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Re: [PATCH 2/2] block: file-posix: Replace posix_fallocate with fallocat
From: |
Daniel P . Berrangé |
Subject: |
Re: [PATCH 2/2] block: file-posix: Replace posix_fallocate with fallocate |
Date: |
Mon, 14 Sep 2020 18:32:01 +0100 |
User-agent: |
Mutt/1.14.6 (2020-07-11) |
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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