[PATCH v5 03/16] md/raid5: set default stripe_size as 4096

Yufen Yu
2020-07-02T12:06:15Z

In RAID5, if issued bio size is bigger than stripe_size, it will be split
in the unit of stripe_size and process them one by one. Even for size
less then stripe_size, RAID5 also request data from disk at least of
stripe_size.

Nowdays, stripe_size is equal to the value of PAGE_SIZE. Since filesystem
usually issue bio in the unit of 4KB, there is no problem for PAGE_SIZE as
4KB. But, for 64KB PAGE_SIZE, bio from filesystem requests 4KB data while
RAID5 issue IO at least stripe_size (64KB) each time. That will waste
resource of disk bandwidth and compute xor.

To avoding the waste, we want to make stripe_size configurable. This patch
just set 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.

To evaluate the new feature, we create raid5 device '/dev/md5' with
4 SSD disk and test it on arm64 machine with 64KB PAGE_SIZE.

1) We format /dev/md5 with mkfs.ext4 and mount ext4 with default
configure on /mnt directory. Then, trying to test it by dbench with
command: dbench -D /mnt -t 1000 10. Result show as:

'stripe_size = 64KB'

Operation Count AvgLat MaxLat
----------------------------------------
NTCreateX 9805011 0.021 64.728
Close 7202525 0.001 0.120
Rename 415213 0.051 44.681
Unlink 1980066 0.079 93.147
Deltree 240 1.793 6.516
Mkdir 120 0.004 0.007
Qpathinfo 8887512 0.007 37.114
Qfileinfo 1557262 0.001 0.030
Qfsinfo 1629582 0.012 0.152
Sfileinfo 798756 0.040 57.641
Find 3436004 0.019 57.782
WriteX 4887239 0.021 57.638
ReadX 15370483 0.005 37.818
LockX 31934 0.003 0.022
UnlockX 31933 0.001 0.021
Flush 687205 13.302 530.088

Throughput 307.799 MB/sec 10 clients 10 procs max_latency=530.091 ms
-------------------------------------------------------

'stripe_size = 4KB'

Operation Count AvgLat MaxLat
----------------------------------------
NTCreateX 11999166 0.021 36.380
Close 8814128 0.001 0.122
Rename 508113 0.051 29.169
Unlink 2423242 0.070 38.141
Deltree 300 1.885 7.155
Mkdir 150 0.004 0.006
Qpathinfo 10875921 0.007 35.485
Qfileinfo 1905837 0.001 0.032
Qfsinfo 1994304 0.012 0.125
Sfileinfo 977450 0.029 26.489
Find 4204952 0.019 9.361
WriteX 5981890 0.019 27.804
ReadX 18809742 0.004 33.491
LockX 39074 0.003 0.025
UnlockX 39074 0.001 0.014
Flush 841022 10.712 458.848

Throughput 376.777 MB/sec 10 clients 10 procs max_latency=458.852 ms
-------------------------------------------------------

It show that setting stripe_size as 4KB has higher thoughput, i.e.
(376.777 vs 307.799) and has smaller latency (530.091 vs 458.852)
than that setting as 64KB.

2) We try to evaluate IO throughput for /dev/md5 by fio with config:

[4KB randwrite]
direct=1
numjob=2
iodepth=64
ioengine=libaio
filename=/dev/md5
bs=4KB
rw=randwrite

[64KB write]
direct=1
numjob=2
iodepth=64
ioengine=libaio
filename=/dev/md5
bs=1MB
rw=write

The result as follow:

+ +
| stripe_size(64KB) | stripe_size(4KB)
+----------------------------------------------------+
4KB randwrite | 15MB/s | 100MB/s
+----------------------------------------------------+
1MB write | 1000MB/s | 700MB/s

The result show that when size of io is bigger than 4KB (64KB),
64KB stripe_size has much higher IOPS. But for 4KB randwrite, that
means, size of io issued to device are smaller, 4KB stripe_size
have better performance.

Normally, default value (4096) can get relatively good performance.
But if each issued io is bigger than 4096, setting value more than
4096 may get better performance.

Here, we just set default stripe_size as 4096, and we will try to support
setting different stripe_size by sysfs interface in the following patch.

Signed-off-by: Yufen Yu <yuyufen@huawei.com>
---
drivers/md/raid5.c | 6 +++---
drivers/md/raid5.h | 1 +
2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 51bc39dab57b..694f6713369d 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -6980,9 +6980,9 @@ static struct r5conf *setup_conf(struct mddev *mddev)
if (conf == NULL)
goto abort;

- conf->stripe_size = PAGE_SIZE;
- conf->stripe_shift = PAGE_SHIFT - 9;
- conf->stripe_sectors = conf->stripe_size >> 9;
+ conf->stripe_size = DEFAULT_STRIPE_SIZE;
+ conf->stripe_shift = ilog2(DEFAULT_STRIPE_SIZE) - 9;
+ conf->stripe_sectors = DEFAULT_STRIPE_SIZE >> 9;

INIT_LIST_HEAD(&conf->free_list);
INIT_LIST_HEAD(&conf->pending_list);
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index e36cf71e8465..98698569370c 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -477,6 +477,7 @@ struct disk_info {
#define NR_HASH (PAGE_SIZE / sizeof(struct hlist_head))
#define HASH_MASK (NR_HASH - 1)
#define MAX_STRIPE_BATCH 8
+#define DEFAULT_STRIPE_SIZE 4096

/* NOTE NR_STRIPE_HASH_LOCKS must remain below 64.
* This is because we sometimes take all the spinlocks
--
2.25.4