linux-ext4 - Re: [RFC][PATCH 0/3] ext4: online defrag (ver 1.0)

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Message-ID: <20090204153205.GF14762@mit.edu>
Date:	Wed, 4 Feb 2009 10:32:05 -0500
From:	Theodore Tso <tytso@....edu>
To:	Greg Freemyer <greg.freemyer@...il.com>
Cc:	Akira Fujita <a-fujita@...jp.nec.com>, linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [RFC][PATCH 0/3] ext4: online defrag (ver 1.0)

On Wed, Feb 04, 2009 at 09:51:07AM -0500, Greg Freemyer wrote:
> 
> If the OHSM team implements a similar ioctl for ext2 and ext3 and
> submits them for mainline at some point, do they have a chance of
> being accepted or are ext2 and ext3 feature frozen?

It seems unlikely it would be accepted.  If the patch could be done in
a way that seriously minimized the chances of destablizing the code,
maybe --- but consider also that the OHSM design is a pretty terrible
hack.  I'm not at all conviced they will be able to stablize it for
production use, and a scheme that involves using dmapi across multiple
block devices.

Note that they apparently need to make other changes to the core
filesystem code besides just the ioctl --- to the block allocation
code, at the very least.

The right answer is really to use a stackable filesystem, and to use
separate filesystems for each different tier, and then build on top of
unionfs to give it its policy support.  I suspect that OHSM will be a
cute student project, but it won't become anything serious given its
architecture/design, unfortunately.

						- Ted
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