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 <p...@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).
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
