Re: [Qemu-devel] [PATCH 1/1] block: change default memory alignment for block requests to 4096

["Denis V. Lunev" <den@openvz.org>]

On 28/01/15 21:49, 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.
>
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Stefan Hajnoczi <stefanha@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;
sorry, the patch is wrong. It breaks 'make check-block'.
I will redo it and perform more testing.

request-alignment related changes are wrong :(
I have run tests without them but added them as
a obvious last minute addition.