Linux MD RAID optimisation

1. report raid rebuild status

cat /proc/mdstat

watch -n1 cat /proc/mdstat

2. raid speed limits.  Increases CPU and RAM utilisation

#report current speed limits

sysctl dev.raid.speed_limit_min

sysctl dev.raid.speed_limit_max

#set speed limits (runtime only default values)

sysctl -w dev.raid.speed_limit_min=1000

sysctl -w dev.raid.speed_limit_max=100000

#set speed limits at boot by editing /etc/sysctl.conf

dev.raid.speed_limit_min=50000

# for 3 disk array

dev.raid.speed_limit_max=500000

# for 4-5 disk array

dev.raid.speed_limit_max=2000000

#uncomment for 6-12 disk array

#dev.raid.speed_limit_max=5000000

3. disable NCQ on raid members

#read current NCQ depth (default 32)

cat /sys/block/sda/device/queue_depth

#disable NCQ (runtime only)

echo 1 > /sys/block/device/queue_depth

#disable NCQ at boot, requires sysfs package to be installed

# edit /etc/sysfs.conf

#disable NCQ on raid members sda, sdb, sdd, sde

block/sda/device/queue_depth=1

block/sdb/device/queue_depth=1

block/sdd/device/queue_depth=1

block/sde/device/queue_depth=1

4. set raid5 stripe cache size

#ram_used = system_page_size * nr_disks * stripe_cache_size

# read current stripe_cache_size on array md0

cat /sys/block/md0/md/stripe_cache_size

#set to 16MiB (runtime only)

echo 16384 > /sys/block/md0/md/stripe_cache_size

#set to 32MiB at boot

#create udev rule /etc/udev/rules.d/60-md-stripe-cache-size

SUBSYSTEM=="block", KERNEL=="md*", ACTION=="add|change", TEST=="md/stripe_cache_size", ATTR{md/stripe_cache_size}="32768"

#for Open Media Vault (OVM default 8MiB) edit /etc/udev/rules.d/99-openmediavault-md-raid-rules

SUBSYSTEM=="block", KERNEL=="md*", ACTION=="add|change", TEST=="md/stripe_cache_size", ATTR{md/stripe_cache_size}="8192"

5. Set read ahead, in 512 byte sectors

blockdev --getra /dev/md0

#set 32MB read ahead (runtime only)

blockdev --setra 65536 /dev/md0