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Performance for RAID 5 SAS and HP 400i controller

Posted on 2013-05-24
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Please verify whehter my assumption is correct or not.

With HP 400i controller, the thoughput is 3Gb/s -> max througput is 375MB/s

Each SAS RAID-5 hdd will have transfer rate ~ 140MB/s, so probably 3 HDDs may overload the controller but the card normal support more than that.

So, what do I miss ?

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Question by:AXISHK
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by:dlethe
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real world with 3HDDs you won't get anything close to 375MB.  Don't worry about it.   Why? Because writes involve reading data from 2 disks and writing to 1.  Then you have to consider filesystem overhead and extra writes any other write requires for the filesystem.

You'll also never sustain 140MB/sec anyway, that is how fast data is read from cache.
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by:AXISHK
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Any material concerned with this type of calculation ? For HP 400i controller (thoughput ~ 375MB/s), what's max disk that it can support ?

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by:andyalder
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Max disks a P400 supports is 18 in a ML370 G5 but that's really down to the number of drive bays available in the server. QuickSpecs also claim 38 disks but there isn't any server that takes that disk arrangement.

I don't know where you get "max througput is 375MB/s", 8*3Gb SAS lanes = 2.4GB/s although the PCIe bus is only 2GB/s.

It really depends what I/O pattern you throw at it though, your 15K disk may be able to read sequentially at 140MB/s so if you had a disk based backup system then you you may be able to stream from a 3 disk RAID 5 at something like 250MB/s but if your operating system is running on those 3 disks then it's not going to be sequential access. Worst case is about 200 IOPS, which means you can read 200 512 byte sectors in 1 second = 100 KBPS per 15K disk. There are some ballpark raw IOPS figures in http://en.wikipedia.org/wiki/IOPS.
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by:AXISHK
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3Gbps  = 3,000,000,000 bps -> 3,000,000 kbps -> 375,000 kB/s -> 375 MB/s

Do you mean there are 8 lanes in P400i with 3Gb/s for each lane ?

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http://h18000.www1.hp.com/products/quickspecs/12400_na/12400_na.html
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Yes, there are 8 * 3Gb SAS lanes. In a simple 8 disk backplane configuration such as in a DL380 there is one lane per disk but the P400 also supports the SAS expander card that fans-out those 8 lanes into 24 lanes to allow 3 disk cages although the per-disk bandwidth is obviously less in that scenario.
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