From a221f5a8ed02690687e6709c49ae0e1e01c5f466 Mon Sep 17 00:00:00 2001

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

Date: Fri, 17 Dec 2021 17:46:54 +0100

Subject: [PATCH 09/12] qemu-img: make is_allocated_sectors() more efficient

RH-Author: Kevin Wolf <kwolf@redhat.com>

RH-MergeRequest: 70: qemu-img convert: Fix sparseness of output image

RH-Commit: [2/2] cc05aa4ac506b57ff9b430c007618cdf1485a03f (kmwolf/centos-qemu-kvm)

RH-Bugzilla: 1882917

RH-Acked-by: Eric Blake <eblake@redhat.com>

RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>

RH-Acked-by: Hanna Reitz <hreitz@redhat.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>

(cherry picked from commit 96054c76ff2db74165385a69f234c57a6bbc941e)

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 f036a1d428..d7ddfcc528 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},

-- 

2.27.0