Comments on: RAID 5 Is A Cruel Mistress

By: Ken Reiss

Ken Reiss — Tue, 26 Feb 2008 04:28:27 +0000

I just ordered a Dell 2950 III with 5x 250 GB SATA in RAID 6. Has anyone else done the same?

Any thoughts re: the performance degradation of RAID 6 vs. RAID 5?

Ken.

By: jimmy

jimmy — Mon, 03 Dec 2007 12:10:48 +0000

The new Dell PERC 6 controller can deal with RAID 6. The PERC 6 is shipping with 3rd gen 29** series

By: Tom

Tom — Fri, 28 Sep 2007 03:13:55 +0000

We mitigate the risk on our critical systems using RAID 1 for the OS volume and RAID 5 for the other volumes. Each volume has a hot spare. Our SQL stuff is clustered with the databases/logs on a SAN which adds yet another layer of protection. The less critical stuff just uses RAID 1 or RAID 5 and we take our chances. In practice, it has worked well for us. We have survived two multi-drive failures this year alone. Of course, we are a smaller IT shop so we limit this strategy to systems that we define as “critical”. That is, systems that are directly responsible for taking money in and getting product out the door.

Another thing, HP’s servers will generate a predictive drive failure warning and they will do a warranty replacement before the drive fails. This is has save by butt many, many times.

By: aharden

aharden — Thu, 27 Sep 2007 17:14:20 +0000

Looks like Dell doesn’t offer RAID 6 on either the PERC 5/i or the PERC 5/E. This is a reason to consider HP. I’ve never been a big fan of Dell’s storage solutions. They sometimes change OEMs (and as a result management tools and array configuration data) between generations, which kills consistency and increases data-loss risk on older arrays as older-generation support dries up.

We’ve deployed hundreds of servers (Dell and HP) and have found that multi-disk communications errors are much more likely to occur on external storage units than the built-in hot-plug storage bays of the tier 1 servers.

By: Bob Plankers

Bob Plankers — Thu, 27 Sep 2007 04:05:47 +0000

Excellent point. But how do you deal with that? Order another server at the same time, which will have drives from the same batch? Wait for two months until you get another batch?

To me it sounds like a mess. Maybe the way out of this is to keep as little data as possible… (pipe dream, I know). :-)

By: Tom

Tom — Thu, 27 Sep 2007 03:55:12 +0000

Another factor to consider is that many times the drives in these arrays are deployed at the same time and happen to be from the same manufacturing batch. An imperfection in a drive due to a manufacturing anomaly would likely apply to the entire batch and cause them to fail after similar usage.

By: wjl (Wolfgang Lonien)

wjl (Wolfgang Lonien) — Tue, 25 Sep 2007 22:33:17 +0000

Distributed data is definitely the way to go. They do this for instance in hospitals I know, just in case that one building would burn down. In my last job with $ex_employer, we always sold two identical servers, configured as a HA cluster.

Of course, you need Gigabit Ethernet or even Fibre if you deal with lots of data.

cheers,
wjl

By: Bob Plankers

Bob Plankers — Tue, 25 Sep 2007 21:09:02 +0000

That’s a great point, and one that I didn’t really consider. The problem is that once the controller marks a drive as dead it’s pretty much “game over.” A communications error also seems to be an argument for reducing the number of disks. The more there are the more chances of one of them causing a problem.

RAID 6 isn’t an option on the Dell PERC5/i, which is the most common. Maybe the follow-on controllers in upcoming revisions of Dell servers will support RAID 6.

By: aharden

aharden — Tue, 25 Sep 2007 20:19:13 +0000

Some configurations support RAID 6, which uses two parity drives. However, most of the double-drive failures I’ve seen involve failed communications between the RAID controller and the disks as opposed to two authentic drive failures occurring within a short window.

One thing I always recommend is a two-drive RAID 1 array dedicated to the OS and possibly applications. Data should be housed on a separate array and everything should be backed up to another location.

On the 10k vs 15k issue, I use a mix of both in both the 2.5″ and 3.5″ form factors and haven’t really seen a higher failure rate with 15k disks.