[Qemu-devel] [PATCH 1/1] block: enforce minimal 4096 alignment in qemu_blockalign

[Denis V. Lunev]

The following sequence
    int fd = open(argv[1], O_RDWR | O_CREAT | O_DIRECT, 0644);
    for (i = 0; i < 100000; i++)
            write(fd, buf, 4096);
performs 5% better if buf is aligned to 4096 bytes rather then to
512 bytes on HDD with 512/4096 logical/physical sector size.

The difference is quite reliable.

On the other hand we do not want at the moment to enforce bounce
buffering if guest request is aligned to 512 bytes. This patch
forces page alignment when we really forced to perform memory
allocation.

Signed-off-by: Denis V. Lunev <address@hidden>
CC: Paolo Bonzini <address@hidden>
CC: Kevin Wolf <address@hidden>
CC: Stefan Hajnoczi <address@hidden>
---
 block.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/block.c b/block.c
index d45e4dd..38cf73f 100644
--- a/block.c
+++ b/block.c
@@ -5293,7 +5293,11 @@ void bdrv_set_guest_block_size(BlockDriverState *bs, int 
align)
 
 void *qemu_blockalign(BlockDriverState *bs, size_t size)
 {
-    return qemu_memalign(bdrv_opt_mem_align(bs), size);
+    size_t align = bdrv_opt_mem_align(bs);
+    if (align < 4096) {
+        align = 4096;
+    }
+    return qemu_memalign(align, size);
 }
 
 void *qemu_blockalign0(BlockDriverState *bs, size_t size)
@@ -5307,6 +5311,9 @@ void *qemu_try_blockalign(BlockDriverState *bs, size_t 
size)
 
     /* Ensure that NULL is never returned on success */
     assert(align > 0);
+    if (align < 4096) {
+        align = 4096;
+    }
     if (size == 0) {
         size = align;
     }
-- 
1.9.1