On 2019/8/30 16:28, NeilBrown wrote:
On Thu, Aug 29 2019, Yufen Yu wrote:
In RAID5, if issued bio has sectors more than STRIPE_SIZE,
it will be split in unit of STRIPE_SIZE. For bio sectors
less then STRIPE_SIZE, raid5 issue bio to disk in the size
of STRIPE_SIZE.
For now, STRIPE_SIZE is equal to the value of PAGE_SIZE.
That means, RAID5 will issus echo bio to disk at least 64KB
when use 64KB page size in arm64.
However, filesystem usually issue bio in the unit of 4KB.
Then, RAID5 will waste resource of disk bandwidth and compute xor.
To avoding the waste, we can add a CONFIG option to set
the default STRIPE_SIZE as 4096. User can also set the value
bigger than 4KB for some special requirements, such as we know
the issued io size is more than 4KB.
Signed-off-by: Yufen Yu <yuyufen@xxxxxxxxxx>
---
drivers/md/Kconfig | 9 +++++++++
drivers/md/raid5.h | 2 +-
2 files changed, 10 insertions(+), 1 deletion(-)
diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index 3834332f4963..4263a655dbbb 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -157,6 +157,15 @@ config MD_RAID456
If unsure, say Y.
+config MD_RAID456_STRIPE_SIZE
+ int "RAID4/RAID5/RAID6 stripe size"
+ default "4096"
+ depends on MD_RAID456
+ help
+ The default value is 4096. Only change this if you know
+ what you are doing.
+
+
config MD_MULTIPATH
tristate "Multipath I/O support"
depends on BLK_DEV_MD
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index cf991f13403e..2fcf5853b719 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -473,7 +473,7 @@ struct disk_info {
*/
#define NR_STRIPES 256
-#define STRIPE_SIZE PAGE_SIZE
+#define STRIPE_SIZE CONFIG_MD_RAID456_STRIPE_SIZE
#define STRIPE_SHIFT (PAGE_SHIFT - 9)
#define STRIPE_SECTORS (STRIPE_SIZE>>9)
#define IO_THRESHOLD 1
--
2.17.2
While I agree that this is a problem that should be fixed, I think your
fix is rather simplistic.
grow_buffers() uses alloc_page() allocate the buffers for each
stripe_head()
With this change, it will allocate 64K buffers and use 4K of them.
This is a waste. Unfortunately fixing it would be a lot more work.
I think that is best way forward though.
Also, I don't think this should be a config option - just make it
always 4096.
Thanks a lot for your suggestion and pointing out what I have ignored.
I will rework it.
Thanks
Yufen
