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 <kw...@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 <ebl...@redhat.com>
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
