[qemu-xen master] qemu-img map: Don't limit block status request size

[patchbot@xxxxxxx]

commit d0ceea88dea053e0c1c038d42ca98782c2e3872d
Author:     Kevin Wolf <kwolf@xxxxxxxxxx>
AuthorDate: Tue Jul 7 16:46:29 2020 +0200
Commit:     Kevin Wolf <kwolf@xxxxxxxxxx>
CommitDate: Tue Jul 14 15:18:59 2020 +0200

    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 <kwolf@xxxxxxxxxx>
    Message-Id: <20200707144629.51235-1-kwolf@xxxxxxxxxx>
    Reviewed-by: Eric Blake <eblake@xxxxxxxxxx>
    Signed-off-by: Kevin Wolf <kwolf@xxxxxxxxxx>
---
 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;
--
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