From d0ceea88dea053e0c1c038d42ca98782c2e3872d Mon Sep 17 00:00:00 2001 From: Kevin Wolf Date: Tue, 7 Jul 2020 16:46:29 +0200 Subject: [PATCH] qemu-img map: Don't limit block status request size 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 Message-Id: <20200707144629.51235-1-kwolf@redhat.com> Reviewed-by: Eric Blake Signed-off-by: Kevin Wolf --- qemu-img.c | 5 +---- 1 file changed, 1 insertion(+), 4 deletions(-) diff --git a/qemu-img.c b/qemu-img.c index 498fbf42fe..4548dbff82 100644 --- a/qemu-img.c +++ b/qemu-img.c @@ -3210,12 +3210,9 @@ 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;