Re: [PATCH v4 4/9] qcow2: Support BDRV_REQ_ZERO_WRITE for truncate

[Vladimir Sementsov-Ogievskiy]

20.04.2020 16:32, Kevin Wolf wrote:
> If BDRV_REQ_ZERO_WRITE is set and we're extending the image, calling
> qcow2_cluster_zeroize() with flags=0 does the right thing: It doesn't
> undo any previous preallocation, but just adds the zero flag to all
> relevant L2 entries. If an external data file is in use, a write_zeroes
> request to the data file is made instead.
>
> Signed-off-by: Kevin Wolf <address@hidden>
> ---
>   block/qcow2.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
>
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 6c6d6101ce..7a70c1c090 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -1726,6 +1726,7 @@ static int coroutine_fn qcow2_do_open(BlockDriverState 
> *bs, QDict *options,
>   
>       bs->supported_zero_flags = header.version >= 3 ?
>                                  BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK : 0;
> +    bs->supported_truncate_flags = BDRV_REQ_ZERO_WRITE;
>   
>       /* Repair image if dirty */
>       if (!(flags & (BDRV_O_CHECK | BDRV_O_INACTIVE)) && !bs->read_only &&
> @@ -4213,6 +4214,14 @@ static int coroutine_fn 
> qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
>           g_assert_not_reached();
>       }
>   
> +    if ((flags & BDRV_REQ_ZERO_WRITE) && offset > old_length) {
> +        ret = qcow2_cluster_zeroize(bs, old_length, offset - old_length, 0);

Hmm. As I understand, qcow2_cluster_zeroize is unprepared to cluster-unaligned 
offset/size. I think we should handle it somehow.

> +        if (ret < 0) {
> +            error_setg_errno(errp, -ret, "Failed to zero out the new area");
> +            goto fail;
> +        }
> +    }
> +
>       if (prealloc != PREALLOC_MODE_OFF) {
>           /* Flush metadata before actually changing the image size */
>           ret = qcow2_write_caches(bs);


--
Best regards,
Vladimir