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qemu-img: rebase: Reduce reads on in-chain rebase

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In the following case:

(base) A <- B <- C (tip)

when running:

    qemu-img rebase -b A C

QEMU would read all sectors not allocated in the file being rebased (C)
and compare them to the new base image (A), regardless of whether they
were changed or even allocated anywhere along the chain between the new
base and the top image (B). This causes many unneeded reads when
rebasing an image which represents a small diff of a large disk, as it
would read most of the disk's sectors.

Instead, use bdrv_is_allocated_above() to reduce the number of
unnecessary reads.

Reviewed-by: Karl Heubaum <karl.heubaum@oracle.com>
Signed-off-by: Sam Eiderman <shmuel.eiderman@oracle.com>
Signed-off-by: Eyal Moscovici <eyal.moscovici@oracle.com>
Message-id: 20190523163337.4497-3-shmuel.eiderman@oracle.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Sam Eiderman 2019-05-23 19:33:36 +03:00 committed by Max Reitz
parent 4ebe061726
commit 863cc78f1b
1 changed files with 24 additions and 1 deletions
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25
qemu-img.c
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@ -3164,7 +3164,7 @@ static int img_rebase(int argc, char **argv)
BlockBackend *blk = NULL, *blk_old_backing = NULL, *blk_new_backing = NULL;
uint8_t *buf_old = NULL;
uint8_t *buf_new = NULL;
BlockDriverState *bs = NULL;
BlockDriverState *bs = NULL, *prefix_chain_bs = NULL;
char *filename;
const char *fmt, *cache, *src_cache, *out_basefmt, *out_baseimg;
int c, flags, src_flags, ret;
@ -3353,6 +3353,12 @@ static int img_rebase(int argc, char **argv)
goto out;
}
/*
* Find out whether we rebase an image on top of a previous image
* in its chain.
*/
prefix_chain_bs = bdrv_find_backing_image(bs, out_real_path);
blk_new_backing = blk_new_open(out_real_path, NULL,
options, src_flags, &local_err);
g_free(out_real_path);
@ -3437,6 +3443,23 @@ static int img_rebase(int argc, char **argv)
continue;
}
if (prefix_chain_bs) {
/*
* If cluster wasn't changed since prefix_chain, we don't need
* to take action
*/
ret = bdrv_is_allocated_above(backing_bs(bs), prefix_chain_bs,
offset, n, &n);
if (ret < 0) {
error_report("error while reading image metadata: %s",
strerror(-ret));
goto out;
}
if (!ret) {
continue;
}
}
/*
* Read old and new backing file and take into consideration that
* backing files may be smaller than the COW image.