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

[Max Reitz]

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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