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Date: Wed, 8 Jan 2014 22:24:35 +0000
From: "Richard W.M. Jones" <rjones@redhat.com>
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Subject: Re: [Qemu-devel] TRIM/DISCARD/UNMAP support on qemu-nbd
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Teng-Feng Yang <shinrairis@gmail.com>, qemu-devel@nongnu.org

On Wed, Jan 08, 2014 at 11:11:35PM +0100, Paolo Bonzini wrote:
> Is guestfish using "discard=on"?

No.

Adding the discard=on parameter does indeed fix this:

13M    /tmp/test1
17M    /tmp/test2

However why isn't this the default?  Is there a case where discard=on
would be undesirable?

It's extremely difficult to know when it's safe to add this parameter.
Qemu gives no indication of when using discard=.. is safe (ie. won't
cause qemu to fail to start up or fail in some other way).  It's even
worse when we have to go via libvirt which itself doesn't expose
qemu's capabilities upwards.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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