SATA PT2

[William Harrington]

On Jun 6, 2007, at 3:28 PM, Phil Thayer wrote:

> I didn't think RAID 5 or RAID 6 kept all parity data on any one
> particular disk.  I thought it was spread across all the disks. In the
> case of RAID 6 I thought that the parity was stored on different disks
> each time but never both on the same disk.
>
> In any case, I think you both may be right.  They are using a  
> different
> parity algorithm for the second parity to be able to recover from a
> multiple disk failure which would cause the loss of multiple bits  
> of the
> data.  My bad on that one.
>
>> -----Original Message-----
>> From: kclug-bounces at kclug.org
>> [mailto:kclug-bounces at kclug.org] On Behalf Of Luke -Jr
>> Sent: Wednesday, June 06, 2007 12:39 PM
>> To: kclug at kclug.org
>> Subject: Re: SATA PT2
>>
>> On Wednesday 06 June 2007 09:47, Phil Thayer wrote:
>>> As for figuring out the second parity calculation on RAID
>> 6, what the
>>> manufacturers are realizing is that they don't necessarily
>> have to have
>>> a different parity algorithm to calculate the second parity.  Simply
>>> putting the same XOR parity data on two separate disks will
>> provide the
>>> same RAID 6 functionality as having a second parity calculation with
>>> lower overhead on controllers.  The old KISS methodology is
>> coming back
>>> into play.  I think you will see more and more of the manufacturers
>>> going this route.
>>
>> That only works if one of the dead disks is a parity disk...
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No, raid 5 + raid 6 would be of no use if one drive only kept the  
parity data. Not sure where you got your info. You need to seriously  
verify your information. RAID 3 has one dedicated disk for parity.  
Raid 5 has parity across all disks. That is when one disk fails  
during the rebuild process of a RAID5 array, it will fail. With  
RAID6, you get recovery when one drive fails, a second will still be  
able to recover the array. RAID 3 keeps parity info on one disk.  
Also, have at least 2 drives available for replacement, and have at  
least 2 backup targets available: via tape, another hard drive all  
equal to or great than the array. I am amazed at how many people  
don't understand the current RAID levels.

This is a decent source with creditable sources. http:// 
www.ecs.umass.edu/ece/koren/architecture/Raid/raidhome.html

A lot of people have no credible information about raid, and "I have  
experience" is not credible. No telling where they got their  
information. Using software RAID rather than hardware RAID will  
affect users' comments. There's a big difference. Now days you want  
to use PCI-X raid controllers.

If you want performance, don't use parity. If you want redundancy use  
RAID5+. You should seriously calculate how much your data is worth.  
If you wish to spend a lot, use both!
Sincerely,

William Harrington