linux-ext4 - 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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Message-ID: <463AD024.6060208@clusterfs.com>
Date:	Fri, 04 May 2007 10:18:12 +0400
From:	Alex Tomas <alex@...sterfs.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Andreas Dilger <adilger@...sterfs.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Marat Buharov <marat.buharov@...il.com>,
	Mike Galbraith <efault@....de>,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [ext3][kernels >= 2.6.20.7 at least] KDE going comatose when
 FS is under heavy write load (massive starvation)

Andrew Morton wrote:
> Yes, there can be issues with needing to allocate journal space within the
> context of a commit.  But

no-no, this isn't required. we only need to mark pages/blocks within
transaction, otherwise race is possible when we allocate blocks in transaction,
then transacton starts to commit, then we mark pages/blocks to be flushed
before commit.

> a) If the page has newly allocated space on disk then the metadata which
>    refers to that page is already in the journal: no new journal space
>    needed.
> 
> b) If the page doesn't have space allocated on disk then we don't need
>    to write it out at ordered-mode commit time, because the post-recovery
>    filesystem will not have any references to that page.
> 
> c) If the page is dirty due to overwrite then no metadata update was required.
> 
> IOW, under what circumstances would an ordered-mode commit need to allocate
> space for a delayed-allocate page?

no need to allocate space within commit thread, I think. only to take care
of the race I described above. in hackish version of data=ordered for delayed
allocation I used counter of submitted bio's with newly-allocated blocks and
commit thread waits for the counter to reach 0.

> 
> However b) might lead to the hey-my-file-is-full-of-zeroes problem.
> 

thanks, Alex

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