JP Vossen on 11 Apr 2013 15:40:49 -0700

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[PLUG] Speaking of Linux RAID1

I just hot-swapped a bad disk on an old PE860 server running CentOS-5.9 and using Linux software RAID1.

I've been getting SMART errors for a couple of weeks so I RMA'd the drive with Western Digital (painless). They 2nd day aired me a replacement, though I have mixed feelings about it. The bad drive is a 1TB Green drive, the new one is a 2TB non-Green. Since it's a RAID1 I can't use the extra TB and in this case the non-Green part doesn't matter. But depending on use-case I may really prefer the Green drives, and if I RMA one I'd really prefer a same-for-same replacement. :-/

I have a script to do the dirty work but it's basically:
	mdadm --manage /dev/md0 --fail /dev/sda1    # /boot
	mdadm --manage /dev/md1 --fail /dev/sda2    # /
	mdadm --manage /dev/md0 --remove /dev/sda1
	mdadm --manage /dev/md1 --remove /dev/sda2

### HOT SWAP SATA drive while the system is still up and running! ###

	/sbin/sfdisk -d /dev/sdb | /sbin/sfdisk /dev/sda
	mdadm --manage /dev/md0 --add /dev/sda1
	mdadm --manage /dev/md1 --add /dev/sda2
	/sbin/grub-install /dev/sda
	/sbin/grub-install /dev/sdb	# Just in case

DO NOT MESS THE ABOVE UP!!! If you get any part of it wrong you will nuke your data! That's why I have a good backup and a script to do it, because what "it" is changes depending on which drive is bad and what partitions it's using for what.

Also note the sfdisk trick of reading the partitions of the good disk and writing to the new one automatically. I forget where I stole that from, but it's awesome.

Other handy stuff:
	## Keep an eye on re-sync progress
	cat /proc/mdstat
	watch -d cat /proc/mdstat

	## Wait for (and time) the re-sync to finish
	time mdadm --wait /dev/md1

If/when the other drive goes and I get a second 2TB replacement, I can swap it in as above, then resize everything using 'fdisk' and the LVM tools then 'resize2fs'. I've already done that once, as this server and OS install started out on 500G. Pretty cool.

None of the above discusses encryption, though that drive *is* encrypted. The encryption (LUKS) is at a different layer and Just Works. And it's nice that I don't have to care about wiping the RMA'd drive!

Also, these mirror drives will Just Work, no matter what. I don't have to care about motherboard RAID support, RAID controllers, or anything else. Even if I split them up (e.g. to clone the server) it all Just Works.

JP Vossen, CISSP            |:::======|
My Account, My Opinions     |=========|
"Microsoft Tax" = the additional hardware & yearly fees for the add-on
software required to protect Windows from its own poorly designed and
implemented self, while the overhead incidentally flattens Moore's Law.
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