Date: Sat, 19 Aug 2006 22:46:03 -0700
From: Andrew Morton <akpm@osdl.org>
To: Ron Yorston <rmy@tigress.co.uk>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] ext2: avoid needless discard of preallocated blocks
Message-Id: <20060819224603.bf687be2.akpm@osdl.org>
In-Reply-To: <200608171945.k7HJjaLk029781@tiffany.internal.tigress.co.uk>
References: <200608171945.k7HJjaLk029781@tiffany.internal.tigress.co.uk>
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On Thu, 17 Aug 2006 20:45:36 +0100
Ron Yorston <rmy@tigress.co.uk> wrote:

> Currently preallocated blocks in ext2 are discarded on every call
> to iput() (by ext2_put_inode() calling ext2_discard_prealloc()).
> 
> An earlier attempt to fix this ("discard ext2 preallocation in last
> iput") moved the ext2_discard_prealloc() call to ext2_clear_inode(),
> but was found to cause filesystem corruption in a test using fsx.
> The problem was that ext2_clear_inode() was writing the inode data
> to disk before calling ext2_discard_prealloc(), so the value of
> i_blocks on disk included the preallocated blocks.
> 
> This patch moves the call to ext2_discard_prealloc() to the new
> function ext2_drop_inode().  This should be both efficient (discard
> happens on only the last call to iput()) and correct (fixes i_blocks
> before writing to disk).  Also, as there is now possibly a longer
> window during which an open file may have an incorrrect block count
> in its on-disk inode, ext2_update_inode adjusts the block count to
> account for preallocated blocks.

Been there, done that.  The problem was that hanging onto the preallocation
will cause separate files to have up-to-seven-block gaps between them.  So
if you put a large number of small files in the same directory, the time to
read them all back is quite significantly impacted: they cover a lot more
disk.
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