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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 19:10:48 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 |
On 27.08.19 18:58, Max Reitz wrote:
> 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.
(Good news, 059 really was the only issue for VMDK.)
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