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

[Richard W.M. Jones]

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

There are a lot of factors for getting unmap/discard/trim to work,
including:

 - guest tools (sg_unmap) or guest filesystem must support it
 - guest kernel must support it
 - host qemu must support it
 - host filesystem/etc must support it

My (possibly weak) understanding of the upstream qemu code is that
unmap/discard/trim is not supported in qcow2.  It is only supported in
raw files when using a POSIX-like host OS which has either of:

 - block devices supporting BLKDISCARDZEROES
 - files on XFS
 - files on other filesystems that support FALLOC_FL_PUNCH_HOLE (eg ext4)

Having said that, I did some tests using libguestfs and I could not
show that unmap was working, either using raw or qcow2 (both on ext4),
with virtio-scsi, and recent kernel & qemu.  I did not see any errors,
but also I don't see what I'm doing wrong.

Attached is my test script.

You will need to compile libguestfs with:

  ./configure --with-extra-packages="sg3_utils"

The results on my machine are:

$ /tmp/sparsetest.sh 
0 /tmp/test1
0 /tmp/test2
Read Capacity results:
   Protection: prot_en=0, p_type=0, p_i_exponent=0
   Logical block provisioning: lbpme=1, lbprz=0
   Last logical block address=204799 (0x31fff), Number of logical blocks=204800
   Logical block length=512 bytes
   Logical blocks per physical block exponent=0
   Lowest aligned logical block address=0
Hence:
   Device size: 104857600 bytes, 100.0 MiB, 0.10 GB
Block limits VPD page (SBC):
  Write same no zero (WSNZ): 1
  Maximum compare and write length: 0 blocks
  Optimal transfer length granularity: 0 blocks
  Maximum transfer length: 0 blocks
  Optimal transfer length: 0 blocks
  Maximum prefetch length: 0 blocks
  Maximum unmap LBA count: 0
  Maximum unmap block descriptor count: 0
  Optimal unmap granularity: 8
  Unmap granularity alignment valid: 0
  Unmap granularity alignment: 0
  Maximum write same length: 0x0 blocks

16M       /tmp/test1           <--- note no sparseness is created
16M       /tmp/test2

Please let us know if you get this working, because I'd really like to
fix virt-sparsify so it can work in-place!

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v

sparsetest.sh
Description: Bourne shell script