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Re: [libvirt] [libvirt PATCH] qemu: Support discard for disk



On Wed, May 15, 2013 at 03:44:11PM +0200, Paolo Bonzini wrote:
> Il 15/05/2013 15:17, Daniel P. Berrange ha scritto:
> > On Wed, May 15, 2013 at 02:56:03PM +0200, Paolo Bonzini wrote:
> >> Il 14/05/2013 14:44, Osier Yang ha scritto:
> >>> QEMU introduced "discard" option for drive since commit a9384aff53,
> >>>
> >>> <...>
> >>> @var{discard} is one of "ignore" (or "off") or "unmap" (or "on") and
> >>> controls whether @dfn{discard} (also known as @dfn{trim} or @dfn{unmap})
> >>> requests are ignored or passed to the filesystem.  Some machine types
> >>> may not support discard requests.
> >>> </...>
> >>>
> >>> This patch exposes the support in libvirt.
> >>>
> >>> QEMU supported "discard" for "-drive" since v1.5.0-rc0:
> >>
> >> I would prefer to use ignore/unmap instead of off/on in libvirt.
> > 
> > If we think there's any chance QEMU might gain a 3rd possible
> > value for this setting, then I tend to agree with you that
> > avoiding off/on would be desirable.
> 
> Yes, the 3rd setting would be "anchor", i.e. when discarding keep any
> preallocation but discard the data.  This corresponds to the XFS ioctl
> XFS_IOC_ZERO_RANGE.

Ok, we should definitely not use  off/on then.

Daniel
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