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

[Peter Lieven]

On 25.11.2013 09:54, Paolo Bonzini wrote:
> 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 <address@hidden>
> > ---
> >   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).
Maybe I should explain the background of this patch. I currently work
on some patches optimizing qemu-img convert. And I found out
that there is significant speed up if the writes to the output
image are aligned to the underlying page boundaries of the iscsi_target.
So I wanted to use bdi->cluster_size for aligning the
requests if its available. The same that already happens for compressed
images.

Maybe I should put together the whole series maybe this gives a better
picture.

Peter