Re: RFC: use TRIM data from filesystems to speed up array rebuild?

[Ric Wheeler <ricwheeler@gmail.com>]

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