mdadm: Linux software RAID

Installation

$ apt-get install mdadm

Usage

Check software RAID array status

Use the following command:

$ cat /proc/mdstat

or

$ mdadm -D /dev/mdX

Output in case of non-existant RAID device:

$ mdadm -D /dev/md0
Personalities :
unused devices: <none>

Output for RAID6 device:

$ mdadm -D /dev/md0
Personalities : [raid6] [raid5] [raid4] [linear] [multipath] [raid0] [raid1] [raid10]
md0 : active raid6 sdc[2] sda[0] sdd[3] sdb[1] sde[4]
      35156259840 blocks super 1.2 level 6, 512k chunk, algorithm 2 [5/5] [UUUUU]

unused devices: <none>

A progress bar is displayed under partitions when a RAID resync process is currently running:

$ mdadm -D /dev/md0
Personalities : [raid6] [raid5] [raid4] [linear] [multipath] [raid0] [raid1] [raid10]
md0 : active raid6 sde[4] sdb[1] sdd[3] sda[0] sdc[2]
      35156259840 blocks super 1.2 level 6, 512k chunk, algorithm 2 [5/5] [UUUUU]
      [=======>.............]  resync = 35.0% (4101838624/11718753280) finish=1179.1min speed=107662K/sec

unused devices: <none>

Speed up software RAID resync

Increase speed limits

Check current speed:

$ sysctl dev.raid.speed_limit_min
$ sysctl dev.raid.speed_limit_max

These values are set in Kibibytes per second (KiB/s).

Increase target rebuild speed:

$ echo 50000 > /proc/sys/dev/raid/speed_limit_min
$ echo 200000 > /proc/sys/dev/raid/speed_limit_max

or

$ sysctl -w dev.raid.speed_limit_min=50000
$ sysctl -w dev.raid.speed_limit_max=200000

Permanently override default values in /etc/sysctl.conf:

$ dev.raid.speed_limit_min = 50000
$ dev.raid.speed_limit_max = 200000

Disable NCQ on all disks

for i in sd[abcde]
do
    echo 1 > /sys/block/$i/device/queue_depth
done

Set read-ahead option

Get the current read-ahead value:

$ blockdev --getra /dev/mdX

Set read-ahead (in 512-byte sectors) per RAID device.

$ blockdev --setra 65536 /dev/mdX

Increase stripe cache size

Records the size (in pages per device) of the stripe cache which is used for synchronising all write operations to the array and all read operations if the array is degraded. By default, the size of the stripe cache is 256 pages and Linux uses 4096B pages. If you use 256 pages for the stripe cache and you have 10 disks, the cache would use 102564096=10MiB of RAM. Only available on RAID5 and RAID6 and boost sync performance by 3-6 times.

$ echo 4096 > /sys/block/mdX/md/stripe_cache_size