Re: defrag deployment status (was Re: [PATCH] ext4: allow defrag (EXT4_IOC_MOVE_EXT) in 32bit compat mode)

[jim owens]

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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