Re: [PATCH 2/2] block: file-posix: Replace posix_fallocate with fallocate

[Nir Soffer]

On Mon, Sep 14, 2020 at 8:32 PM Daniel P. Berrangé <berrange@redhat.com> wrote:
>
> On Mon, Aug 31, 2020 at 05:01:27PM +0300, Nir Soffer wrote:
> > If fallocate() is not supported, posix_fallocate() falls back to
> > inefficient allocation, writing one byte for every 4k bytes[1]. This is
> > very slow compared with writing zeros. In oVirt we measured ~400%
> > improvement in allocation time when replacing posix_fallocate() with
> > manually writing zeroes[2].
> >
> > We also know that posix_fallocated() does not work well when using OFD
> > locks[3]. We don't know the reason yet for this issue yet.
> >
> > Change preallocate_falloc() to use fallocate() instead of
> > posix_falloate(), and fall back to full preallocation if not supported.
> >
> > Here are quick test results with this change.
> >
> > Before (qemu-img-5.1.0-2.fc32.x86_64):
> >
> > $ time qemu-img create -f raw -o preallocation=falloc /tmp/nfs3/test.raw 6g
> > Formatting '/tmp/nfs3/test.raw', fmt=raw size=6442450944 
> > preallocation=falloc
> >
> > real 0m42.100s
> > user 0m0.602s
> > sys 0m4.137s
> >
> > NFS stats:
> > calls      retrans    authrefrsh    write
> > 1571583    0          1572205       1571321
> >
> > After:
> >
> > $ time ./qemu-img create -f raw -o preallocation=falloc /tmp/nfs3/test.raw 
> > 6g
> > Formatting '/tmp/nfs3/test.raw', fmt=raw size=6442450944 
> > preallocation=falloc
> >
> > real 0m15.551s
> > user 0m0.070s
> > sys 0m2.623s
> >
> > NFS stats:
> > calls      retrans    authrefrsh    write
> > 24620      0          24624         24567
> >
> > [1] 
> > https://code.woboq.org/userspace/glibc/sysdeps/posix/posix_fallocate.c.html#96
> > [2] https://bugzilla.redhat.com/1850267#c25
> > [3] https://bugzilla.redhat.com/1851097
>
> This bug appears to be private to RH employees only, so rather than link
> to it, please summarize any important facts in it for benefit of nonn-RH
> QEMU contributors.

Thanks, I missed that detail when linking to the bug. The bug is public now.

> > Signed-off-by: Nir Soffer <nsoffer@redhat.com>
> > ---
> >  block/file-posix.c                     | 32 +++++++++-----------------
> >  docs/system/qemu-block-drivers.rst.inc | 11 +++++----
> >  docs/tools/qemu-img.rst                | 11 +++++----
> >  qapi/block-core.json                   |  4 ++--
> >  4 files changed, 25 insertions(+), 33 deletions(-)
>
> Regards,
> Daniel
> --
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