Re: Software RAID and TRIM

[Roberto Spadim <roberto@spadim.com.br>]

some ideas....

maybe for a test only...
we could send trim commands on raid1 arrays only or 'raid0 linear'
since they don´t stripe, this could be 'easy' to develop
when filesystem send trim, we send it to down device (/dev/sdX99)
there´s a problem of offset (for raid1) maybe some devices just work
with 4096bytes blocks on trim command, maybe not
we could implement and put in a beta/alpha realease to test like ext4
guys are doing with discard command (it´s a user option today)


2011/6/29 Scott E. Armitage <launchpad@scott.armitage.name>:
> On Wed, Jun 29, 2011 at 7:10 AM, Tom De Mulder <tdm27@cam.ac.uk> wrote:
>> However, that might not necessarily be a problem; tools exist that can be run manually (slightly fsck-like) and tell the drive which blocks can be erased.
>
> For RAID5/6 at least, md will still require knowledge of what stripes
> are and are not in use by the filesystem. In the current
> implementation, the entire array must be consistent, regardless of
> whether or not a particular block is in use. As far as my
> understanding goes, any level of TRIM support for parity arrays would
> be a fundamental shift in the way md treats the array.
>
> The simplest solution I see is to do as Niel suggested, and mimic TRIM
> support at the RAID level, and pass commands down as necessary. An
> alternative solution would be to add a second TRIM layer, where md
> maintains a list of what is or is not in use, and once an entire
> stripe has been discarded by the filesystem, it can send a single TRIM
> command to each member drive to drop the entire stripe contents. This
> adds abstraction for the filesystem layer, allowing it to treat the
> RAID array like a regular SSD, but adds significant complexity to md
> itself.
>
> -Scott
>
> p.s. Sorry if you receive this twice; Majordomo rejected the first one
> on HTML subpart basis.
>
> --
> Scott Armitage, B.A.Sc., M.A.Sc. candidate
> Space Flight Laboratory
> University of Toronto Institute for Aerospace Studies
> 4925 Dufferin Street, Toronto, Ontario, Canada, M3H 5T6
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-- 
Roberto Spadim
Spadim Technology / SPAEmpresarial
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