Re: Accepting discard to free space from disk images

[Roman Mamedov <rm@romanrm.net>]

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On Mon, 9 Feb 2015 08:39:41 +0000
Hugo Mills <hugo@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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