Hardware RAID 5 Performance Configurations?

gandy909 asks: "I am facing the need to replace a major server in the next few months due to both EOL status and disk I/O bottleneck issues with the array containing the data. The server is configured with a 2 channel array controller. It is a RAID 5 array and has 4 drives, 2 per channel (2 data, 1 parity, and 1 hot spare). Obvious performance benefits in replacing the server are quadrupling CPU speed and doubling the memory. Other side benefits I will gain with the new drives, that I think should help my performance, are moving from u160 to u320, and going from 7200 RPM to either 10k or 15k RPM." How would you configure a larger array to best increase its performance? "Having googled around, the consensus is that increasing the number of drives is the preferred way to attack the I/O bottleneck. What I don't find much help on is determining the configuration of the larger array. Assuming I am going to be using a 12 drive array I have come up with the following possible configurations:

1 2 channel controller, 6 drives per channel
1 4 channel controller, 3 drives per channel
2 2 channel controllers, 3 drives per channel
3 2 channel controllers, 2 drives per channel

Would any one of those configurations provide better performance than the others, or would they all even out considering other factors?"

Comments

Get away from RAID 5

[Fished]

Not enough information - is this data warehousing? Transaction processing? Mostly reads, or a lot of writes as well?

But generally, I don't recommend RAID 5 for performance critical situations. It's great for data warehouse, but if you lose one drive their goes your performance. Also, realize that, often, the place where you can really boost performance is in the database, not the hardware. How's your query optimization? Do you have appropriate indexes? Is the code accessing the database efficient?

Well, knowing 0 about your business requirements.

[dnight]

I'd put 2GB fibre HBA's in the server(s) and attach an OpenSAN RAID box from Winchester Systems with dual-homed fibre to the array. That'll take care or your bottleneck and help your future scaling requirements.

Depends on usage patterns

[cookd]

First, 3 drives in RAID-5 is not very useful. You get a lot of the disadvantages with few of the benefits. Having more drives really helps throughput. So go for more smaller drives over fewer faster drives for RAID-5.

Second, RAID-5 is great for read speeds, but less great for write speeds. A good caching controller will help hide this, but a small write operation requires a read from each disk in the set before the write can be completed (in order to recompute parity for the stripe). If this is mostly reading, or if most writes are large (not small and random), RAID-5 will work fine (data storage, data mining, etc). If writes are frequent (transactions), RAID-5 is painful. RAID-10 might be better.

Re:Depends on usage patterns

[cookd]

And to answer the question you actually ASKED...

Fast drives can sustain about 60 MB/s nowadays, so don't put more than (max channel rate)/(max drive rate) = (320)/(60) = around 5 drives on a single channel.

As for multiple controllers, if they aren't present for redundancy purposes, you're probably just as well off using just one.

But then again, I'm just a Slashdotter who hasn't kept up with the latest specs, so I'm pretty much just making stuff up. Free advice is worth every penny.