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My question is: when is it appropriate to simply create one large array with fast read & write performance, such as RAID 10, instead of creating smaller arrays for specific applications?

It seems to me that if my overall I/O requirements are not terribly heavy, a single array with excellent read and write performance could provide better performance overall to all applications except at those (perhaps rare) times when applications with different access patterns peaked at the same time (copying large amounts of large files while a database is getting slammed).

If I dedicate a pair spindles to a particular task, such as transaction logs, and they're not even breaking a sweat with the workload...why not just put that workload onto a larger RAID 10? Those spindles would then be able to contribute to other workloads instead of sitting around scratching themselves 60% of the time.

PS, in my particular case the cost overhead of RAID 10 isn't a factor, because I'm looking at creating multiple RAID 1 arrays and one smallish RAID 5. Going RAID 10 for the amount of space I need would be comparable.

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4 Answers 4

up vote 9 down vote accepted

Knowing how to setup your storage is all about measuring and budgeting the IOPS and bandwidth. (I'm being simplistic here because the size of the percentage mix of read/write, average IOs, RAID stripe size, and cache hit percentages matter greatly. If you can get those numbers you can make your calculations even more accurate.)

There's a really nice little IO calculator here that I frequently use when planning out storage. wmarow's storage directory is also nice for getting some fairly contemporary disk performance numbers.

If I dedicate a pair spindles to a particular task, such as transaction logs, and they're not even breaking a sweat with the workload...why not just put that workload onto a larger RAID 10?

Remember that putting sequential IO onto a spindle with random IO makes that sequential IO random. Your transaction log disks may look like they're not breaking a sweat because you're seeing sequential IO operations. Sequential reads and writes to a RAID-1 volume will be quite fast, so if you're basing "not breaking a sweat" on disk queue length, for example, you're not getting the whole story.

Measure or calculate the maximum possible random IOPS for the intended destination volume, take a baseline of the current workload on that volume, and then decide if you have enough headroom to put those transaction log IOPS into the remaining random IO in the destination volume. Also, be sure to budget the space necessary for the workload (obviously). If you're so inclined, build in a percentage of additional "headroom" in your IO workload / space allocation.

Continue with this methodology for all of the other workloads that you want to put into the destination RAID-10 volume. If you run out of random IOPS then you're piling too much into the volume-- add more disks or put some of the workload on dedicated volumes. If you run out of space, add more disks.

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One problem is that with my current configuration, I can really only get a good measurement on total IOPS, which is only around 300 (60% read) with an average of 14K bytes per transfer. The large RAID 10 array I'm considering should be able to do 758 random 16k IOPS (60% and factoring out cache). This calculation doesn't take into consideration the mixed workloads, however, and that's what I'm struggling with. –  Boden Jan 7 '10 at 18:14

Sure - why not. thing is, remember to build your large multi-disc array correctly, get it wrong and you're in for a long rebuild time.

RAID 10 comes in 2 flavours:

  • RAID 1+0 where you build 2 RAID 1 arrays, then stripe them.
  • RAID 0+1 where you build 2 RAID 0 arrays, then mirror them.

Once a disc dies, in one you'll just have to rebuild a single raid1 array. With the other you'll have to rebuild the entire raid0 array.

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I've read more articles lately saying that RAID 5 is a bad way to go for reliability. I believe it more after having a disk go bad in a RAID 5 array, replaced the drive, and it couldn't recover because a second disk had a latent unrecoverable read error that forced us to reformat and restore from backup.

As drives are getting larger, the odds of having an undetected unrecoverable read error on these huge disks is also increasing, and with RAID 5 the reliability simply isn't cutting it anymore. If you're not mirroring, apparently advice is to go with RAID 10.

Unless you're doing something that is very sensitive to speed issues I wouldn't worry about breaking things into small arrays or anything like that. Put it into a RAID 10 and see if your performance is up to snuff. With proper caching and memory in the system it should be fine. Numbers for benchmarks will never quite be what you will actually get because it depends on your actual usage, actual load, disk drive performance, controller caching, disk caching, fragmentation, etc.

The best rule of thumb is to simplify the configuration as much as possible because even if you sink a day or a week in sweating over this setup in a year or two you're going to have to troubleshoot some issue on this server that will leave you wondering why you set it up the way you did if you added any unnecessary complexity to the configuration.

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Yeah, I moved away from RAID 5 to RAID 6, and the write penalty is killing me. I like the idea of one big RAID 10. What can I use to benchmark different types of simultaneous I/O load? –  Boden Jan 7 '10 at 17:26
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A stopwatch is the most accurate :-) –  Bart Silverstrim Jan 7 '10 at 17:33
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Don't underestimate the importance of drives and controller. We have a Dell NAS with RAID 5 and it is choked when users are logging in and pulling profiles from it and we concluded that it was because the drives aren't 10,000 RPM, after researching a bit into the performance monitor numbers. We ended up moving certain data to a separate server to offload some of the workload. –  Bart Silverstrim Jan 7 '10 at 17:35
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Honestly, I don't understand why anyone would use RAID 5 in a corporate environment. We're RAID 10 all the way. –  Beep beep Jan 7 '10 at 17:50
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Because at the time RAID 5 wasn't evil :-) RAID 10 was considered luxury unneeded at our level of business, I suppose you could say. Prices have really fallen, times have shifted, and we still support what is "working" in the server room. –  Bart Silverstrim Jan 7 '10 at 18:15

How predictable do you want the latencies for your applications to be?

Say, for instance, you have one app that is really latency sensitive, and it shares a filesystem with your financial database. At fiscal closing, all of a sudden you get calls about the latency sensitive app timing out. How you going to figure that out?

On the other hand, I'm a huge fan of simplifying everything, so I'd verify that you don't have any exceptional requirements, and consolidate and simplify your configs if your requirements don't fall into any weird special cases.

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