Re: [Qemu-devel] [RFC][PATCH 1.8] block/iscsi: set bdi->cluster_size

[Paolo Bonzini <pbonzini@redhat.com>]

Il 25/11/2013 09:29, Peter Lieven ha scritto:
> this patch aims to set bdi->cluster_size to the internal page size
> of the iscsi target so that enabled callers can align requests
> properly.
> 
> Signed-off-by: Peter Lieven <pl@kamp.de>
> ---
>  block/iscsi.c |    6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/block/iscsi.c b/block/iscsi.c
> index b7b5238..3a4c5d4 100644
> --- a/block/iscsi.c
> +++ b/block/iscsi.c
> @@ -1569,6 +1569,12 @@ static int iscsi_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
>      IscsiLun *iscsilun = bs->opaque;
>      bdi->unallocated_blocks_are_zero = !!iscsilun->lbprz;
>      bdi->can_write_zeroes_with_unmap = iscsilun->lbprz && iscsilun->lbp.lbpws;
> +    bdi->cluster_size = iscsilun->bl.opt_xfer_len * iscsilun->block_size;
> +    /* if opt_xfer_len is not specified guess the internal page size of the
> +     * iscsi target by the means of opt_unmap_gran */
> +    if (bdi->cluster_size == 0) {
> +        bdi->cluster_size = iscsilun->bl.opt_unmap_gran * iscsilun->block_size;
> +    }
>      return 0;
>  }
>  
> 

This is not what cluster_size does; cluster_size is the granularity of
allocations, and hence COW requests.  In that sense, opt_unmap_gran is
probably a better choice than opt_xfer_len, especially if you want to
set the default cluster size of a new image (e.g. qcow2) so that the
format itself will not try to discard partial clusters.

However, I'm not sure overloading cluster_size is a good thing; you can
already use discard_alignment.  Also, I think you need some clamping
(e.g. if <4KB or >1MB).

Paolo