Re: [Qemu-devel] fstrim & upstream kernel not working

[Richard W.M. Jones]

On Fri, Mar 14, 2014 at 01:30:40PM +0100, Paolo Bonzini wrote:
> Il 13/03/2014 22:49, Richard W.M. Jones ha scritto:
> >- Is there any reason why virtio-scsi doesn't emulate WRITE SAME?
> 
> Yes, the reason is that you're using QEMU 1.7. :)
> 
> >- Can you see where ext4 issues the zeroout/write same call?  AFAICT
> >it is still issuing discards, but these are getting turned into
> >zeroout/write same by some sort of block layer magic that I can't
> >quite follow.
> 
> That's provisioning_mode, which is writesame_16 with QEMU 1.7 and
> unmap with QEMU 2.0.

Got it.

This morning I was trying kernel from git + qemu from git together.
This works, sort of.

Firstly I tightened up the automated tests[1] of trimming.  Previously
we just tested that >= 1 block was freed in the host file.  Now I'm
checking that >= 512 KB is freed.  This change revealed that fstrim
was only trimming about 64 KB from the host file (although -o discard
and blkdiscard tests[1] work as expected).

I worked around this in any case by rearranging the test [2]:

Doing:

  rm /a_big_file
  fstrim /
  sync
  umount /
  [shut down qemu]

would only trim 64 KB on the host.

Doing:

  rm /a_big_file
  umount /                         # added
  mount -o nodiscard /dev/sda /    # added
  fstrim /
  sync
  umount /
  [shut down qemu]

would trim the expected amount (around 10 MB).

I've no idea why this is (looks like an ext4/kernel bug to me), but in
any case the tests now use the second method[2].

Rich.

[1] https://github.com/libguestfs/libguestfs/tree/master/tests/discard
[2] 
https://github.com/libguestfs/libguestfs/commit/accf1b66aa835714690a2979e990c49243875dab

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-top is 'top' for virtual machines.  Tiny program with many
powerful monitoring features, net stats, disk stats, logging, etc.
http://people.redhat.com/~rjones/virt-top