linux-kernel - Re: defrag deployment status (was Re: [PATCH] ext4: allow defrag (EXT4_IOC_MOVE

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Message-ID: <4B964BBD.3070707@gmail.com>
Date:	Tue, 09 Mar 2010 08:23:09 -0500
From:	jim owens <owens6336@...il.com>
To:	Greg Freemyer <greg.freemyer@...il.com>
CC:	David Newall <davidn@...idnewall.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Jeff Garzik <jeff@...zik.org>, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org, Akira Fujita <a-fujita@...jp.nec.com>
Subject: Re: defrag deployment status (was Re: [PATCH] ext4: allow defrag
 	(EXT4_IOC_MOVE_EXT) in 32bit compat mode)

After thinking about it overnight, I realized I think in terms
of 1 drive is 1 filesystem.  That is a fatal trap for defragment.

> When I only worried about a few OEM drives, I used to read the zone
> geometry from the drive to see where each speed transition was as the
> density decreased.  But that is just not worth the effort in linux
> filesystems IMO, it is enough to pack low.

So I retract that we don't care about zone geometry, we need to
care deeply, but not in the sense of how moving short distances
on a drive affects the performance.  What we need to ensure is
that the placer algorithm does not span across partitions as in:

["/" 100GB created] [300GB other] [100G LVM added to "/"]

so the filesystem thinks it is 200GB contiguous and the
defragmenter thinks address 90GB is closer to address 110 GB
than 90GB is to 50GB.

jim
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