Re: [Qemu-devel] [PATCH v3 1/2] block: posix: Always allocate the first block

[Max Reitz <mreitz@redhat.com>]

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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 <nsoffer@redhat.com>
>> ---
>>  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 <mreitz@redhat.com>
> 
> 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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