[linux-lvm] LVM thin pool discard_max_bytes reports 0

Timur Alperovich timur at maginatics.com
Thu Aug 7 03:48:47 UTC 2014

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 (

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.

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

Thank you,
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