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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:

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.


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.


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 :)


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