Sometimes when assembling a raid array, you may find that it is going to take hours, even worse, days to complete. Luckily there are ways to increase the assemble time with some useful commands which should decrease the time considerably.
To increase the rebuild speed, enter:
echo value > /proc/sys/dev/raid/speed_limit_min
OR
sysctl -w dev.raid.speed_limit_min=value
In this example, set it to 60000 K/Sec, enter:
echo 60000 > /proc/sys/dev/raid/speed_limit_min
OR
sysctl -w dev.raid.speed_limit_min=60000
If you want to override the default settings you could add these two lines to /etc/sysctl.conf:
dev.raid.speed_limit_min = 60000
dev.raid.speed_limit_max = 200000
Bitmap Option
Bitmaps optimize rebuild time after a crash, or after removing and re-adding a device.
Turn it on by typing the following command:
mdadm –grow –bitmap=internal /dev/md0
Once array rebuild or fully synced, disable bitmaps:
mdadm –grow –bitmap=none /dev/md0
The net result
The rebuild rate jumps from 4k to 61k:
cat /proc/mdstat
Sample outputs:
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] [multipath]
md5 : active raid1 sde2[2](S) sdd2[3](S) sdc2[4](S) sdb2[1] sda2[0]
530048 blocks [2/2] [UU]
md0 : active raid6 sde3[4] sdd3[3] sdc3[2] sdb3[1] sda3[0]
5855836800 blocks level 6, 64k chunk, algorithm 2 [5/5] [UUUUU]
[============>........] resync = 61.7% (1205475036/1951945600) finish=242.9min speed=61204K/sec
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