Re: Accepting discard to free space from disk images

[Hugo Mills <hugo@carfax.org.uk>]

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On Mon, Feb 09, 2015 at 02:12:21PM +0500, Roman Mamedov wrote:
> 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)?

   I used SCSI, so maybe that's the problem. I'll try with IDE.

   Hugo.

-- 
Hugo Mills             | I was cursed with poetry very young. It creates
hugo@... carfax.org.uk | unrealistic expectations.
http://carfax.org.uk/  |                                   Victor Frankenstein
PGP: 65E74AC0          |                                        Penny Dreadful

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