On Mon, 9 Feb 2015 08:39:41 +0000 Hugo Mills <h...@carfax.org.uk> wrote:
> > I use KVM (QEMU) with discard pass-through from the VM guest
> > ("discard=unmap"
> > option), with the VM images stored on Btrfs. It works just fine, the disk
> > space used for the image file does shrink when the guest OS issues discards
> > on
> > its FS. Maybe there is a difference in how KVM and the 'loop' module submit
> > discards to Btrfs?
>
> This is interesting, because I've tried exactly that setup (KVM,
> discard=unmap) and it's not worked for me.
Did you use the emulated IDE interface in KVM? Others such as virtio, ahci and
maybe SCSI may not support discard. Did you get a successful result issuing
TRIM within the guest (e.g. fstrim -v / succeeds and doesn't -ENOSUPP)?
--
With respect,
Roman
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