[rescue] Re: [SunHELP] Help with RAID

Phil Stracchino alaric at caerllewys.net
Wed Jun 4 14:50:54 CDT 2003

On Wed, Jun 04, 2003 at 03:17:32PM -0400, robert brooke gravitt wrote:
> Ok,
> I've looked through the Sun doc on DiskSuite, but I'm still a little 
> clueless on what to do.
> I've got a Sun SPARCStorage array with 23 Disks in it ( 23 x 1 Gig.) 
> What i'd like to do is create a striped, mirrored volume for storage. 
> I'd like to make 2 10 Gig mirrors to mount and use for a CVS root. Any 
> help will be appreciated. I looked through a tutorial for mirroring 
> your root disk, but I just don't understand how to concatenate the 
> disks and then mirror them.

Fortunately, what you're trying to do is something in which Solaris
Disksuite knows to Do The Right Thing.  You see, the way most people
think of doing this is to create two stripes and then mirror them, but
the correct way is to create N mirrors and stripe them.  Disksuite knows
this, so when you ask for the former, it's bright enough to do the
latter.  (This, at least, is my understanding as explained to me by a
technical wizard in Sun's High Performance Computing group.)

Here's an example of the metainit commands to do it:

metainit d11 1 10 c1t{0,1,2,3,4,5,6,7,8,9}d0s0 -i 128k
metainit d12 1 10 c2t{0,1,2,3,4,5,6,7,8,9}d0s0 -i 128k
metainit d10 -m d11
metattach d10 d12

Oh yeah, don't use the default 16k interlace factor.  It's too small for
optimal efficiency.  You'll get better performance with the 128k
interlace noted above.

Although this looks like two stripes mirrored to each other, internally
Disksuite implements this as a stripe of ten individual two-disk mirrors.

Why does it do this? Redundancy.  With two stripes mirrored to each
other, if one single disk in each stripe fails, you're offline and your
data is history.  With a stripe of ten individual submirrors, you can
survive failures of half your total disks without data loss, so long as
you don't lose both disks of any mirrored pair, and the performance is
identical to doing it the obvious, easy (but wrong) way.

Calculating the statistical odds of losing both disks of one mirror vs.
losing one disk from each stripe is left as an exercise for the reader.

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