linux-ext4 - delayed allocation blocks not flushed?

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Date:	Mon, 27 May 2013 09:54:13 +0200
From:	Bert De Jonghe <Bert.DeJonghe@...lidata.com>
To:	linux-ext4@...r.kernel.org
CC:	Bastiaan Stougie <bstougie@...lidata.com>
Subject: delayed allocation blocks not flushed?

Hi,

On an otherwise idle system, create an ext4 filesystem on /dev/sdi, 
mount it and wait more than dirty_writeback_centisecs.

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?

Please Cc on reply as we're not on the list.

Many thanks & best regards,
Bert.

View attachment "sync_filesystem.patch" of type "text/x-patch" (218 bytes)