linux-ext4 - Re: delayed allocation blocks not flushed?

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Date:	Mon, 27 May 2013 22:35:01 +0200
From:	Jan Kara <jack@...e.cz>
To:	Bert De Jonghe <Bert.DeJonghe@...lidata.com>
Cc:	linux-ext4@...r.kernel.org,
	Bastiaan Stougie <bstougie@...lidata.com>
Subject: Re: delayed allocation blocks not flushed?

  Hello,

On Mon 27-05-13 09:54:13, Bert De Jonghe wrote:
> On an otherwise idle system, create an ext4 filesystem on /dev/sdi,
> mount it and wait more than dirty_writeback_centisecs.
  What kernel version are you using?

> Then create a small file (echo something > file);  wait
> (dirty_expire_centisecs x 2), expect delayed_allocation_blocks to
> fall to zero but it remains at 2 over here.
> 
> Create another small file (echo something_else > file2); again wait
> (dirty_expire_centisecs x 2); check delayed_allocation_blocks which
> is now at 4.
>
> Wait a weekend more:
> 
>    # date; cat /sys/fs/ext4/sdi/delayed_allocation_blocks
>    Fri May 24 15:33:21 UTC 2013
>    4
> 
>    # date; cat /sys/fs/ext4/sdi/delayed_allocation_blocks
>    Mon May 27 07:14:30 UTC 2013
>    4
> 
> Doing a manual sync flushes out all blocks, also adding a sync after
> mounting solves the not flushing behaviour (possible patch
> attached).
> 
> Has anyone noticed this before and is this expected behaviour? It
> looks like these blocks are not flushed out to disk thus remain in
> memory and will be lost upon power failure?
  This would be a bug. I have tried to recreate this with my 3.7.10 kernel
but I failed. File gets flushed to disk after 30s or so. What kernel
version are you using?

Also I don't see why syncing after mount would help to resolve the problem
that files are not flushed to disk in a timely manner...

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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