SUMMARY -- Disksuite Question

All:

Original post is below. As expected, I received several answers before I
had time to get a fresh cup of coffee. Thanks to Darren Dunham, Jay
Lessert, Charles Homan, Brett Monroe, Gary Chambers and Martin Hepworth
for responding so quickly.

Bottom line is that I can build the disks as I described, but will hurt
my performance, or as one responder so eloquently put  it -- only every
time the RAID5 is accessed. Kind of what I thought might happen,
confirmation is good. One responder mentioned that he has 10 disks in a
RAID5 array, but no one mentioned a specific upper limitation. One
responder pointed out that with the striping, I will probably get to
peak I/O performance at 4-5 disks anyway (plus mirrors). And all
mentioned the higher CPU usage for RAID5 -- this was good to learn.
Luckily, I have lots of CPU as well (I told my problems were enviable),
but will be watching it more carefully as I start with RAID5. Finally, I
got a reference to an Oracle tuning PDF on Sun's site, though not the
specific URL.

So, I will keep my disks separate, try the RAID5, watching my CPU and go
find the PDF referenced for further Oracle tuning with regards to Sun
disks. Naturally, I'll do the research before the build. Thanks again to
all.

Original post:

> Sun Managers:
> 
> I have a rather enviable problem -- I am about to have too much disk 
> space. Currently, we have 6x36 GB disks in a SunFire v880. For better 
> I/O performance, we are adding another 6x36 GB disks. I don't need all

> of this disk space for the Oracle databases, but do want the extra 
> performance. I'm not a real I/O expert, so could use some suggestions 
> here.
> 
> For Oracle, we do lots of writes, fewer reads, so do stripes
> and mirror on
> them -- RAID 0+1. I have another application with lots of images to
> store. I would like to store the images in this extra disk 
> space. Since
> this use would be lots of storage and few reads, I would like to use
> RAID5. Even with all of this extra disk space, I'm rather miserly and
> want to create as much as possible.
> 
> So, the question is: Can I use some of my disk slices across all 12 
> disks to do RAID 0+1 for Oracle stuff and other slices across all 12 
> disks (well, probably fewer, but you get the point here) to build 
> RAID5 arrays? Am I going to kill my performance for Oracle? Would I
> be better
> off, saying dedicating 8 disks to Oracle and then doing the other 4
> disks in a RAID5?
> 
> Another question. Any ideas on what the optimum or maximum number of 
> disks is for RAID5? It seems that using 3 disks and losing one-third 
> of your usable space is just a waste when you can put 5 or 6
> disks into an
> array and lose 16-20%. But how far can you go with that, 
> particularly in
> a low-hit area? Can you put, say 10 disks into a RAID5?
> 
> Thanks very much for any assistance.

Bruce Purcell
Enrollment Services
Director, PM/TS
bpurcell@csuhayward.edu
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