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From a221f5a8ed02690687e6709c49ae0e1e01c5f466 Mon Sep 17 00:00:00 2001
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From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
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Date: Fri, 17 Dec 2021 17:46:54 +0100
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Subject: [PATCH 09/12] qemu-img: make is_allocated_sectors() more efficient
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RH-Author: Kevin Wolf <kwolf@redhat.com>
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RH-MergeRequest: 70: qemu-img convert: Fix sparseness of output image
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RH-Commit: [2/2] cc05aa4ac506b57ff9b430c007618cdf1485a03f (kmwolf/centos-qemu-kvm)
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RH-Bugzilla: 1882917
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RH-Acked-by: Eric Blake <eblake@redhat.com>
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RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
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RH-Acked-by: Hanna Reitz <hreitz@redhat.com>
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Consider the case when the whole buffer is zero and end is unaligned.
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If i <= tail, we return 1 and do one unaligned WRITE, RMW happens.
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If i > tail, we do on aligned WRITE_ZERO (or skip if target is zeroed)
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and again one unaligned WRITE, RMW happens.
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Let's do better: don't fragment the whole-zero buffer and report it as
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ZERO: in case of zeroed target we just do nothing and avoid RMW. If
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target is not zeroes, one unaligned WRITE_ZERO should not be much worse
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than one unaligned WRITE.
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Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
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Message-Id: <20211217164654.1184218-3-vsementsov@virtuozzo.com>
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Tested-by: Peter Lieven <pl@kamp.de>
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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(cherry picked from commit 96054c76ff2db74165385a69f234c57a6bbc941e)
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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---
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qemu-img.c | 23 +++++++++++++++++++----
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tests/qemu-iotests/122.out | 8 ++------
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2 files changed, 21 insertions(+), 10 deletions(-)
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diff --git a/qemu-img.c b/qemu-img.c
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index f036a1d428..d7ddfcc528 100644
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--- a/qemu-img.c
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+++ b/qemu-img.c
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@@ -1171,19 +1171,34 @@ static int is_allocated_sectors(const uint8_t *buf, int n, int *pnum,
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}
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}
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+ if (i == n) {
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+ /*
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+ * The whole buf is the same.
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+ * No reason to split it into chunks, so return now.
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+ */
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+ *pnum = i;
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+ return !is_zero;
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+ }
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+
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tail = (sector_num + i) & (alignment - 1);
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if (tail) {
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if (is_zero && i <= tail) {
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- /* treat unallocated areas which only consist
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- * of a small tail as allocated. */
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+ /*
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+ * For sure next sector after i is data, and it will rewrite this
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+ * tail anyway due to RMW. So, let's just write data now.
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+ */
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is_zero = false;
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}
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if (!is_zero) {
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- /* align up end offset of allocated areas. */
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+ /* If possible, align up end offset of allocated areas. */
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i += alignment - tail;
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i = MIN(i, n);
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} else {
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- /* align down end offset of zero areas. */
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+ /*
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+ * For sure next sector after i is data, and it will rewrite this
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+ * tail anyway due to RMW. Better is avoid RMW and write zeroes up
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+ * to aligned bound.
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+ */
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i -= tail;
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}
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}
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diff --git a/tests/qemu-iotests/122.out b/tests/qemu-iotests/122.out
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index 69b8e8b803..e18766e167 100644
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--- a/tests/qemu-iotests/122.out
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+++ b/tests/qemu-iotests/122.out
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@@ -201,9 +201,7 @@ convert -S 4k
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{ "start": 8192, "length": 4096, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET},
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{ "start": 12288, "length": 4096, "depth": 0, "present": false, "zero": true, "data": false},
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{ "start": 16384, "length": 4096, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET},
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-{ "start": 20480, "length": 46080, "depth": 0, "present": false, "zero": true, "data": false},
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-{ "start": 66560, "length": 1024, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET},
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-{ "start": 67584, "length": 67041280, "depth": 0, "present": false, "zero": true, "data": false}]
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+{ "start": 20480, "length": 67088384, "depth": 0, "present": false, "zero": true, "data": false}]
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convert -c -S 4k
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[{ "start": 0, "length": 1024, "depth": 0, "present": true, "zero": false, "data": true},
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@@ -215,9 +213,7 @@ convert -c -S 4k
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convert -S 8k
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[{ "start": 0, "length": 24576, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET},
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-{ "start": 24576, "length": 41984, "depth": 0, "present": false, "zero": true, "data": false},
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-{ "start": 66560, "length": 1024, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET},
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-{ "start": 67584, "length": 67041280, "depth": 0, "present": false, "zero": true, "data": false}]
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+{ "start": 24576, "length": 67084288, "depth": 0, "present": false, "zero": true, "data": false}]
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convert -c -S 8k
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[{ "start": 0, "length": 1024, "depth": 0, "present": true, "zero": false, "data": true},
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--
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2.27.0
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