Re: Questions (RAID, SCSI, etc) - ANY help appreciated!

On Thu, May 30, 2002 at 06:03:01PM -0500, Vitaly McLain wrote:
> 1. How exactly does RAID 0+1 work? Let's say I have two 18Gb drives in a
> RAID 0+1 configuration; does that mean I'll basically have one 36Gb drive
> (RAID 0) *and* still have mirroring (RAID 1)?

Nope.  There's no magic, sorry: if you want 36GB of safe mirrored storage
you have got to start out with 72GB of raw disk.  Make that 4 drives and
this will work.  Unless you're dealing with making use of existing hardware,
though, you might want to skip the striping and just get larger drives.  I
just saw an announcement for Seagate's newest 10K and 15K Cheetah series
drives - up to 70-some GB and 100-something GB (at 15K and 10K,
respectively).

http://linas.org/linux/raid.html - not quite up to date; background, lots

> 2. Let's say I have a SCSI controller (Embedded Adaptec AIC-7899 dual
> channel Ultra3 (Ultra160)/LVD SCSI). If I have two SCSI drives, of the same
> size, what else do I need for hardware RAID? One RAID controller, or
> something else?

For hardware RAID, you need a RAID controller.  With any number of non-RAID
controllers, you can only do software RAID.  Of course, it's really all
software RAID at bottom, just some versions move the RAID code off the main
CPU.  :-)

> 3. In a RAID 5 configuration, let's say I have 3 18Gb drives -- they will
> show up as one 18Gb drive, right? (I *think* I understand the
> mirroring/parity bit concept.) What if I add another drive (a 4th drive),
> will it still go into the same array which will show up as one 18Gb drive? I
> thought I had the answer to this, but I couldn't convince my employeer.

Nope.  RAID 5 (and 4 and most anything else above 1 except I've seen 10 used
as if it's a shorthand for 1+0) gets you N - 1 drives' worth of user data
space, so if you build the array with three drives you get 36 GB, and 54 GB
with four.  OTOH, if you build it with three now and add a fourth drive
later, you'll still have 36 GB, but you can enjoy the peace of mind that a
hot spare brings.  Unless you backup all 36 GB, rebuild, and restore, of
course.

> 4. What's the deal with software RAID? Is there a significant performance
> hit? And, if you don't mind answering...are there any "free" (free beer)
> software RAID solutions for Windows 2000? I admit I haven't had a chance to
> Google about this question, yet, but if anyone wants to offer any insight,
> that'd be fantastic.

"Software RAID" runs the RAID control code on the main CPU instead of on a
dedicated outboard CPU.  There is inevitably some additional overhead for
this; more with RAID 5 than mirroring, probably.  And there's more I/O
traffic passing through the controller, of course - twice as much for writes
in mirroring.

Windows had software RAID back in NT 4, I think.  I have no idea about W2K,
but with all that bulk I should hope it had it hidden somewhere inside.

> 5. Some SCSI drives are advertised as 68pin, others as 80pin. Does each
> particular SCSI controller have a particular pinout, or what? Should I just
> research the controller mentioned above to figure out what sort of pinout I
> need?

Yes and yes.  If you're going with a SCSI array, do remember to locate a
reliable source of goats first!  They're never available in a hurry when you
need them...

-- 
You arguably have quite a few inalienable rights,
but being taken seriously isn't one of them.
Neither is being respected.  -- Rick Moen  <linuxmafia.com/~rick/faq/>