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How does using a SAS RAID controller with a RAID expander affect performance?

Posted on 2014-07-20
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Last Modified: 2014-07-21
•      Server Chassis is Supermicro  SuperStorage Server SSG-6027R-E1R12L http://www.supermicro.com/products/system/2U/6027/SSG-6027R-E1R12L.cfm.
•      Raid Controller is a single  LSI 9240-8I
•      Hard drive configuration is 12 WD2003FYYS 2TB 7200RPM drives

Only the LSI Controller is used. It is attached to the 12 bay back plane containing the 12 SATA drives using 1 SAS connector.  All 12 drives are detected and run fine in RAID5. There are 2 virtual volumes, one 100GB and one consumes the rest of the available drive space. I am not seeing performance issues, but the server is not under any load.
So effectively only one of the 2 SAS ports are being used on the LSI controller to drive 12 SATA drives. This appears to be by design on the supermicro MB. The backplane only has one SAS connector. What I cannot find is where the server throughput bottleneck may be.  The controller is designed for 8 drives at 6Gb/s transfer, so is it running at 150% capacity (4.5Gb/s)  ? Or since only one SAS connecter is being used, is it running at 300% capacity (2Gb/s)?
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I haven't seen this particular backplane but you say only one port* is in use. You haven't said if that is a wide port though.  As far as the 9240-8I is concerned there are 4 lanes per connector so if there's one cable between the backplane and the card you probably have a wide port of 4 lanes giving 24Gbps which is far more than you can get from 12 SATA disks unless they are SSDs.

*Look up the definition of port in the SAS manual - you'll see it's one or more data lanes.

BTW, 12 SATA disks in RAID 5 will probably fail to rebuild when one fails, at least if they're high capacity near-line disks.
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by:jrvandy
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Andyalder
You are correct on the Port designation. I should have said that it is using only 1 of the 2 SAS Connectors.  What you also said about the math makes since. Even 4 ports at 6Gb/s is 24Gb/s of bandwidth. So a WD2003FYYS 2TB drive has been tested at 1.2Gb/s (thank you Tomshardware) , so 12 of them assuming a RAID0 with no overhead could push only 14.4Gb/s.
Now for your other point that 12 SATA disks in RAID5 would fail to rebuild; I am very interested to what you base this on? I have certainly seen large arrays with 8 disks take a week to rebuild, but “probably fail” is certainly concerning.
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"probably" is a bit of an exaggeration but with one disk missing the possibility of a bad block on one of the remaining disks increases not only with the number of disks but also the capacity of the disks. www.zdnet.com/blog/storage/why-raid-5-stops-working-in-2009/162 goes into the maths of the failed rebuild due to UREs. The controller may well skip past the URE and continue the rebuild so it looks like it is completed but there's still a punctured stripe, when you come to read the data that's on the stripe that didn't rebuild the controller has to send a read error message to the OS. If you value the data and hate restoring from backup use RAID 6.
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