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1) Let's say I have an array of twelve disc drives in a RAID-1 formation. That's effectively six drives presented for use. What's the maximum read rate I should be able to get out of that array? Let's say each drive maxes out at 40 MB/sec. Does that mean I should be able to read (up to) 240 MB/sec from that array? Or will I only ever be able to get a maximum of 40 MB/sec read rate, regardless of how many drives in the array?

2) If the answer to the first question is 240 MB/sec and yet I'm getting only 40 MB/sec out of that array, what could be the problem? Where do I start diagnosing this?

3) As you've probably guessed by now, this is an actual problem I'm having right now. Up until recently I was able to read up to 200 MB/sec out of this array, but then, after a simple re-boot of the server, I'm getting just 35 MB/sec. What could possibly have caused this? I'm baffled. There doesn't appear to be anything physically wrong with the hardware. All appears to be working normally. What am I missing? Some "parallelize/striping" parameter that has reset itself? A dead battery in the RAID controller? The RAID array in question is directly attached within a PowerEdge server, which in turn is used to host a SQL Server instance. The RAID array stores the files for just one database.

Any and all help gratefully received!

kme757
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1 Answers1

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The Read performance of RAID-1 is equal to the sum of all member disk since any read request can be handled by any of the drives. This is however, dependent on I/O Load.

The Write performance is only equal to the speed of the a single disk. If your disks have different write speeds, this is based on the lowest speed.

If the issue is slower reads, then it's possible that (a) one or more of the disks are degraded. In such case, the RAID would still work but that would impact the performance of the system until the disks are replaced. (b) You have other I/O going on in the background that's eating up your disk performance. You can try to check using top command in Linux or task manager in Windows.

jarvis
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