[linux-lvm] LVM thin pool discard_max_bytes reports 0

[Timur Alperovich]

I have managed to answer my own questions.

On Wed, Aug 6, 2014 at 8:48 PM, Timur Alperovich <timur at maginatics.com>
wrote:

> Hi there,
>
> I was trying to understand why fstrim would fail on an LVM thin
> provisioned volume and figured out that the discard_max_bytes is reported
> as 0 in /sys/block/dm-N/queue/discard_max_bytes (I checked the device that
> corresponded to the LVM pool that the volume is created within). The
> discard options are also disabled for the thin volume itself. I'm observing
> this issue on an Ubuntu 12.04 system, running kernel 3.8.0-32, and LVM2
> 2.02.98 backported to Ubuntu 12.04 (
> https://launchpad.net/~timur-alperovich/+archive/ubuntu/ppa/+packages).
>
> I did a small experiment where I created another LVM pool within the same
> volume group and the discard value was set to 65536 for that device. I'm
> not sure if it matters, but the system is an running on ESXi 5.0 host.
> While I don't necessarily expect discard to propagate to the host, I was
> hoping to reclaim the block in the thin pool itself.
>
> Could anyone shed some light on why discard may be disabled on an LVM thin
> pool device? I tried looking for any bugs specific to TRIM or discard
> subsequent to 2.02.98, but could not find anything definitive.
>

I noticed the following in the 2.02.98 changelog:
"Reinstate correct default to ignore discards for thin metadata from old
tools."

and this looks like the associated commit:
commit 109b3bb49b31d5fa7f6d0dd23345d703ab866ad6
Author: Zdenek Kabelac <zkabelac at redhat.com>
Date:   Tue Aug 21 15:51:54 2012 +0200

    thin: discard

The commit changes the default discard value to "ignore" and this can be
confirmed with lvs -o +discards


> Also, is there a way to enable discard without having to recreate the thin
> pool?
>

Yes, it can be changed with lvchange --discards passdown (being mindful of
https://bugzilla.redhat.com/show_bug.cgi?id=903411)


>
> Thank you,
> Timur
>

Cheers,
Timur
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listman.redhat.com/archives/linux-lvm/attachments/20140806/3e8bbcbc/attachment.htm>