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

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Date:	Sat, 2 Feb 2008 00:26:00 +0300
From:	Al Boldi <a1426z@...ab.com>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Jan Kara <jack@...e.cz>, Andreas Dilger <adilger@....com>,
	Chris Snook <csnook@...hat.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] ext3: per-process soft-syncing data=ordered mode

Chris Mason wrote:
> On Thursday 31 January 2008, Jan Kara wrote:
> > On Thu 31-01-08 11:56:01, Chris Mason wrote:
> > > On Thursday 31 January 2008, Al Boldi wrote:
> > > > The big difference between ordered and writeback is that once the
> > > > slowdown starts, ordered goes into ~100% iowait, whereas writeback
> > > > continues 100% user.
> > >
> > > Does data=ordered write buffers in the order they were dirtied?  This
> > > might explain the extreme problems in transactional workloads.
> >
> >   Well, it does but we submit them to block layer all at once so
> > elevator should sort the requests for us...
>
> nr_requests is fairly small, so a long stream of random requests should
> still end up being random IO.
>
> Al, could you please compare the write throughput from vmstat for the
> data=ordered vs data=writeback runs?  I would guess the data=ordered one
> has a lower overall write throughput.

That's what I would have guessed, but it's actually going up 4x fold for 
mysql from 559mb to 2135mb, while the db-size ends up at 549mb.

This may mean that data=ordered isn't buffering redundant writes; or worse.


Thanks!

--
Al

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