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Does it impact performance if I have 24 small drives in Raid10 vs. 4 larger drives?

The root of the question is does it have any performance or redundancy benefit if I fill out all 24 of my bays with alot of smaller drives or just stick to 4 drives or 8 drives but larger disks?

Dave M
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Mark
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  • By filling all 24 of your bays with smaller drives, it makes adding capacity in the future very difficult and very expensive. I would probably aim for a middle ground - perhaps 12 or 18 disks in RAID10, leaving room for expansion if required? – Mark Henderson Dec 15 '14 at 00:49
  • Also on that note, I've done a lot of work with large arrays of spinny disks - to get the the same performance as a single SSD takes about 12 spinny drives, and to match 4 SSD's in RAID10 is basically impossible with spinny disks – Mark Henderson Dec 15 '14 at 00:50
  • Can you provide specifics on the hardware, storage involved and the disk types you're considering? – ewwhite Dec 15 '14 at 13:56

2 Answers2

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Each drive adds its performance to the sum, so more drives will be faster than less drives, all else being equal. The size doesn't directly have an effect, though. If you have a choice between 2.5 inch 300GB 10k RPM drives and 2.5 inch 900GB 10k RPM drives, they'll perform about the same, drive for drive. The difference is when you figure it per TB. So 10 TB of 900GB drives in a raid will be slower than 10TB of 300GB drives, all else being equal.

Some other things you should consider: 7.2k RPM drives are slower than 10k or 15k drives, and are also much larger, usually. They are much slower per spindle when it comes to doing seeks (like random reads) but only a little slower per spindle when it comes to streaming IO (sequential IO that doesn't require the heads to move much). SSDs, which have no moving parts, are a little faster than a regular spindle drive in pure MB/s when the IO can be streamed, but they don't slow down when there's seeking to be done, so where a spindle would start to be limited by the number of IO/s it can do, SSDs aren't, really.

Basil
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  • SSD are not "a little faster". They are 100 times faster as a guideline number. I recently replaced 8 10x drives (2.5") with 2 SSD and performance went through the roof. If you need IOPS then anything except a SSD is too exensive. – TomTom Dec 15 '14 at 13:54
  • @TomTom perhaps Basil's phrasing wasn't optimal, but his statement was accurate: in terms of sequential reads, SSDs are generally only a little improved over their HDD counterparts; eg your typical 7200rpm HDD may manage ~ 160MB/sec sequential whereas a typical SSD may manage ~250MB/sec sequential. Certainly not the 100 times that you claim, but certainly an improvement. Perhaps you were referring to latency? – BE77Y Dec 15 '14 at 14:10
  • Yes, but a Raid of that size normally NEVER has sequential stuff. Sequential is limited to workstations. Show me a single server that has sequential access. I would say 199 of 200 server level machines have random access. And the last one also has during backup ;) – TomTom Dec 15 '14 at 14:12
  • @Tomtom, I was specifically talking about streaming IO. I went on to describe their speed compared to seeking spindles later. – Basil Dec 15 '14 at 18:11
  • Streaming IO still does not have to be non-random. Streaming IO can easily be 100 streams in parallel. – TomTom Dec 15 '14 at 18:12
  • I'm not editing my answer, it's clear what I was talking about. – Basil Dec 15 '14 at 18:14
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Yes. More spindle = higher IOPS. Provided that the smaller drives are similar, performance-wise, to the larger drives (i.e., similar interface and similar RPM).

pepoluan
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