RAID 5 and Performance Degradation Concerns
My manager (not a DBA) has put together a spec for a new disk array (Hewlett Packard XP 1024) on which we'll be running our 600GB OLTP database (64bit 8i and HP-UX 11i) and has asked for my opinions. My most immediate concern, and one which I've mentioned to him, is that it only has RAID level 1 or 5. Now we definitely need some hardware level striping so RAID 5 is our only option. I'm no storage expert, but understand that Oracle and RAID 5 is generally percieved to be a bad idea on a write heavy application due to the maintenance of parity information. He's responded that HP claim that on this array RAID 5 performs just as well as RAID 0+1 (my preferred option, we currently have an EMC Symmetrix and utilise this). I'm meeting some HP reps on Monday and want to grill them on this, but I was wondering if anybody out there has any practical experience with Oracle and this product, or any opinions on HPs claims.
Any guidance would be much appreciated.
I will quote from the Oracle 9i bible(Performance Tuning Tips and Techniques)....p.65
"Raid 1+0 This level provides mirrored disks and striping. This incorporates the advantage of the first two RAID levels by providing the disk I/O benifit of RAID0 to the redundancy provided by RAID1.For high read/write environments such as OLAP,where sporadic access to data is the norm, this RAID level is recommended".
"RAID5 ...It is a low-cost solution that is inefficient for write-intensive applications."
--If you can get the book over the weekend read chapter 3.--
Able was I ere I saw Elba
No matter what HP says about RAID 5, it will hurt perf for an OLTP system.
Re: RAID 5 and Performance Degradation Concerns
Tell them to put up or shut up. Have them give you two boxes to evaluate, one configured with RAID 1+0 and the other with RAID 5 and run your application on them. If there's no difference, go with the RAID 5.
Originally posted by hacketta1
He's responded that HP claim that on this array RAID 5 performs just as well as RAID 0+1 (my preferred option, we currently have an EMC Symmetrix and utilise this).
One more test you can do.
When DB is running, ask the HP guy to pull out 2 disks randommly selected in the striped set, and now you see your DB is up and running. Or the disk array itself is crashed.
Thanks for your responses. I like the idea of doing some tests of my own; not sure if they'll agree though. One thing I'm thinking is that the array will have at a minimum an 8GB cache, so if this could be configured as write-only it may go someway to circumventing the RAID-5 write penalty.
Something else that's vexing me is that it apparantly this thing has a minimum stripe size of 9GB. Again, I'm no expert in these things but I can't really see how this would help eliminate hot-spots.
We eval stuff all the time. If they don't like the idea, tell them you are looking at adding some EMC arrays...
Originally posted by hacketta1
I like the idea of doing some tests of my own; not sure if they'll agree though.
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