I recently came across some videos for 2010's linux.conf.au presentations. I haven't watched all of them however a couple of them look interesting. One that I have watched is Going mad with MDADM. I learnt a few things from this presentation about software raid, most of which are covered in the md.txt file distributed with the kernel tree. The most useful piece of info was how to request a raid array check:
[root@mercury ~]# echo check > /sys/block/md1/md/sync_action
You can then check the status of the check via /proc/mdstat as normal:
[mike@mercury|~] $ cat /proc/mdstat
Personalities : [raid1] [raid10]
md1 : active raid10 sda3[0] sdd3[3] sdc3[2] sdb3[1]
484231040 blocks 64K chunks 2 near-copies [4/4] [UUUU]
[=>...................] check = 6.3% (30890816/484231040) finish=73.7min...
md0 : active raid1 sda1[0] sdd1[3] sdc1[2] sdb1[1]
128384 blocks [4/4] [UUUU]
unused devices: <none>
Checking a modestly sized array can take some time. In the case of a large array and a RAID level that uses parity such as 5 or 6 it is reported that it can take a very long time to do a check. Resyncs are even worse as they involve writing as well as reading.
Check the log file for the status of the completed check:
[mike@mercury|~/data/scans] $ grep 'kernel: md' /var/log/messages.log | tail -n 5
Apr 10 16:27:58 mercury kernel: md: data-check of RAID array md1
Apr 10 16:27:58 mercury kernel: md: minimum _guaranteed_ speed: 1000 KB/sec/disk.
Apr 10 16:27:58 mercury kernel: md: using maximum available idle IO bandwidth (but not more than 200000 KB/sec) for data-check.
Apr 10 16:27:58 mercury kernel: md: using 128k window, over a total of 484231040 blocks.
Apr 10 18:07:10 mercury kernel: md: md1: data-check done.