linux-ext4 - Re: delalloc makes data=writeback safer

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Message-ID: <20100615140801.GC18513@atrey.karlin.mff.cuni.cz>
Date:	Tue, 15 Jun 2010 16:08:02 +0200
From:	Jan Kara <jack@...e.cz>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: 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.
  Well, we turn off delayed allocation when a filesystem gets close to
ENOSPC so it's not completely reliable. But besides this, your observation
looks correct to me.

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