Re: [PATCH v5 07/10] block: introduce preallocate filter

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

21.08.2020 17:11, Vladimir Sementsov-Ogievskiy wrote:
> It's intended to be inserted between format and protocol nodes to
> preallocate additional space (expanding protocol file) on writes
> crossing EOF. It improves performance for file-systems with slow
> allocation.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   docs/system/qemu-block-drivers.rst.inc |  26 +++
>   qapi/block-core.json                   |  20 +-
>   block/preallocate.c                    | 291 +++++++++++++++++++++++++
>   block/Makefile.objs                    |   1 +
>   4 files changed, 337 insertions(+), 1 deletion(-)
>   create mode 100644 block/preallocate.c
> 
> diff --git a/docs/system/qemu-block-drivers.rst.inc b/docs/system/qemu-block-drivers.rst.inc
> index b052a6d14e..5e8a35c571 100644
> --- a/docs/system/qemu-block-drivers.rst.inc
> +++ b/docs/system/qemu-block-drivers.rst.inc
> @@ -952,3 +952,29 @@ on host and see if there are locks held by the QEMU process on the image file.
>   More than one byte could be locked by the QEMU instance, each byte of which
>   reflects a particular permission that is acquired or protected by the running
>   block driver.
> +

[..]

> +
> +static coroutine_fn int preallocate_co_preadv_part(
> +        BlockDriverState *bs, uint64_t offset, uint64_t bytes,
> +        QEMUIOVector *qiov, size_t qiov_offset, int flags)
> +{
> +    return bdrv_co_preadv_part(bs->file, offset, bytes, qiov, qiov_offset,
> +                               flags);
> +}
> +
> +static int coroutine_fn preallocate_co_pdiscard(BlockDriverState *bs,
> +                                               int64_t offset, int bytes)
> +{
> +    return bdrv_co_pdiscard(bs->file, offset, bytes);
> +}
> +
> +static bool coroutine_fn do_preallocate(BlockDriverState *bs, int64_t offset,
> +                                       int64_t bytes, bool write_zero)
> +{
> +    BDRVPreallocateState *s = bs->opaque;
> +    int64_t len, start, end;

int64_t old_data_end = s->data_end;

> +
> +    if (!s->active) {
> +        return false;
> +    }
> +
> +    if (s->data_end >= 0) {
> +        s->data_end = MAX(s->data_end,
> +                          QEMU_ALIGN_UP(offset + bytes, BDRV_SECTOR_SIZE));
> +    }
> +
> +    len = bdrv_getlength(bs->file->bs);
> +    if (len < 0) {
> +        return false;

return old_data_end >= 0 && offset >= old_data_end;


And with this small improvement we make the following test 20% faster (ssd, ext4):

./qemu-img create -f qcow2 $img 16G; ./qemu-img bench -c 150000 -d 64 -f qcow2  -s 16k -t none -n -w $img;

(assume additional patch which inserts the filter).

Great! So, preallocation is generally good idea, not only for vstorage.

What about inserting the filter by default?

I'll come tomorrow with more complete test results.



-- 
Best regards,
Vladimir