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

[Paolo Bonzini <pbonzini@redhat.com>]

On 29/01/2015 11:50, 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 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 <den@openvz.org>
> CC: Paolo Bonzini <pbonzini@redhat.com>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  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;
>      }
> 

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>