[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] fstrim & upstream kernel not working
From: |
Richard W.M. Jones |
Subject: |
Re: [Qemu-devel] fstrim & upstream kernel not working |
Date: |
Fri, 14 Mar 2014 12:42:16 +0000 |
User-agent: |
Mutt/1.5.20 (2009-12-10) |
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