[linux-lvm] [dm-devel] do not disable ext4 discards on first discard failure? [was: Re: dm snapshot: ignore discards issued to the snapshot-origin target]

[Lukas Czerner]

On Mon, 2 May 2011, Martin K. Petersen wrote:

> >>>>> "Lukas" == Lukas Czerner <lczerner at redhat.com> writes:
> 
> Lukas,
> 
> Lukas> [root at trim ~]# lvcreate -L 3500M vg_test
> Lukas>   Logical volume "lvol0" created
> 
> Ok, so here's what I think is going on. You're creating a linear target
> which happens to fit inside the first PV.
> 
> Here's two devices. One that supports discard_zeroes_data=1 (8:17) and
> one that doesn't (8:49).
> 
> # dmsetup table 
> foo-bar: 0 1032192 striped 2 32 8:17 384 8:49 384
> foo-baz: 0 106496 linear 8:17 516480
> 
> # grep . /sys/block/d*/queue/discard_z*
> /sys/block/dm-0/queue/discard_zeroes_data:0
> /sys/block/dm-1/queue/discard_zeroes_data:1
> 
> LV foo/bar (dm-0) is striped, straddling two devices with incompatible
> values. Hence 0.
> 
> LV foo/baz (dm-1) is linear and fits inside the first PV. Thus it has
> discard_zeroes_data=1.

Well that's why I have included /proc/partition, so you can see that
this is not happening, because both partitions are 1.86GB long and the LV I
am creating from those is 3.42GB long.

[root at trim ~]# dmsetup table
vg_test-lvol0: 0 3899392 linear 8:49 2048
vg_test-lvol0: 3899392 3268608 linear 8:19 2048


Nevertheless there is something weird going on, because even when I
create striped volume I get this:


[root at trim ~]# dmsetup table
vg_test-lvol0: 0 7176192 striped 2 16 8:49 2048 8:19 2048
[root at trim ~]# ls -lah /dev/mapper/vg_test-lvol0 
lrwxrwxrwx. 1 root root 7  3. kvě 04.42 /dev/mapper/vg_test-lvol0 ->
../dm-0
[root at trim ~]# grep . /sys/block/dm-0/queue/discard_*
/sys/block/dm-0/queue/discard_granularity:512
/sys/block/dm-0/queue/discard_max_bytes:4294966784
/sys/block/dm-0/queue/discard_zeroes_data:1


And of course (8:49) is a partition on the SSD which does zero data and
(8:19) is a partition on the spinning device which does not zero data,
nor does it support discard. And this is on yesterdays kernel.

[root at trim ~]# uname -r
2.6.39-rc5+

So assuming that you're trying recent kernel as well, someone is doing
something different :)

Thanks!
-Lukas