunable remains useful for misbehaving apps.

Al, can you try a patch posted to linux-fsdevel and linux-ext4 from
Hisashi Hifumi <hifumi.hisashi@xxxxxxxxxxxxx> to see if this improves
your situation?  Dated Mon, 04 Feb 2008 19:15:25 +0900.

    [PATCH] ext3,4:fdatasync should skip metadata writeout when overwriting

It may be that we already have a solution in that patch for database
workloads where the pages are already allocated by avoiding the need
for ordered mode journal flushing in that case.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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