Raid 5 versus 3 computer backup

[blu at vl.com (Tom Metro)]

Seth Gordon wrote:
> (1) I think RAID 5 (and up) is really only worth doing if (a) every hour
> of downtime costs you real money or (b) you are exercising the drives
> heavily enough that you can *expect* one of them to fail every few months.

Another advantage to RAID, as Rich Braun has pointed out in previous 
postings, is that it can help ease the effort to expand the capacity of 
a storage system. As he explained, you can do a rolling upgrade. Hot 
swapping each disk with a larger disk over time, and periodically 
growing the file system.

At minimum this saves you the need to create a whole new parallel file 
system and copying everything to it each time you upgrade capacity. 
Although that much you can do just using LVM without RAID. If you do 
everything just right, RAID permits you to do capacity upgrades without 
disrupting a running system, and I think with less effort than using LVM 
alone, while providing some added reliability benefits.

However, note that RAID5 isn't currently a good choice for this, as you 
can't grow a RAID5 set using mdadm. The documentation suggests you can, 
but I tried it, and it didn't work. Seems others have the same problem. 
Supposedly it works fine for RAID1. And eventually the problem will 
probably get fixed.


> (2) Whether or not you have RAID, you need backups.  (I speak from
> experience....)

Right. RAID is a strategy for increasing uptime, not securing data.

I use rsync mirroring among multiple machines for data backup. And one 
of these days, I'll get around to setting up off-site mirroring.

  -Tom

-- 
Tom Metro
Venture Logic, Newton, MA, USA
"Enterprise solutions through open source."
Professional Profile: http://tmetro.venturelogic.com/

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