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Re: [dm-devel] RFC: use TRIM data from filesystems to speed up array rebuild?



On 09/04/2012 04:24 PM, NeilBrown wrote:
On Tue, 04 Sep 2012 15:11:26 -0400 Ric Wheeler <ricwheeler gmail com> wrote:

On 09/04/2012 02:06 PM, Chris Friesen wrote:
Hi,

I'm not really a filesystem guy so this may be a really dumb question.

We currently have an issue where we have a ~1TB RAID1 array that is mostly
given over to LVM.  If we swap one of the disks it will rebuild everything,
even though we may only be using a small fraction of the space.

This got me thinking.  Has anyone given thought to using the TRIM information
from filesystems to allow the RAID code to maintain a bitmask of used disk
blocks and only sync the ones that are actually used?

Presumably this bitmask would itself need to be stored on the disk.

Thanks,
Chris

Device mapper has a "thin" target now that tracks blocks that are allocated or
free (and works with discard).

That might be a basis for doing an focused RAID rebuild,
I wonder how....
Maybe the block-later interface could grow something equivalent to
"SEEK_HOLE" and friends so that the upper level can find "holes" and
"allocated space" in the underlying device.
I wonder if it is time to discard the 'block device' abstraction and just use
files every .... but I seriously doubt it.

NeilBrown

I don't think that we have to go to that extreme, but I think it would be very useful to see if the device mapper people have ideas in how the thin target might be used in combination with MD :)

ric


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