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Date: Fri, 14 Mar 2014 13:30:40 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] fstrim & upstream kernel not working
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To: "Richard W.M. Jones" <rjones@redhat.com>, qemu-devel@nongnu.org

Il 13/03/2014 22:49, Richard W.M. Jones ha scritto:
> I got fstrim happily working in Fedora 20, but it's not working with
> the upstream kernel.  The message is:
>
>   fstrim -v /sysroot/
>   [   45.541339] sda: WRITE SAME failed. Manually zeroing.
>   /sysroot/: 47.2 MiB (49466368 bytes) trimmed
>
> While this isn't technically an error, it of course doesn't trim
> anything.  In fact the host disk grows after the fstrim.
>
> A couple of questions:
>
> - 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.

Paolo

> kernel: 3.14.0-0.rc6.git2.1.fc21
> qemu: 1.7.0-5.fc21.x86_64