qemu-img: make is_allocated_sectors() more efficient
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>
diff --git a/qemu-img.c b/qemu-img.c
index 21ba1e6..6fe2466 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1171,19 +1171,34 @@
}
}
+ 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;
}
}