linux-ext4 - Re: [PATCH 08/49] e2fsck: don't rehash inline directories

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Message-ID: <20140313053848.GD9070@birch.djwong.org>
Date:	Wed, 12 Mar 2014 22:38:48 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 08/49] e2fsck: don't rehash inline directories

On Wed, Mar 12, 2014 at 11:52:48PM -0400, Theodore Ts'o wrote:
> On Mon, Mar 10, 2014 at 11:54:49PM -0700, Darrick J. Wong wrote:
> > If a directory's contents are stored entirely inside the inode,
> > there's no index to rebuild and no dirblock checksum to recompute.
> > As far as I know these are the only two reasons to call dir rehash.
> 
> Well, actually, there is a third reason to rehash directories, and
> that is to reorganize a directory to optimize out deleted entries that
> are scattered in the middle of the directory.

Ooh, I forgot about that. :/

> That being said, it's more critical for inline directories, since we
> very much want to keep them from spilling over to an external block,
> this process of compressing out deleted space is something that should
> be done in real time as we operate on the directory, by the kernel,
> and not just at fsck time.
> 
> The only reason why we don't do this today is because if the directory
> is open for scanning using opendir/readdir, if we reorganize a
> directory block, it could end up corrupting the readdir --- and for
> non-inline directories, it's much less important.
> 
> What I think would might make sense is to have the kernel track
> whether the directory has been opened for reading, and if it hasn't,
> then it would be safe to try compressing all of the directory entries
> in the block so that the free space is in a single unused directory
> entry at the end of the block.  We could try doing this "dynamic
> compression" of directory free space both at unlink(2) time, and also
> when we try inserting a directory entry into the block and there is
> apparently no space in the directory block.
> 
> So I'm fine with skipping the rehashing of inline directories now, but
> this is a future, relatively small, kernel project we might want to
> think about for ext4.

Probably we ought to fix up rehash.c to be able to compress directory entries
too.  The only reason I kicked them here was that somehow an inline data dir
would end up on the rehash list, causing the block iteration to fail and e2fsck
stops cold.

--D
> 
> Cheers,
> 
> 							- Ted
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