Re: How to stop QEMU punching holes in backing store

[Nir Soffer]

On Sat, Jun 29, 2024 at 12:19 AM Nir Soffer <nsoffer@redhat.com> wrote:
>
> On Wed, Jun 19, 2024 at 2:18 PM Nir Soffer <nsoffer@redhat.com> wrote:
> > On 19 Jun 2024, at 8:54, Justin <justin0@safe-mbox.com> wrote:
> >
> >    I've run strace and I see calls to fallocate with these flags:
> >    FALLOC_FL_KEEP_SIZE|FALLOC_FL_PUNCH_HOLE
> >
> >    I've tried passing these options: discard=off,detect-zeroes=off but
> >    this does not help. This is the full set of relevant options I'm
> >    using:
> >
> >    -drive 
> > file=/vms/vm0/drive,format=raw,if=virtio,discard=off,detect-zeroes=
> >    off
> >
> > You don't need to disable detect-zeros - in my tests it makes dd 
> > if=/dev/zero
> > 5 times faster (770 MiB/s -> 3 GiB/s) since zero writes are converted to
> > fallocate(FALLOC_FL_KEEP_SIZE|FALLOC_FL_ZERO_RANGE).
> >
> > The issue seems to be ignoring the discard option when opening the image,
> > and is fixed by this:
> > https://lists.nongnu.org/archive/html/qemu-block/2024-06/msg00198.html
> >
> > Thanks. When might this patch (or something similar) be merged?
> >
> > The patch need more work, when the work is done and qemu maintainers are 
> > happy it is a good estimate :-)
>
> Justin, v3 should be ready now, do you want to help by testing it?
>
> v3:
> https://lists.nongnu.org/archive/html/qemu-block/2024-06/msg00644.html
>
> You can also pull it from:
> https://gitlab.com/nirs/qemu/-/tree/consider-discard-option

The fix is included in QEMU 9.1.0-rc0