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In a 4 disk RAID 1/0 configuration, how many spindles service read requests?

Posted on 2010-08-20
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In a 4 disk RAID 1/0 configuration, how many spindles service "read" requests?

I have calculated the I/O capacity of my drives based on vendor specs and I was drawing up a chart to depect various RAID configurations and their Read/Write IOPs for a presentation I have next week.  I have seen multiple blogs and forum posts that say to calculate cummulative "read" IOPs in a given RAID setup just add up the IOPs for all the spindles.  But something makes me think that RAID 1/0 would only use one side of the mirror to perform the read operation.  Am I wrong?
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Question by:VFS-User
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by:briandunkle
ID: 33489659
There isn't an absolute answer for this, because any read speed increase from RAID 1 is based on the OS and hardware implementation. In theory, RAID 1 could read different data from the two members, but in most cases it doesn't - most implementations just slave the two members together completely.
RAID 1+0 (a RAID 0 striped array composed of two or more RAID 1 sets) makes it even less likely - most likely the OS will treat it simply as RAID 0.

So the answer is that most likely, you will see the same speed you would from a simple RAID 0: in this case, 2 spindles for most operations.
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by:dkikalis
ID: 33490819
It depends on the brand of controller as the firmware. If you have a current raid controller SAN or NAS the answer should be that a read operation can read from all four spindles at the same time so a large fine transfer will see th I/9 at the limit of the four drives (assuming no other bottlenecks). Remember however that the most time consuming part of data transfer is the seek and the hover time so if your data is badly fragemented you may not see that type of throughput.  Most higher end controllers can be set to buffer tHe reads in an attempt to reduce latency.
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by:VFS-User
ID: 33500456
I should have put more details in my original post.  To be clear these are brand new servers with brand new latest release controllers connecting to a Clariion CX4-240.  The RAID is handed to VMWare as a single LUN which in turn is handing it to the guest OS, in this case SUN.  There are no other LUNs on the RAID group.  I am already aware of how seek time and other mitigating factors may impact I/O.  I just need to know if a (4) disk RAID 1/0 will use 2 spindles or 4 spindles to service (read) requests.
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dkikalis earned 500 total points
ID: 33501318
For large pure reads you will be reading from all 4 spindles.
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by:briandunkle
ID: 33502626
All comes down to the Clariion, obviously. Looked on their site but didn't find clear specs on how they handle RAID 1/0.
I would expect it to read all 4, but I also wouldn't make any guarantees until I had word from EMC.
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