Re: discard/trim support in device mapper?

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Mar 22 2010 at  8:15am -0400,
Andreas Beckmann <beckmann@cs.uni-frankfurt.de> wrote:

> Hi,
> 
> since 2.6.33 the ext4 file system supports emitting DISCARD/TRIM/...
> commands that notify the disk about deleted blocks that may be reused by
> the disk for whatever it wants. This is especially helpful for SSDs to
> reduce performance degradation due to internal fragmentation ...
> 
> What are the plans / is the status for adding DISCARD support to the
> device mapper? Especially for RAID-0/RAID-1 configurations with multiple
> SSDs this will be really helpful. The DISCARD requests received from the
> file system layer just need to be translated/duplicated to the sector
> ranges on the physical disks and then the appropriate commands for the
> disks need to be generated.
> 
> If there is anything to be tested, I'm willing to help. I have a machine
> with 4 SuperTalent FTM56GX25H SSDs (firmware 1916) that happily do TRIM
> if they are used as single disks, but not in a 1TB RAID-0 configuration
> managed via LVM.
> 
> Experiments have shown significant performance improvements (factor 4-5)
> for some heavy I/O loads on single SSD (250 GB) using DISCARD, I'd like
> to achieve them for a larger data set (1 TB), too.

We are aware of the benefits associated with issuing discards to SSDs.
Discard support for DM will be a focused priority for me starting
mid-April.

Until then I hope to establish a baseline understanding on the scope of
the project.  In particular I intend to get back with Martin Petersen to
understand how his work is going on making "write same and discard
requests first class citizens so they can be split and merged like
regular write requests".

Martin noted his associated block layer changes are fairly invasive.
This should help indictate why DM (and MD) doesn't have proper discard
(WRITE SAME/UNMAP or TRIM) support yet.

Mike