linux-ext4 - delalloc makes data=writeback safer

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Message-ID: <877hm8jt4g.fsf@openvz.org>
Date:	Wed, 09 Jun 2010 16:19:27 +0400
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	linux-ext4@...r.kernel.org
Subject: delalloc makes data=writeback safer

Everybody know that writeback mode for ext3 is potentially result in
stale data in case of unclean umount. The same is true for ext4 with
nodelalloc.
But as far as i can see things is getting better in case of delalloc.
In case of delalloc we have following calltrace:
[writeback thread]
ext4_da_writepages
  journal_start()
  ext4_map_blocks() -> real block allocation
  mpage_da_submit_io() -> submit_bh() -> submit_bio()
  journal_stop()
So journal will be closed only after data was issued. 
And if journal->j_dev == journal->j_fs_dev (which is usually the case)
data will be serialized with metadata. So it looks like it is impassible
to see stale data even after unclean umount.

Do I miss something? May be this was already discussed before, 
if so please post a link to previous discussion.
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