linux-kernel - Re: [ext3][kernels >= 2.6.20.7 at least] KDE going comatose when FS is under heavy write load (massive starvation)

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Date:	Fri, 27 Apr 2007 13:09:06 -0600
From:	Zan Lynx <zlynx@....org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mike Galbraith <efault@....de>,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [ext3][kernels >= 2.6.20.7 at least] KDE going comatose when
	FS is under heavy write load (massive starvation)

On Fri, 2007-04-27 at 11:31 -0700, Andrew Morton wrote:
[snip]
> ext3's problem here is that a single fsync() requires that ext3 sync the
> whole filesystem.  Because
> 
> - a journal commit can contain metadata from multiple files, and if we
>   want to journal one file's metadata via fsync(), we unavoidably journal
>   all the other file's metadata at the same time.
> 
> - ordered mode requires that we write a file's data blocks prior to
>   journalling the metadata which refers to those blocks.
> 
> net result: syncing anything syncs the whole world.
> 
> There are a few areas in which this could conceivably be tuned up: if a
> particular file doesn't currently have any metadata in the commit, we don't
> actually need to sync its data blocks: we could just transfer them into
> next commit.  Hard, unlikely to be of benefit.
[snip]

How about mixing the ordered and data journal modes?  If the data blocks
would fit, have fsync write them into the journal as is done in
data=journal mode.  Then that file data is committed to disk as fsync
requires, but it shouldn't require flushing all the previous metadata to
get an ordered guarantee.

Or so it seems to me.
-- 
Zan Lynx <zlynx@....org>

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