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

[Daniel P . Berrangé]

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.

> 
> 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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