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Re: [Qemu-block] [PATCH v3 1/2] block: posix: Always allocate the first
From: |
Max Reitz |
Subject: |
Re: [Qemu-block] [PATCH v3 1/2] block: posix: Always allocate the first block |
Date: |
Tue, 27 Aug 2019 18:58:10 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 |
On 27.08.19 03:05, Nir Soffer wrote:
> When creating an image with preallocation "off" or "falloc", the first
> block of the image is typically not allocated. When using Gluster
> storage backed by XFS filesystem, reading this block using direct I/O
> succeeds regardless of request length, fooling alignment detection.
>
> In this case we fallback to a safe value (4096) instead of the optimal
> value (512), which may lead to unneeded data copying when aligning
> requests. Allocating the first block avoids the fallback.
>
> Since we allocate the first block even with preallocation=off, we no
> longer create images with zero disk size:
>
> $ ./qemu-img create -f raw test.raw 1g
> Formatting 'test.raw', fmt=raw size=1073741824
>
> $ ls -lhs test.raw
> 4.0K -rw-r--r--. 1 nsoffer nsoffer 1.0G Aug 16 23:48 test.raw
>
> And converting the image requires additional cluster:
>
> $ ./qemu-img measure -f raw -O qcow2 test.raw
> required size: 458752
> fully allocated size: 1074135040
>
> When using format like vmdk with multiple files per image, we allocate
> one block per file:
>
> $ ./qemu-img create -f vmdk -o subformat=twoGbMaxExtentFlat test.vmdk 4g
> Formatting 'test.vmdk', fmt=vmdk size=4294967296 compat6=off
> hwversion=undefined subformat=twoGbMaxExtentFlat
>
> $ ls -lhs test*.vmdk
> 4.0K -rw-r--r--. 1 nsoffer nsoffer 2.0G Aug 27 03:23 test-f001.vmdk
> 4.0K -rw-r--r--. 1 nsoffer nsoffer 2.0G Aug 27 03:23 test-f002.vmdk
> 4.0K -rw-r--r--. 1 nsoffer nsoffer 353 Aug 27 03:23 test.vmdk
>
> I did quick performance test for copying disks with qemu-img convert to
> new raw target image to Gluster storage with sector size of 512 bytes:
>
> for i in $(seq 10); do
> rm -f dst.raw
> sleep 10
> time ./qemu-img convert -f raw -O raw -t none -T none src.raw dst.raw
> done
>
> Here is a table comparing the total time spent:
>
> Type Before(s) After(s) Diff(%)
> ---------------------------------------
> real 530.028 469.123 -11.4
> user 17.204 10.768 -37.4
> sys 17.881 7.011 -60.7
>
> We can see very clear improvement in CPU usage.
>
> Signed-off-by: Nir Soffer <address@hidden>
> ---
> block/file-posix.c | 51 +++++++++++++++++++
> tests/qemu-iotests/059.out | 2 +-
> tests/qemu-iotests/{150.out => 150.out.qcow2} | 0
> tests/qemu-iotests/150.out.raw | 12 +++++
> tests/qemu-iotests/175 | 19 ++++---
> tests/qemu-iotests/175.out | 8 +--
> tests/qemu-iotests/178.out.qcow2 | 4 +-
> tests/qemu-iotests/221.out | 12 +++--
> tests/qemu-iotests/253.out | 12 +++--
> 9 files changed, 99 insertions(+), 21 deletions(-)
> rename tests/qemu-iotests/{150.out => 150.out.qcow2} (100%)
> create mode 100644 tests/qemu-iotests/150.out.raw
Reviewed-by: Max Reitz <address@hidden>
Maybe it’ll break the vmdk iotests when using a non-default subformat;
but currently running the iotests for non-default VMDK subformats is
broken anyway, so it doesn’t matter.
Max
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