linux-ext4 - Re: The meaning of data=ordered as it relates to delayed allocation

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Date:	Tue, 20 Jan 2009 03:10:41 +0800
From:	Andreas Dilger <adilger@....com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: The meaning of data=ordered as it relates to delayed allocation

On Jan 18, 2009  19:52 -0500, Theodore Ts'o wrote:
> An Ubuntu user recently complained about a large number of recently
> updated files which were zero-length after an crash.  I started looking
> more closely at that, and it's because we have an interesting
> interpretation of data=ordered.  It applies for blocks which are already
> allocated, but not for blocks which haven't been allocated yet.  This
> can be surprising for users; and indeed, for many workloads where you
> aren't using berk_db some other database, all of the files written will 
> be newly created files (or files which are getting rewritten after
> opening with O_TRUNC), so there won't be any difference between
> data=writeback and data=ordered.
> 
> So I wonder if we should either:
> 
> (a) make data=ordered force block allocation and writeback --- which
>     should just be a matter of disabling the
>     redirty_page_for_writepage() code path in ext4_da_writepage()

That would re-introduce the "Firefox" problem where fsync of one file forces
all other files being written to flush their data blocks to disk.

> (b) add a new mount option, call it data=delalloc-ordered which is (a)

I'd prefer a better name, like "flushall-ordered" or similar, because to
me "delalloc-ordered" would imply the current behaviour.

> (c) change the default mount option to be data=writeback

That can expose garbage data to the user, which the current behaviour
does not do.

> (d) Do (b) and make it the default
> 
> (e) Keep things the way they are
> 
> Thoughts, comments?   My personal favorite is (b).   This allows users
> who want something that works functionally much more like ext3 to get
> that, while giving us the current speed advantages of a more aggressive
> delayed allocation.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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