On 28/01/15 23:07, Paolo Bonzini wrote:
On 28/01/2015 19:49, Denis V. Lunev wrote:
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 10% 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.
The 10% difference, however, is probably not enough to cover the cost of
providing a bounce buffer if a guest is (rightfully) using a 512-byte
aligned buffer: bs->bl.opt_mem_alignment is in fact badly named and it
should be bs->bl.min_mem_alignment instead.
Instead, you probably should patch bdrv_opt_mem_align to return at least
4096, and leave the detection logic intact. This will let
qemu_blockalign return a properly aligned buffer to qemu-img and other
in-process allocations, without negatively affecting the guest.
Thanks,
Paolo
ok, this looks good to me :)
Signed-off-by: Denis V. Lunev <d...@openvz.org>
CC: Kevin Wolf <kw...@redhat.com>
CC: Stefan Hajnoczi <stefa...@redhat.com>
---
block.c | 4 ++--
block/raw-posix.c | 4 ++--
2 files changed, 4 insertions(+), 4 deletions(-)
diff --git a/block.c b/block.c
index d45e4dd..bc5d1e7 100644
--- a/block.c
+++ b/block.c
@@ -543,7 +543,7 @@ void bdrv_refresh_limits(BlockDriverState *bs, Error **errp)
bs->bl.max_transfer_length = bs->file->bl.max_transfer_length;
bs->bl.opt_mem_alignment = bs->file->bl.opt_mem_alignment;
} else {
- bs->bl.opt_mem_alignment = 512;
+ bs->bl.opt_mem_alignment = 4096;
}
if (bs->backing_hd) {
@@ -966,7 +966,7 @@ static int bdrv_open_common(BlockDriverState *bs,
BlockDriverState *file,
bs->open_flags = flags;
bs->guest_block_size = 512;
- bs->request_alignment = 512;
+ bs->request_alignment = 4096;
bs->zero_beyond_eof = true;
open_flags = bdrv_open_flags(bs, flags);
bs->read_only = !(open_flags & BDRV_O_RDWR);
diff --git a/block/raw-posix.c b/block/raw-posix.c
index ec38fee..d1b3388 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -266,7 +266,7 @@ static void raw_probe_alignment(BlockDriverState *bs, int
fd, Error **errp)
if (!s->buf_align) {
size_t align;
buf = qemu_memalign(MAX_BLOCKSIZE, 2 * MAX_BLOCKSIZE);
- for (align = 512; align <= MAX_BLOCKSIZE; align <<= 1) {
+ for (align = 4096; align <= MAX_BLOCKSIZE; align <<= 1) {
if (pread(fd, buf + align, MAX_BLOCKSIZE, 0) >= 0) {
s->buf_align = align;
break;
@@ -278,7 +278,7 @@ static void raw_probe_alignment(BlockDriverState *bs, int
fd, Error **errp)
if (!bs->request_alignment) {
size_t align;
buf = qemu_memalign(s->buf_align, MAX_BLOCKSIZE);
- for (align = 512; align <= MAX_BLOCKSIZE; align <<= 1) {
+ for (align = 4096; align <= MAX_BLOCKSIZE; align <<= 1) {
if (pread(fd, buf, align, 0) >= 0) {
bs->request_alignment = align;
break;