On Sat, 25 Mar 2017 15:17:11 -0400
Hendrik Boom <hend...@topoi.pooq.com> wrote:
> If I were to manage to reconnect the absent drive, how would the
> boot-time RAID assembly work? (/boot is on the RAID). Would it be
> able to figure out which of the two drives is up-to-date, and
> therefore which one to consider defective and not use?
>
> Do I need to wipe the missing drive completely before I connect it?
> (I have another machine to do this on, using a USB-to-SATA interface).
Picked up from somewhere, and used a couple times already. Long !
Cheers,
Ron.
--
One of the sanest, surest and most generous joys of life
comes from being happy over the good fortune of others.
-- Robert A. Heinlein
-- http://www.olgiati-in-paraguay.org --
How To Replace The HDD in a RAID 1 Array
1 Preliminary Note
In this example I have two hard drives, /dev/sda and /dev/sdb,
with the partitions /dev/sda1 and /dev/sda2 as well as /dev/sdb1 and /dev/sdb2.
/dev/sda1 and /dev/sdb1 make up the RAID1 array /dev/md0.
/dev/sda2 and /dev/sdb2 make up the RAID1 array /dev/md1.
/dev/sda1 + /dev/sdb1 = /dev/md0
/dev/sda2 + /dev/sdb2 = /dev/md1
/dev/sdb has failed, and we want to replace it.
2 How Do I Tell If A Hard Disk Has Failed?
If a disk has failed, you will probably find a lot of error messages in the log
files, e.g. /var/log/messages or /var/log/syslog.
You can also run
cat /proc/mdstat
and instead of the string [UU] you will see [U_] if you have a degraded RAID1
array.
3 Removing The Failed Disk
To remove /dev/sdb, we will mark /dev/sdb1 and /dev/sdb2 as failed and remove
them from their respective RAID arrays (/dev/md0 and /dev/md1).
First we mark /dev/sdb1 as failed:
mdadm --manage /dev/md0 --fail /dev/sdb1
The output of
cat /proc/mdstat
should look like this:
server1:~# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid5] [raid4] [raid6]
[raid10]
md0 : active raid1 sda1[0] sdb1[2](F)
24418688 blocks [2/1] [U_]
md1 : active raid1 sda2[0] sdb2[1]
24418688 blocks [2/2] [UU]
unused devices: <none>
Then we remove /dev/sdb1 from /dev/md0:
mdadm --manage /dev/md0 --remove /dev/sdb1
The output should be like this:
server1:~# mdadm --manage /dev/md0 --remove /dev/sdb1
mdadm: hot removed /dev/sdb1
And
cat /proc/mdstat
should show this:
server1:~# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid5] [raid4] [raid6]
[raid10]
md0 : active raid1 sda1[0]
24418688 blocks [2/1] [U_]
md1 : active raid1 sda2[0] sdb2[1]
24418688 blocks [2/2] [UU]
unused devices: <none>
Now we do the same steps again for /dev/sdb2 (which is part of /dev/md1):
Then power down the system:
shutdown -h now
and replace the old /dev/sdb hard drive with a new one (it must have at least
the same size as the old one
if it's only a few MB smaller than the old one then rebuilding the arrays will
fail).
4 Adding The New Hard Disk
After you have changed the hard disk /dev/sdb, boot the system.
The first thing we must do now is to create the exact same partitioning as on
/dev/sda.
We can do this with the command sgdisk from the gdisk package.
If you havent installed gdisk yet, run this command to install it on Debian and
Ubuntu:
apt-get install gdisk
The next step is optional but recomended. To ensure that you have a backup of
the partition scheme,
you can use sgdisk to write the partition schemes of both disks into a file. I
will store the backup in the /root folder.
sgdisk --backup=/root/sda.partitiontable /dev/sda
sgdisk --backup=/root/sdb.partitiontable /dev/sdb
In case of a failure you can restore the partition tables with the
--load-backup option of the sgdisk command.
Now copy the partition scheme from /dev/sda to /dev/sdb run:
sgdisk -R /dev/sdb /dev/sda
afterwards, you have to randomize the GUID on the new hard disk to ensure that
they are unique
sgdisk -G /dev/sdb
You can run
sgdisk -p /dev/sda
sgdisk -p /dev/sdb
to check if both hard drives have the same partitioning now.
Next we add /dev/sdb1 to /dev/md0 and /dev/sdb2 to /dev/md1:
mdadm --manage /dev/md0 --add /dev/sdb1
server1:~# mdadm --manage /dev/md0 --add /dev/sdb1
mdadm: re-added /dev/sdb1
mdadm --manage /dev/md1 --add /dev/sdb2
server1:~# mdadm --manage /dev/md1 --add /dev/sdb2
mdadm: re-added /dev/sdb2
Now both arays (/dev/md0 and /dev/md1) will be synchronized. Run
cat /proc/mdstat
to see when it's finished.
During the synchronization the output will look like this:
cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid5] [raid4] [raid6]
[raid10]
md0 : active raid1 sda1[0] sdb1[1]
24418688 blocks [2/1] [U_]
[=>...................] recovery = 9.9% (2423168/24418688)
finish=2.8min speed=127535K/sec
md1 : active raid1 sda2[0] sdb2[1]
24418688 blocks [2/1] [U_]
[=>...................] recovery = 6.4% (1572096/24418688)
finish=1.9min speed=196512K/sec
unused devices: <none>
When the synchronization is finished, the output will look like this:
cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid5] [raid4] [raid6]
[raid10]
md0 : active raid1 sda1[0] sdb1[1]
24418688 blocks [2/2] [UU]
md1 : active raid1 sda2[0] sdb2[1]
24418688 blocks [2/2] [UU]
unused devices: <none>
That's it, you have successfully replaced /dev/sdb!
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