[Qemu-devel] [PULL 1/4] qcow2: do not allocate extra memory

[Max Reitz <mreitz@redhat.com>]

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

There are no needs to allocate more than one cluster, as we set
avail_out for deflate to one cluster.

Zlib docs (http://www.zlib.net/manual.html) says:
"deflate compresses as much data as possible, and stops when the input
buffer becomes empty or the output buffer becomes full."

So, deflate will not write more than avail_out to output buffer. If
there is not enough space in output buffer for compressed data (it may
be larger than input data) deflate just returns Z_OK. (if all data is
compressed and written to output buffer deflate returns Z_STREAM_END).

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 1468515565-81313-1-git-send-email-vsementsov@virtuozzo.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/qcow.c  | 2 +-
 block/qcow2.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/block/qcow.c b/block/qcow.c
index 0c7b75b..6f9b2e2 100644
--- a/block/qcow.c
+++ b/block/qcow.c
@@ -983,7 +983,7 @@ static int qcow_write_compressed(BlockDriverState *bs, int64_t sector_num,
         return ret;
     }
 
-    out_buf = g_malloc(s->cluster_size + (s->cluster_size / 1000) + 128);
+    out_buf = g_malloc(s->cluster_size);
 
     /* best compression, small window, no zlib header */
     memset(&strm, 0, sizeof(strm));
diff --git a/block/qcow2.c b/block/qcow2.c
index a6bca73..b326d0b 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2612,7 +2612,7 @@ static int qcow2_write_compressed(BlockDriverState *bs, int64_t sector_num,
         return ret;
     }
 
-    out_buf = g_malloc(s->cluster_size + (s->cluster_size / 1000) + 128);
+    out_buf = g_malloc(s->cluster_size);
 
     /* best compression, small window, no zlib header */
     memset(&strm, 0, sizeof(strm));
-- 
2.9.0