how to get discard working on KVM block devices?

Hi,

I'm playing with dm-thin target, and I'd like to get discard working from my test KVM virtuals..

I've got thinprov pool, on top of which I've got test logical volume. On host, I can issue discard (using fstrim) on it, and thin pool usage lowers correctly, therefore I think it's working well...

However, when I export such block device to KVM (using either virtio or scsi), discard seems not to be working - I can neither issue fstrim, nor mount ext4 with discard option (well, I can mount it, but it reports discard is not supported by underlying device, fstrim reports FITRIM ioctl failed: Operation not supported).

I tried with kernel 3.8.2 on both host and guest, and qemu-1.4.0.

Is there anything special needed to do to get discard working?

thanks a lot in advance for any reply

BR

nik

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