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qemu-img map: Don't limit block status request size

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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 was 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>
Message-Id: <20200707144629.51235-1-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Kevin Wolf 2020-07-07 16:46:29 +02:00
parent 4b196cd16d
commit d0ceea88de
1 changed files with 1 additions and 4 deletions
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5
qemu-img.c
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@ -3210,12 +3210,9 @@ static int img_map(int argc, char **argv)
curr.start = start_offset; curr.start = start_offset;
while (curr.start + curr.length < length) { while (curr.start + curr.length < length) {
int64_t offset = curr.start + curr.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); ret = get_block_status(bs, offset, n, &next);
if (ret < 0) { if (ret < 0) {
error_report("Could not read file metadata: %s", strerror(-ret)); error_report("Could not read file metadata: %s", strerror(-ret));
goto out; goto out;