Re: [Qemu-devel] TRIM/DISCARD/UNMAP support on qemu-nbd

[Stefan Hajnoczi]

On Mon, Dec 30, 2013 at 07:58:29PM +0800, Teng-Feng Yang wrote:
> I have been studying QCOW2 file format for a couple of days, and I am
> a little bit confused about whether QCOW2 supports UNMAP or not.

Discard is an area that has seen a lot of development activity over the
past year or two.  That means it's still relatively new, you may find
outdated information online, etc.

> As I surf through internet, some mailing list discussion had mentioned
> that qemu-nbd and nbd module both support UNMAP command.

Yes:
 * qemu-nbd since QEMU 1.1 supports TRIM
 * nbd.ko since Linux 3.7 supports discard

> So I follow the steps below on my machine (Ubuntu 13.10 with linux
> kernel 3.12) to test if qemu-nbd and QCOW2 do support UNMAP.
> 
> 1. Create a qcow2 file via qemu-img
>     > sudo qemu-img create -f qcow2 -o cluster_size=524288 base.qcow2 1G
> 
> 2. Connect this qcow2 file with qemu-nbd
>     > sudo qemu-nbd -c /dev/nbd0 base.qcow2 --discard=unmap
> 
> 3. Use sg_unmap command to issue UNMAP command to this NBD
>     > sudo sg_unmap --lba=0 --num=1 /dev/nbd0
> 
> Everytime I get the following error message:
> 
>     unmap cdb: 42 00 00 00 00 00 00 00 18 00
> unmap: pass through os error: Inappropriate ioctl for device
> UNMAP failed (use '-v' to get more information)
> 
> I also try to format this nbd device with EXT4 and mount it, but still
> cannot perform fstrim on the mount point.

NBD isn't a SCSI device so sending UNMAP commands doesn't work.  I think
you need to issue ioctl(BLKDISCARD) instead.  See blkdiscard(8).

Also, make sure to use qemu.git/master if you want qcow2 discard
support.  I didn't check the details but the unmap implementation for
qcow2 has recently been added/modified.

Stefan