On Fri, Mar 03, 2017 at 02:03:31PM +1100, Neil Brown wrote:

On Fri, Feb 17 2017, Shaohua Li wrote:

quoted

Bump the flush stripe batch size to 2048. For my 12 disks raid
array, the stripes takes:
12 * 4k * 2048 = 96MB

This is still quite small. A hardware raid card generally has 1GB size,
which we suggest the raid5-cache has similar cache size.

The advantage of a big batch size is we can dispatch a lot of IO in the
same time, then we can do some scheduling to make better IO pattern.

Last patch prioritizes stripes, so we don't worry about a big flush
stripe batch will starve normal stripes.

Signed-off-by: Shaohua Li <redacted>
---
 drivers/md/raid5-cache.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
index 3f307be..b25512c 100644
--- a/drivers/md/raid5-cache.c
+++ b/drivers/md/raid5-cache.c

@@ -43,7 +43,7 @@
 /* wake up reclaim thread periodically */
 #define R5C_RECLAIM_WAKEUP_INTERVAL (30 * HZ)
 /* start flush with these full stripes */
-#define R5C_FULL_STRIPE_FLUSH_BATCH 256
+#define R5C_FULL_STRIPE_FLUSH_BATCH 2048

Fixed numbers are warning signs... I wonder if there is something better
we could do?   "conf->max_nr_stripes / 4" maybe?  We use that sort of
number elsewhere.
Would that make sense?

The code where we check the batch size (in r5c_do_reclaim) already a check:
total_cached > conf->min_nr_stripes * 1 / 2
so I think that's ok, no?

Thanks,
Shaohua