[RESEND PATCH] xen/blkfront: increase the default value of xen_blkif_max_segments

[RESEND PATCH] xen/blkfront: increase the default value of xen_blkif_max_segments

[Bob Liu]

The default max-segments of indirect requests was 32, so IO operations with
bigger block size(>32*4k) would be split and make performance drop.

Nowadays backend device usually support 512k max_sectors_kb on desktop,
and usually larger on server machines connected with high-end storage system.
The default max size(128k) is not appropriate anymore, this patch increases it
to 128(128*4k=512k).

Signed-off-by: Bob Liu <bob.liu@oracle.com>
---
 drivers/block/xen-blkfront.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 2236c6f..1bf2429 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -94,9 +94,9 @@ static const struct block_device_operations xlvbd_block_fops;
  * by the backend driver.
  */
 
-static unsigned int xen_blkif_max_segments = 32;
+static unsigned int xen_blkif_max_segments = 128;
 module_param_named(max, xen_blkif_max_segments, int, S_IRUGO);
-MODULE_PARM_DESC(max, "Maximum amount of segments in indirect requests (default is 32)");
+MODULE_PARM_DESC(max, "Maximum amount of segments in indirect requests (default is 128)");
 
 #define BLK_RING_SIZE __CONST_RING_SIZE(blkif, PAGE_SIZE)
 
-- 
1.7.10.4

Re: [RESEND PATCH] xen/blkfront: increase the default value of xen_blkif_max_segments

[Roger Pau Monné]

El 21/01/15 a les 8.56, Bob Liu ha escrit:
> The default max-segments of indirect requests was 32, so IO operations with
> bigger block size(>32*4k) would be split and make performance drop.
> 
> Nowadays backend device usually support 512k max_sectors_kb on desktop,
> and usually larger on server machines connected with high-end storage system.
> The default max size(128k) is not appropriate anymore, this patch increases it
> to 128(128*4k=512k).
> 
> Signed-off-by: Bob Liu <bob.liu@oracle.com>

Looks fine to me, the Linux backend is also prepared to handle it
without any modifications. As I said in the last version, I would add
the following to the commit message:

This change implies we can now have 32*128+32 = 4128 in-flight grants,
which greatly surpasses the default amount of persistent grants blkback
can handle, so the LRU in blkback will kick in.

Roger.