RAID-6 support in kernel?

hi all

I'n working on server setup with some 16 disks in RAID-5; one of them a 
spare. After a little reading, I find myself longing for support for 
RAID-6 support in kernel, giving the opportunity to allow for two failed 
drives without a chrash (see links about RAID-6 below if interested).

I am aware of that not all kernel hackers like such configurations, and
that some will rather see small RAID-configurations connected with VLM.  
I beleive there is a reason for using RAID-6, and RAID-controller vendors
(such as Compaq) are already using them, so why shouldn't linux do so
also? With a high number of cheap IDE drives, the chance of one failing is 
quite high, so why not RAID-6? At least for a system doing most reads...

thanks

roy

RAID-6 layout: http://www.acnc.com/04_01_06.html

-- 
Roy Sigurd Karlsbakk, Datavaktmester

Computers are like air conditioners.
They stop working when you open Windows.

On Mon, 3 Jun 2002, Roy Sigurd Karlsbakk wrote:

I am aware of that not all kernel hackers like such configurations, and
that some will rather see small RAID-configurations connected with VLM.  
I beleive there is a reason for using RAID-6, and RAID-controller vendors
(such as Compaq) are already using them, so why shouldn't linux do so
also? With a high number of cheap IDE drives, the chance of one failing is 
quite high, so why not RAID-6? At least for a system doing most reads...

See the following thread from March 2002 on linux-raid:
http://groups.google.com/groups?hl=en&lr=&safe=off&th=804941541a023c63&seekm=linux.raid.Pine.LNX.4.44.0203261239110.12942-100000

You can always fake this effect by combining two 8-disk RAID-5s into a
RAID-0. It's not technically RAID-6, but can withstand a 2-disk failure,
although not _any_ 2-disk failure. However, it's my understanding that
RAID-6 cannot withstand _any_ two disk failure either (see the above
thread). 

I also suspect that the use of dual RAID-5s combined with the CPU overhead
of ATA will kill most systems under any kind of load. For that matter, the
2x parity hit from RAID-6 probably wouldn't make you CPU too happy either,
even if there was a kernel driver that implemented it.

---
Derek Vadala, derek@cynicism.com, http://www.cynicism.com/~derek

You can always fake this effect by combining two 8-disk RAID-5s into a
RAID-0. It's not technically RAID-6, but can withstand a 2-disk failure,
although not _any_ 2-disk failure. However, it's my understanding that
RAID-6 cannot withstand _any_ two disk failure either (see the above
thread). 

It'll waste 9 drives, giving me a total capacity of 7n instead of 14n. 
And, by definition, RAID-6 _can_ withstand _any_ two-drive failure.

I also suspect that the use of dual RAID-5s combined with the CPU overhead
of ATA will kill most systems under any kind of load. For that matter, the
2x parity hit from RAID-6 probably wouldn't make you CPU too happy either,
even if there was a kernel driver that implemented it.

With a 1500MHz Athlon on a typical file server where there's not much 
writes, the CPU is sitting there chrunching RC5-64 som 99,95 % of the 
time. I don't think it'll make much differnce with today's CPUs

roy

-- 
Roy Sigurd Karlsbakk, Datavaktmester

Computers are like air conditioners.
They stop working when you open Windows.

On Mon, 3 Jun 2002, Roy Sigurd Karlsbakk wrote:

It'll waste 9 drives, giving me a total capacity of 7n instead of 14n. 
And, by definition, RAID-6 _can_ withstand _any_ two-drive failure.

This is certainly not true. 

Combining N RAID-5 into a stripe wastes on N disks. 

If you combine two it wastes 2 disks, etc.

That is, for each RAID-5 you waste a single disk worth of storage for
partiy. I don't know what equation you're using where you get 9 drives
from.

As far as it's ability to withstand _any_ 2-disk failure... I'm not sure
what you mean by definition. RAID-6 implemations don't follow a standard
because there isn't one. Depending on how it's implemented, RAID-6 is not
necessarily able to withstand a filaure of any two disks. We can argue as
much as you want, but I'm not willing to invest the time.

With a 1500MHz Athlon on a typical file server where there's not much 
writes, the CPU is sitting there chrunching RC5-64 som 99,95 % of the 
time. I don't think it'll make much differnce with today's CPUs

It's up to you to decide if the performance trade-off is worthwhile. I
merely trying to point out that system with 2 RAID-5 is likely to incur
the same CPU hit as a single RAID-6, implemented in the kernel. 


---
Derek Vadala, derek@cynicism.com, http://www.cynicism.com/~derek

On Sun, 2 Jun 2002, Derek Vadala wrote:

You can always fake this effect by combining two 8-disk RAID-5s into a
RAID-0. It's not technically RAID-6, but can withstand a 2-disk failure,
although not _any_ 2-disk failure. However, it's my understanding that
RAID-6 cannot withstand _any_ two disk failure either (see the above
thread). 

I think (hope) you meant 1+5, which will stand any three disk failure, and
up to 1+N/2 if just the right drives fail. They never do, of course.
 

I also suspect that the use of dual RAID-5s combined with the CPU overhead
of ATA will kill most systems under any kind of load. For that matter, the
2x parity hit from RAID-6 probably wouldn't make you CPU too happy either,
even if there was a kernel driver that implemented it.

I doubt it. Unless you run a system with heavy CPU demand there are lots
of cycles for this stuff. I run 0+1 several places and I don't see serious
CPU load. I would be very interested in RAID-6 in the kernel, but I have
the feeling that RAID-6 means diferent things to diferent people, judging
from posts here and articles online. I haven't found the performance info
you, I assume I will.

-- 
bill davidsen [off-list ref]
  CTO, TMR Associates, Inc
Doing interesting things with little computers since 1979.