Am 20.04.2017 um 10:38 hat jemmy858...@gmail.com geschrieben:
> From: Lidong Chen <lidongc...@tencent.com>
>
> when the buffer is zero, blk_co_pwrite_zeroes is more effectively than
> blk_co_pwritev with BDRV_REQ_WRITE_COMPRESSED. this patch can reduces
> the time when converts the qcow2 image with lots of zero.
>
> Signed-off-by: Lidong Chen <lidongc...@tencent.com>
Good catch, using blk_co_pwrite_zeroes() makes sense even for compressed
images.
> diff --git a/qemu-img.c b/qemu-img.c
> index b220cf7..0256539 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -1675,13 +1675,20 @@ static int coroutine_fn
> convert_co_write(ImgConvertState *s, int64_t sector_num,
> * write if the buffer is completely zeroed and we're allowed to
> * keep the target sparse. */
> if (s->compressed) {
> - if (s->has_zero_init && s->min_sparse &&
> - buffer_is_zero(buf, n * BDRV_SECTOR_SIZE))
> - {
> - assert(!s->target_has_backing);
> - break;
> + if (buffer_is_zero(buf, n * BDRV_SECTOR_SIZE)) {
> + if (s->has_zero_init && s->min_sparse) {
> + assert(!s->target_has_backing);
> + break;
> + } else {
> + ret = blk_co_pwrite_zeroes(s->target,
> + sector_num << BDRV_SECTOR_BITS,
> + n << BDRV_SECTOR_BITS, 0);
> + if (ret < 0) {
> + return ret;
> + }
> + break;
> + }
> }
If s->min_sparse == 0, we may neither skip the write not use
blk_co_pwrite_zeroes(), because this requires actual full allocation
with explicit zero sectors.
Of course, if you fix this, what you end up with here is a duplicate of
the code path for non-compressed images. The remaining difference seems
to be the BDRV_REQ_WRITE_COMPRESSED flag and buffer_is_zero() vs.
is_allocated_sectors_min() (because uncompressed clusters can be written
partially, but compressed clusters can't).
So I suppose that instead of just fixing the above bug, we could actually
mostly unify the two code paths, if you want to have a try at it.
Kevin
