[PULL 14/16] qemu-img: make is_allocated_sectors() more efficient

[Kevin Wolf <kwolf@redhat.com>]

From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

Consider the case when the whole buffer is zero and end is unaligned.

If i <= tail, we return 1 and do one unaligned WRITE, RMW happens.

If i > tail, we do on aligned WRITE_ZERO (or skip if target is zeroed)
and again one unaligned WRITE, RMW happens.

Let's do better: don't fragment the whole-zero buffer and report it as
ZERO: in case of zeroed target we just do nothing and avoid RMW. If
target is not zeroes, one unaligned WRITE_ZERO should not be much worse
than one unaligned WRITE.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20211217164654.1184218-3-vsementsov@virtuozzo.com>
Tested-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 qemu-img.c                 | 23 +++++++++++++++++++----
 tests/qemu-iotests/122.out |  8 ++------
 2 files changed, 21 insertions(+), 10 deletions(-)

diff --git a/qemu-img.c b/qemu-img.c
index 21ba1e6800..6fe2466032 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1171,19 +1171,34 @@ static int is_allocated_sectors(const uint8_t *buf, int n, int *pnum,
         }
     }
 
+    if (i == n) {
+        /*
+         * The whole buf is the same.
+         * No reason to split it into chunks, so return now.
+         */
+        *pnum = i;
+        return !is_zero;
+    }
+
     tail = (sector_num + i) & (alignment - 1);
     if (tail) {
         if (is_zero && i <= tail) {
-            /* treat unallocated areas which only consist
-             * of a small tail as allocated. */
+            /*
+             * For sure next sector after i is data, and it will rewrite this
+             * tail anyway due to RMW. So, let's just write data now.
+             */
             is_zero = false;
         }
         if (!is_zero) {
-            /* align up end offset of allocated areas. */
+            /* If possible, align up end offset of allocated areas. */
             i += alignment - tail;
             i = MIN(i, n);
         } else {
-            /* align down end offset of zero areas. */
+            /*
+             * For sure next sector after i is data, and it will rewrite this
+             * tail anyway due to RMW. Better is avoid RMW and write zeroes up
+             * to aligned bound.
+             */
             i -= tail;
         }
     }
diff --git a/tests/qemu-iotests/122.out b/tests/qemu-iotests/122.out
index 69b8e8b803..e18766e167 100644
--- a/tests/qemu-iotests/122.out
+++ b/tests/qemu-iotests/122.out
@@ -201,9 +201,7 @@ convert -S 4k
 { "start": 8192, "length": 4096, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET},
 { "start": 12288, "length": 4096, "depth": 0, "present": false, "zero": true, "data": false},
 { "start": 16384, "length": 4096, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 20480, "length": 46080, "depth": 0, "present": false, "zero": true, "data": false},
-{ "start": 66560, "length": 1024, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 67584, "length": 67041280, "depth": 0, "present": false, "zero": true, "data": false}]
+{ "start": 20480, "length": 67088384, "depth": 0, "present": false, "zero": true, "data": false}]
 
 convert -c -S 4k
 [{ "start": 0, "length": 1024, "depth": 0, "present": true, "zero": false, "data": true},
@@ -215,9 +213,7 @@ convert -c -S 4k
 
 convert -S 8k
 [{ "start": 0, "length": 24576, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 24576, "length": 41984, "depth": 0, "present": false, "zero": true, "data": false},
-{ "start": 66560, "length": 1024, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 67584, "length": 67041280, "depth": 0, "present": false, "zero": true, "data": false}]
+{ "start": 24576, "length": 67084288, "depth": 0, "present": false, "zero": true, "data": false}]
 
 convert -c -S 8k
 [{ "start": 0, "length": 1024, "depth": 0, "present": true, "zero": false, "data": true},
-- 
2.31.1