[rescue] Re: [SunHELP] Help with RAID

Curtis H. Wilbar Jr. rescue at hawkmountain.net
Thu Jun 5 01:55:59 CDT 2003

>From: Robert Novak <rnovak at indyramp.com>
>On Thu, 5 Jun 2003, Curtis H. Wilbar Jr. wrote:
>> Ummm... everywhere I've ever done this, you create either two stripes
>> which you then mirror, or two concats and then mirror.... I've never
>> heard of someone concatenating a bunch of little mirrors...
>Logically it makes more sense to concatenate a "bunch of little
>mirrors"... think about it.

agreed... no argument there... never had really thought about it...
probably because most of the boxes I had ODS on either did mirroring,
or I had a mirror of a stripe of two drives.... (so 4 drives, 2 stripes
of 2, then mirrored).... in that config you can lose two drives whether
you had RAID 0+1 or RAID 1+0.... so I rarely ran a config that would
have benefited from the difference enough to have really thought about it...
logically config wise, it was always configed as a mirror of stripes.

><theory mode, not SDS applied mode here>


>> >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
>> This is creating a mirror of two stripes !
>Yeah, it is. But Solstice Disksuite treats it as a stripe of mirrors
>under certain configuration conditions.

I understand that now...

>Go read http://www.psychofx.com/chris/unix/sun/dsraid10.txt
>and http://sunsolve.sun.com/pub-cgi/retrieve.pl?doc=finfodoc/45222 (you
>may have to google for the cache version of the latter--just go to
>google.com and put this url in the search bar to find the cached copy).

read the sunsolve article... it was written less than 1 yr ago... most of
the DiskSuite systems I was running prior to then... so I had never
seen or heard of the article.

It doesn't state at which version (or for all time) that ths internal
optimization applies.

[ summary of refrenced URL removed :-) ]

>> >Although this looks like two stripes mirrored to each other, internally
>> >Disksuite implements this as a stripe of ten individual two-disk mirrors.
>> Say what ?  Is this in the docs somewhere.... I've never heard of this.
>> It also runs contradictory to what you are setting up.... you are first
>> creating the ODS stripes, then creating an ODS mirror.  Internally if
>> it treats it as something other than this would be quite odd to handle.
>You should read Brian Wong's book. Configuration and Capacity Planning for
>Solaris Servers http://www.amazon.com/exec/obidos/tg/detail/-/0133499529

Will have to check that out thanks...

>This is where I first saw the concept, and at one point I had an indirect
>argument with Brian through a friend about it. Took me a couple of years
>to find a better answer than "it just does"... but, well, it just does.
>> Are you sure about this ?  I'd like to know more about it and the
>> source, becasue I worked for Sun, have friends that work for Sun, have
>> used Disk Suite since the SunOS 4.X days and I have never heard of this.
>I worked for Sun, have friends that work for Sun, have used Disksuite
>since the Solaris 2.5.1 days, and I've heard of it. None of the former
>details really matter with regard to the latter.  The topic has come up
>here and/or on sunhelp@ before too.

guess I haven't been around here long enough.  I worked for Sun in
SunService.  I did not support DiskSuite except a couple of support
calls back when I was first hired, and that was ODS on SunOS 4.X.

I primarily used ODS after I left Sun.... I had still never heard of
this... but again, most of the stuff I was configuring was at most
4 disks in a 2x2 config, in which case 0+1 vs 1+0 really wouldn't
make a difference.

>> OK... this makes sense... but I'd still think you need to create the
>> 10 submirrors which you would then stripe.  I doubt ODS is going to do
>> it automagically as such hiding the fact that it has done so...
>That's exactly what it does. You can't add a metadevice to a stripe. You
>can only add a slice to a stripe. You can add a stripe to a mirror though.
>SDS hides this from you and just does it in the conditions mentioned

I think all in all it would have made more since for ODS to allow you
to construct it outright so it appears that way in the system...  since
it can't, it  is nice that it optimizes it that way for you itself... 
but it never gives you a clue (unless you see disk failures in both
your stripes and start to question why the volume still works!).

>> I question the chances of either happening before the first disk is
>> replaced (at least in a production environment).
>I've had three disks die in the time it took to get the fourth replaced,
>and I was using a disk array that used four-drive magazines. That was not
>pleasant. So while it's unlikely, it happens. And given the opportunity to
>prepare for it happening, I would take that opportunity if possible. I
>felt that way before I had a big bad batch of 73gb disks, but now that
>I've run into it, I feel even more sure of that stance.

Well... lots of unlikely things happen... one could never prepare for all
of them... but it is nice that ODS optimizes this... but if you don't meet
the conditions, it won't tell you, and you won't get the optimization... so
this is one downfall about having it automagically do it rather than
representing things as they are really being done and/or specifically allowing
you to define the structure in this manner.

>rescue list - http://www.sunhelp.org/mailman/listinfo/rescue

-- Curt

Curtis Wilbar
Hawk Mountain Networks
rescue at hawkmountain.net

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