RAID1 – How to Fail a Drive Marked as Removed

raidraid1Ubuntuubuntu-9.04

I have (had) a RAID 1 array (2 disk mirror) and one of the disks, sda, failed. So I've replaced the bad disk with a new one, but seem to be stuck on how to get the second drive back up and running as part of the array.

The system is running Ubuntu Server 9.04 and was configured as follows:

MD0 => sda1,sdb1

MD1 => sda3,sdb3

MD2 => sda2,sdb2

 mdadm --detail /dev/md0

shows two drives:

0 /dev/sdb1 "Active Sync"

1 [nothing] "Removed"

MD1 and MD2 look the same.

The tutorial I found says to mark each partition as failed using the command:

mdadm --manage /dev/md0 --fail /dev/sda1

But, since the drive is not there, I get:

mdadm: cannot find /dev/sda1: No such file or directory

Can I skip the failing step? Or is there some other way to fail a partition that's no longer present? Or if I copy the partition table from the good old drive to the new one, will it automatically pick up that it's the replacement?

I'm new at this and don't want to screw it up. 🙂

Best Answer

You shouldn't need to fail them. Since they should have already been failed when you first noticed the issue and the RAID members are now removed. There are just a few steps to get it back up and running.

Setup partitions on the replacement disk. These partitions should be identical in size to that of the failed and currently active disk, and should be marked as partition type "Linux RAID Autodetect" (0xFD). You can simplify this by copying the partition table with sfdisk.
```
sfdisk -d /dev/sdb | sfdisk /dev/sda
```
If the disk has been used before then you may want to ensure that any existing softRAID information is removed before you begin again.
```
mdadm --zero-superblock /dev/sda
```
Install an MBR onto the new disk so that it is bootable. Do this from the grub shell. Assumes that /dev/sda is the first disk.
```
root (hd0,0)
setup (hd0)
quit
```

Add new partitions back to the arrays.

mdadm --add /dev/md0 /dev/sda1
mdadm --add /dev/md1 /dev/sda3
mdadm --add /dev/md2 /dev/sda2

Monitor the status of their reconstruction by viewing /proc/mdstat. You can automate this with.
```
watch -n10 cat /proc/mdstat
```

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Yes, you need to start the array in degraded mode using sdc drive. Then, you can re-add the failed drive sdd.

I usually use the command mdadm with option --assemble to assemble and start the array in degraded mode using one disk only. Something like this:

$ mdadm --assemble /dev/md1 /dev/sda2

You may need to use -f option to force starting degraded array.

If that works fine, you can proceed by re-adding the replaced drive.

$ mdadm --re-add /dev/md1 /dev/sdb2

I used these commands many times before and did not experience any data loss. They worked smoothly when the drives are OK.

Precaution: Please backup your disks before running such commands to avoid any possible data loss.

Linux – mdadm raid 1 grub only on sda

This should be your problem

root (hd1,0)
 Filesystem type is ext2fs, partition type 0x83

Take the following steps:

Create the 2 /boot partitions on /dev/sda1 and /dev/sdb1 - type fd(Linux autodetect raid) - use your favorite tool(fdisk, cfdisk, gparted,...) (fd00 for GPT)
Remember to turn on the bootable flag on both partitions, sda1 and sdb1 (not for GPT)

Force the disks to be a brand new raid:

mdadm --zero-superblock /dev/sda1 
mdadm --zero-superblock /dev/sdb1

While creating the raid metadata that will be your /boot partition, use the version 0.9. Linux cannot autodetect newer versions (without a ramdisk).
```
mdadm --create /dev/md0 --level=1 --raid-disks=2 /dev/sda1 /dev/sdb1 --metadata=0.9
```
Format using ext2 or ext3
Install your Linux of choice, WITHOUT formating the /boot

After your distro first boot:

Fix your /etc/fstab to point /boot to /dev/md0(maybe it will not be necessary)

Install grub on the 2 disks MBR

# grub /dev/sda
 grub> root (hd0,0)
 grub> setup (hd0)
 grub> quit
 quit

# grub /dev/sdb
 grub> root (hd1,0)
 grub> setup (hd1)
 grub> quit
 quit

Edit your bootloader(instructions to Grub1)
Search the "default" line and add the "fallback" option bellow
```
vi /boot/grub/menu.lst
default 0
fallback 1
```

Add another entry to your bootloader(again, in my case i've choosen grub1 since its less complicated and it's good enough to my needs), one of each pointing to the different boot partitions that are members of the raid:

title           Debian GNU/Linux, kernel 2.6.32-5-686  (default)
root            (hd0,0)
kernel          /vmlinuz-2.6.32-5-686 root=/dev/mapper/vg-root ro quiet
initrd          /initrd.img-2.6.32-5-686

title           Debian GNU/Linux, kernel 2.6.32-5-686  (fallback)
root            (hd1,0)
kernel          /vmlinuz-2.6.32-5-686 root=/dev/mapper/vg-root ro quiet
initrd          /initrd.img-2.6.32-5-686

Note that in my case, i have a LVM layer on my / md raid.

Done. This should be enough to you to have a "redundant" bootloader.

Best Answer

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