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Date: Tue, 07 Jan 2014 15:48:54 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] TRIM/DISCARD/UNMAP support on qemu-nbd
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To: "Richard W.M. Jones" <rjones@redhat.com>
Cc: Teng-Feng Yang <shinrairis@gmail.com>, qemu-devel@nongnu.org

Il 02/01/2014 17:15, Richard W.M. Jones ha scritto:
> 
> 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)

It doesn't have to support BLKDISCARDZEROES, only BLKDISCARD.  I test it
with scsi_debug using both lbprz=0 and lbprz=1 (which becomes
BLKDISCARDZEROES unset and set respectively).

Otherwise this is correct.

> 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.

Can you share more?

Paolo