[Sussex] Recovering from RAID 5

[Steve Dobson]

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Hi John

I'm afraid I don't have a recovery solution for you.  I read this
article [1] a year or two ago and was convinced by it.  I will admit
that I haven't checked the maths behind it, but the principles do appear
sound to me.  As disk capacity increases for a given format then they
must be packing the tracks closer and closer together.  I can't see this
improving the read failure rate.  Couple this with the requirements that
RAID 5 requires for recovery and complete RAID 5 failure during rebuild
becomes a significant increasing probability as disk capacity increases.

I have a very simple backup policy:  I backup to separate disk only that
which is important to me, really important stuff gets backed up off
site.  This is because I consider backup to be an irrelevant task.

That's right, irrelevant!

Recovery is important.  And I have found when you think about system
recovery rather than system backup you come to very different answers to
the same questions.  Here is a sample:

1). Should I backup the whole system?

Backup Centric: Yes:  If you can backup everything every night why not?
It makes recovery easier - it's all there on the one tape.

Recovery Centric:  No!  Reading from tapes is sloooooooooow!  If you
have incrementals it's even worse.  If it takes me less time to install
and configure a system from scratch than it does to recover just the OS
from backup then why bother backing up the OS?

2). Do I backup DMZ systems?

Backup Centric: Of course, I backup everything.

Recovery Centric: Never.  Why?  Viruses, worms and crackers.  If a DMZ
machine is suspect then which backup do I recover from?  When did the
machine become infected or compromised?  How do I know I am restoring a
proper snapshot of the system?  Best option is to install from a known
good source.

Sure, backup the /etc directories so you can quickly re-configure a
replacements, but DMZ systems are there to sacrifice themselves in order
to protected the really important stuff (which is not stored in the DMZ).

3). What should I backup.

Backup Centric: Everything!

Recovery Centric: Only that which is important and I can't recover any
other way.  For example, why backup to tape your music, that's already
backed up on your CDs.  (Downloads are a different kettle of fish).

If you really need the data fast then have a clone of the data on a
second server (rsync over night or more frequently if you wish) and you
can quickly swap the backup in to replace the dead server should
continuance be that important.

Well, that's my two pennies worth.

Steve

[1]

http://www.zdnet.com/blog/storage/why-raid-5-stops-working-in-2009/162
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