[TriLUG] New system configuration thoughts

Thu May 3 10:43:00 EDT 2007

RAID 6 is RAID 5 with an extra parity disk so you can actually lose two entire disks from an array without data loss. I would still recommend a hot spare for a RAID 6 array as well to allow the rebuild process to begin immediately. There is more overhead, of course with RAID 6, as two entire disks have to be written out, in parity, across the array; but it prevents the possibility of losing the entire array from a two-disk loss. I'm not sure how often this actually occurs, but if I had a lot of very important data on a RAID 5 array, I'd definitely keep it backed up on disk or tape at all times just to be sure. Even with a hot spare in a RAID 5 array, there is a 12-24 hour (depending on the size of the disks) window where you could lose the entire array if another drive drops out during the rebuild process. 

To get RAID 6, you need to have a RAID 6 capable Host Bus Adapter (HBA). I have set them up on HP DL3xx servers that come with a built-in HBA and also on Adaptec SATA HBA's. Most newer RAID cards have RAID 6 capability. SCSI drives are definitely the way to go for enterprise-class storage (particularly with high I/O activity such as databases) as the MTBF (mean time between failure) rate is MUCH better with SCSI than (S)ATA drives (approx. 3 years for SCSI vs. 1 year for SATA on average I think.) In other words, plan on buying three SATA drives for every one U320 SCSI drive. Also, be very careful about the SATA drives you purchase if putting them in a RAID array as there is a very big difference between desktop-grade and enterprise (RAID-level)-grade SATA drives. Unfortunately, the vendors probably don't know and manufacturers don't make this point clear, but SATA is reasonably new to the enterprise and you can get stuck with really crappy drives if you buy the wrong kind. 

Until you do actual benchmarking with a particular drive and card, it's difficult to know what you're real throughput will be. You also need a fast processor and bus on the motherboard or the money spent on faster drives and HBA may be wasted. It's difficult to make recommendations when it comes to RAID as there are so many different scenarios. If you were running a large database you'd want to set up your array in a very different manner from setting up a file server, etc. Just do your research before putting anything in to production.

Kevin

----- Original Message ----
From: Jim Ray <jim at neuse.net>
To: Triangle Linux Users Group discussion list <trilug at trilug.org>
Sent: Thursday, May 3, 2007 5:30:22 AM
Subject: RE: [TriLUG] New system configuration thoughts

I RAID6 the same as RAID5 plus a hot spare?  What is your favorite flavor of
RAID card and hard disk?

I'm partial to those serial attached SCSI drives with the 15,000 rpm speed.

Regards,

Jim

Jim Ray, President
Neuse River Networks
tel: 919-838-1672 cell: 919-606-1772
http://www.Neuse.Net

Connecting You to the World since 1997

> -----Original Message-----
> From: trilug-bounces at trilug.org [mailto:trilug-bounces at trilug.org] On
Behalf Of Kevin
> Jones
> Sent: Wednesday, May 02, 2007 10:54 PM
> To: Triangle Linux Users Group discussion list
> Subject: Re: [TriLUG] New system configuration thoughts
> 
> I'll attest to that as it happened to me. Since that event I have never
trusted RAID 5 again
> and have opted for RAID 6 whenever possible. Of course, if you can't do
RAID 6 always,
> always have a hot spare online so the rebuild can begin immediately.
> 
> Kevin
> 
> ----- Original Message ----
> From: Brian Henning <Brian.Henning at datadirect.com>
> To: Triangle Linux Users Group discussion list <trilug at trilug.org>
> Sent: Wednesday, May 2, 2007 7:53:23 PM
> Subject: RE: [TriLUG] New system configuration thoughts
> 
> Kevin Flanagan supposed:
> 
> > How is RAID mirroring not redundancy?  It's redundancy where you don't
> have
> > a system outage in order to replace a disk that has failed.  Odds are
> in
> > your favor that you won't have 2 failures at the same time.  Mirror
> first,
> > _then_ you replicate the data to another system.
> 
> Ever heard the suggestion about always replacing your headlights in
> pairs?  The odds of having two failures (of equally-aged,
> equally-stressed hardware, as in a RAID mirroring setup) at the same
> time are fairly high.
> 
> ~B
> 
> 
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