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

[Paolo Bonzini <pbonzini@redhat.com>]

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

> 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;
>