SATA controllers

[Liam Proven]

On Sat, Mar 13, 2010 at 2:37 PM, Tom H <tomh0665 at gmail.com> wrote:
>>> I'm speccing a new machine and as part of it I'd like to have a
>>> linux-controlled 4-disk RAID 6 array using SATA 3 Gbps disks (aka STA 2).
>
>> You want more than 4 disks for RAID6. Seriously.
>
> A valid point! I had COMPLETELY missed that... :)

:¬)

>> RAID6 uses 2 parity drives; this means you get the capacity of (N-2)
>> drives, where N is the number of drives. Ergo, use 4 drives, you only
>> get the capacity of 2. This is pointless, because if you lose half the
>> capacity, you would get /much/ better performance from RAID10 (a
>> mirror of stripes) or RAID 0+1 (a stripe set of mirror pairs).
>> (I may have got the definitions of 0+1 and 10 transposed, but it's not
>> really important at this point!)
>
> You are right and I am 99% sure that you have the definitions
> transposed; but I would have to look it up to be 100% sure because I
> have the same problem with these 1+0 and 0+1 things.

In the past, I must admit, I had never paid much attention to RAID
0+1/10, because it seemed to me to be squandering space for
performance. However, disks today are getting very big, very cheaply -
they have long ago outstripped code or program data size and are now
really just keeping a bit ahead of the curve of the space used by
digitally-encoded hi-def video. Which I don't use, so I no longer
care! :¬)

The thing is, and it is seldom appreciated, that as disks have been
growing bigger, faster than Moore's Law, I believe, the number of
errors per drive is rising too. Drives have been getting steadily less
& less reliable since the mid-1990s, in my (quite considerable)
experience. Certainly it's not improving as fast as capacities; one
example of this is the price disparity between enterprise-class
(typically SAS) and consumer-class (SATA) drives. It took years for
enterprise drives to pass 35/70GB when home users could get units 10×
that capacity.

Indeed some industry people are now starting to say that the RAID5/6
distributed-parity concept is reaching the end of the line:
http://www.enterprisestorageforum.com/article.php/3839636

But given that you can buy a terabyte of disk for like £100 or
something now & 1.5-2TB drives are quite affordable, suddenly, RAID
0+1/10 seems to make rather more sense!

The question that bothers me, though, is what the hell do you back up
a 4TB array onto? That's why I'm not touching them yet, personally. My
biggest RAID is ~400GB and that is still a ton of room for me and my
stuff, such as my *complete* personal email archive going back to
1994, which still fits comfortably on a £10 USB thumbdrive.

-- 
Liam Proven • Profile: http://www.linkedin.com/in/liamproven
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