linux-ext4 - Re: [PATCH 01/11 RESEND] libe2p: Add new function get_fragment

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Date:	Fri, 01 Jul 2011 17:34:09 +0900
From:	Kazuya Mio <k-mio@...jp.nec.com>
To:	Greg Freemyer <greg.freemyer@...il.com>
CC:	Andreas Dilger <aedilger@...il.com>,
	Eric Sandeen <sandeen@...hat.com>, "Ted Ts'o" <tytso@....edu>,
	ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 01/11 RESEND] libe2p: Add new function get_fragment_score()

2011/06/28 22:53, Greg Freemyer wrote:
> While you're thinking about the issue:
>
> As I hope I've said before, for sparse file I think e4defrag should
> score and defrag one block_group at a time.  Thus if a VM backing
> storage file has 100 block_groups (as I'm using the term), then it
> should score each of the 100 separately and if needed defrag them one
> at a time.
>
> I can see no benefit from treating a large sparse file as monolithic
> for the decision process.

If e4defrag does defrag one block_group at a time, this block_group may be
allocated far away from the other block_groups. If so, seek time increases
even if the number of extents is less than before.

Of course, I'm aware of the advantage of your suggestion. I may also try to
consider it to defrag only a part of a file in the future, but before that
I think I should do the cleanup and bugfix.

> fyi: Is there an agreed on term for what I'm calling a block_group.  I
> believe e4defrag uses the term "extent group" in the comments, but
> sparse files exist in non-extent based filesystems, so it's not a very
> portable name.

e4defrag supports only an extent based filesystem, so I think it's no problem.
And I associate "block_group" with the physical layout of the blocks on the
disk. I guess we shouldn't use the same word with different meanings.

Regards,
Kazuya Mio
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