Re: [PATCH] qemu-img map: Don't limit block status request size

[Eric Blake]

On 7/7/20 9:46 AM, Kevin Wolf wrote:
> Limiting each loop iteration of qemu-img map to 1 GB was arbitrary from
> the beginning, though it only cut the maximum in half then because the
> interface a signed 32 bit byte count. These days, bdrv_block_status()
> supports a 64 bit byte count, so the arbitrary limit is even worse.
>
> On file-posix, bdrv_block_status() eventually maps to SEEK_HOLE and
> SEEK_DATA, which don't support a limit, but always do all of the work
> necessary to find the start of the next hole/data. Much of this work may
> be repeated if we don't use this information fully, but query with an
> only slightly larger offset in the next loop iteration. Therefore, if
> bdrv_block_status() is called in a loop, it should always pass the
> full number of bytes that the whole loop is interested in.
>
> This removes the arbitrary limit and speeds up 'qemu-img map'
> significantly on heavily fragmented images.
>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>   qemu-img.c | 5 +----
>   1 file changed, 1 insertion(+), 4 deletions(-)

Do you want this in 5.1?  It seems like a nice optimization.

> diff --git a/qemu-img.c b/qemu-img.c
> index bdb9f6aa46..74946f81ca 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -3217,12 +3217,9 @@ static int img_map(int argc, char **argv)
>       curr.start = start_offset;
>       while (curr.start + curr.length < length) {
>           int64_t offset = curr.start + curr.length;
> -        int64_t n;
> +        int64_t n = length - offset;
>   
> -        /* Probe up to 1 GiB at a time.  */
> -        n = MIN(1 * GiB, length - offset);
>           ret = get_block_status(bs, offset, n, &next);
> -
>           if (ret < 0) {
>               error_report("Could not read file metadata: %s", strerror(-ret));
>               goto out;

Reviewed-by: Eric Blake <eblake@redhat.com>

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org