(Sorry for the mixup in sending this by direct email instead of posting
to the list.)
On 17/02/11 00:32, Stan Hoeppner wrote:
David Brown put forth on 2/15/2011 7:39 AM:
This brings up an important point - no matter what sort of system you get (home
made, mdadm raid, or whatever) you will want to do some tests and drills at
replacing failed drives. Also make sure everything is well documented, and well
labelled. When mdadm sends you an email telling you drive sdx has failed, you
want to be /very/ sure you know which drive is sdx before you take it out!
This is one of the many reasons I recommended an enterprise class vendor
solution. The Nexsan unit can be configured for SMTP and/or SNMP and/or pager
notification. When a drive is taken offline the drive slot is identified in the
GUI. Additionally, the backplane board has power and activity LEDs next to each
drive. When you slide the chassis out of the rack (while still fully
operating), and pull the cover, you will see a distinct blink pattern of the
LEDs next to the failed drive. This is fully described in the documentation,
but even without reading such it'll be crystal clear which drive is down. There
is zero guess work.
The drive replacement testing scenario you describe is unnecessary with the
Nexsan products as well as any enterprise disk array.
I'd still like to do a test - you don't want to be surprised at the
wrong moment. The test lets you know everything is working fine, and
gives you a feel of how long it will take, and how easy or difficult it is.
But I agree there is a lot of benefit in the sort of clear indications
of problems that you get with that sort of hardware rather a home made
system.
You also want to consider your raid setup carefully. RAID 10 has been mentioned
here several times - it is often a good choice, but not necessarily. RAID 10
gives you fast recovery, and can at best survive a loss of half your disks - but
at worst a loss of two disks will bring down the whole set. It is also very
inefficient in space. If you use SSDs, it may not be worth double the price to
have RAID 10. If you use hard disks, it may not be sufficient safety.
RAID level space/cost efficiency from a TCO standpoint is largely irrelevant
today due to the low price of mech drives. Using the SATABeast as an example,
the cost per TB of a 20TB RAID 10 is roughly $1600/TB and a 20TB RAID 6 is about
$1200/TB. Given all the advantages of RAID 10 over RAID 6 the 33% premium is
more than worth it.
>
I don't think it is fair to give general rules like that. In this
particular case, that might be how the sums work out. But in other
cases, using RAID 10 instead of RAID 6 might mean stepping up in chassis
or controller size and costs. Also remember that RAID 10 is not better
than RAID 6 in every way - a RAID 6 array will survive any two failed
drives, while with RAID 10 an unlucky pairing of failed drives will
bring down the whole raid. Different applications require different
balances here.
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