Re: Veritas's implementation/interpretation of RAID 0+1 and RAID 10

From: Chuck Swiger <chuck_at_codefab.com>
Date: Tue, 20 Aug 2002 22:09:11 +0000 (UTC)
Message-ID: <ajuem7$ml4$1@shot.codefab.com>

In comp.sys.sun.admin Darren Dunham <ddunham_at_redwood.taos.com> wrote:
> In comp.unix.solaris NetComrade <andreyNSPAM_at_bookexchange.net> wrote:
> mirror-stripe is the classic (easy) mirror of stripes.. stripe-mirror
> is the layered stripe of mirrors. It looks like that page is fine
> except for the 1+0 and 0+1 headings. I don't like them anyway because
> they're too easy to get backwards.

Obviously true, given this thread. :-)

>> The way it's supposed to be:
>> 
>> RAID 0+1
>> http://www.acnc.com/04_01_0p1.html

>
> from that page: RAID 0+1 has the same fault tolerance as RAID level 5
>
> Really?

Nope. Consider a 4 disk RAID set; using RAID-5, the system can deal with a single disk failure without losing data, but losing a second disk means losing everything.

> Why would the applications between 1+0 and 0+1 be different?

Define '+' as mirroring, and '*' as striping:

(a+b) * (c+d) is a stripe on mirrored disks, or RAID-1,0.

If disk A fails, the set can also deal with a failure of either disk C or D and stay up. Only if B fails will it lose data.

(a*b) + (c*d) is a mirror consisting of two stripes, or RAID-0,1.

If disk A fails, the set cannot lose either disk C or disk D, but a failure of B won't take down the system. To sum up:

RAID-5:  100% chance of data loss if two disks fail.
RAID-1,0: 33%    "   
RAID-0,1: 66%    "   

Note that this is assuming the RAID-0,1 system does not understand that A ~= C and B ~= D. DiskSuite is supposed to be smart enough so it will use B in place of D in order to keep the filesystem available, thus giving it the same failure probabilities as the 1,0 case.

-Chuck

       Chuck Swiger | chuck_at_codefab.com | All your packets are belong to us.
       -------------+-------------------+-----------------------------------
       "The human race's favorite method for being in control of the facts
        is to ignore them."  -Celia Green

Received on Tue Aug 20 2002 - 17:09:11 CDT