The default queue_limits::max_segments value (BLK_MAX_SEGMENTS = 128)
unnecessarily limits bio sizes to 512k (assuming 4k pages). rbd, being
a virtual block device, doesn't have any restrictions on the number of
physical segments, so bump max_segments to max_hw_sectors, in theory
allowing a sector per segment (although the only case this matters that
I can think of is some readv/writev style thing). In practice this is
going to give us 1M bios - the number of segments in a bio is limited
in bio_get_nr_vecs() by BIO_MAX_PAGES = 256.
Note that this doesn't result in any improvement on a typical direct
sequential test. This is because on a box with a not too badly
fragmented memory the default BLK_MAX_SEGMENTS is enough to see nice
rbd object size sized requests. The only difference is the size of
bios being merged - 512k vs 1M for something like
$ dd if=/dev/zero of=/dev/rbd0 oflag=direct bs=$RBD_OBJ_SIZE
$ dd if=/dev/rbd0 iflag=direct of=/dev/null bs=$RBD_OBJ_SIZE
Signed-off-by: Ilya Dryomov <[email protected]>
---
drivers/block/rbd.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
index 89fe8a4bc02e..bc88fbcb9715 100644
--- a/drivers/block/rbd.c
+++ b/drivers/block/rbd.c
@@ -3791,6 +3791,7 @@ static int rbd_init_disk(struct rbd_device *rbd_dev)
/* set io sizes to object size */
segment_size = rbd_obj_bytes(&rbd_dev->header);
blk_queue_max_hw_sectors(q, segment_size / SECTOR_SIZE);
+ blk_queue_max_segments(q, segment_size / SECTOR_SIZE);
blk_queue_max_segment_size(q, segment_size);
blk_queue_io_min(q, segment_size);
blk_queue_io_opt(q, segment_size);
--
1.9.3
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