How to fsck a mounted raid device /dev/md2 ?

Hi,

I would like to request an assistant.

It seems that my /var/log/messages having the following error message :

Jan  1 11:24:26 srv9 kernel: raid1: sdb: unrecoverable I/O read error for block 286895360

My cat /proc/mdstat shows as follows :

root@srv9 [~]# cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sdb1[1] sda1[0]
      104320 blocks [2/2] [UU]

md1 : active raid1 sdb2[1] sda2[0]
      2096384 blocks [2/2] [UU]

md2 : active raid1 sda3[2](S) sdb3[1]
      310367680 blocks [2/1] [_U]


My mdadm --detail /dev/md2 shows as follows :

root@srv9 [/]# mdadm --detail /dev/md2
/dev/md2:
        Version : 0.90
  Creation Time : Fri May 30 17:11:49 2008
     Raid Level : raid1
     Array Size : 310367680 (295.99 GiB 317.82 GB)
  Used Dev Size : 310367680 (295.99 GiB 317.82 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 2
    Persistence : Superblock is persistent

    Update Time : Tue Jan  1 23:24:58 2013
          State : clean, degraded
 Active Devices : 1
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 1

           UUID : 273f6c1b:0e438923:11f1305b:25e0df37
         Events : 0.9388326

    Number   Major   Minor   RaidDevice State
       0       0        0        0      removed
       1       8       19        1      active sync   /dev/sdb3

       2       8        3        -      spare   /dev/sda3



SUMMARY
=========

1. /var/log/messages shows sdb which having the error.
2. /proc/mdstat shows sda3[2](S) device which is degraded and becoming spare.
3. /dev/sdb3 is my actual /dev/md2 current active mounted partition .

I believe my /dev/sdb3 is having filesystem error that need to be fixed but it is actually current active raid partition.

May i know the simplest way to fsck the drive ? If possible without boot it using linux rescue disk ?

I found the following article :

http://wiki.sabayon.org/index.php?title=HOWTO:_Repair_filesystem_using_fsck_on_a_raid_setup

But in the article does not mentioned that i need to unmounted the /dev/md2 device first , afaik the fsck must be run on unmounted device to avoid data corruption.

FYI, the /dev/sdb3 is still active mounted as /dev/md2 .

Should i fsck /dev/sda3 or /dev/sdb3 ? and how ?


Appreciates anybody assistant on this.

Thank you.