[PATCH] dm snapshot: add discard support to the snapshot-origin target [was: Re: Testing TRIM with LVM]

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Apr 12 2011 at  9:47pm -0400,
DarkNovaNick@gmail.com <DarkNovaNick@gmail.com> wrote:

> I get 0 back when I do "cat
> /sys/block/sda/queue/discard_zeroes_data" so does this mean that my
> drive doesn't zero out the TRIMed blocks? I don't really care if it
> does or does not, I just want to make sure the drive is getting the
> TRIM commands as the benchmarks I've seen show performance
> degradation if they aren't used.

Right, means TRIM doesn't zero the TRIM'd blocks.

But otherwise it looks like you're in good shape relative to TRIM.
 
> Regarding snapshot targets, the only instance of "discard not
> supported" came as I did a snapshot today:
> 
> Apr 12 15:27:58 darknovanick kernel: [65147.211211] EXT4-fs warning
> (device dm-0): release_blocks_on_commit:2672: discard not supported,
> disabling
> Apr 12 15:28:16 darknovanick kernel: [65166.103061] EXT4-fs (dm-1):
> 72 orphan inodes deleted
> Apr 12 15:28:16 darknovanick kernel: [65166.103062] EXT4-fs (dm-1):
> recovery complete
> Apr 12 15:28:17 darknovanick kernel: [65166.232257] EXT4-fs (dm-1):
> mounted filesystem with ordered data mode. Opts: (null)
> 
> This snapshot was of a LV on the SSD, but the snapshot itself was
> being stored on a mechanical hard drive. Does this message mean that
> discard was just disabled for the snapshot itself (which was deleted
> a half hour later), or is it now disabled for the entire volume now
> that I've done a snapshot, even though its been removed?

Unfortunately, the failed discard to the snapshot-origin device disables
ext4's discards for the entire volume (even after you remove the
snapshot).

But the following patch fixes that:

Subject: dm snapshot: add discard support to the snapshot-origin target

Allow the snapshot-origin target to pass discards to the origin device
(after any copyout of the region being discarded; but generally speaking
that copyout won't be needed as the region will have already been copied
to the cow).

So for example, when you remove a file from an ext4 mounted origin
volume it triggers copyout to the snapshot.  When the DISCARD is then
issued during the subsequent journal commit no copyout is needed.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 drivers/md/dm-snap.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index a2d3309..028d216 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -2076,6 +2076,7 @@ static int origin_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 
 	ti->private = dev;
 	ti->num_flush_requests = 1;
+	ti->num_discard_requests = 1;
 
 	return 0;
 }
@@ -2153,7 +2154,7 @@ static int origin_iterate_devices(struct dm_target *ti,
 
 static struct target_type origin_target = {
 	.name    = "snapshot-origin",
-	.version = {1, 7, 1},
+	.version = {1, 8, 0},
 	.module  = THIS_MODULE,
 	.ctr     = origin_ctr,
 	.dtr     = origin_dtr,