Having a mini-SAS connection for 4 HDDs is complete overkill.
No hard drive exists which can saturate a single 6 Gpbs channel, this is something which only a handfull of SSDs can do -- and only under very selective conditions (sequential reads). The highest transfer rate that I have seen is less than 200MBs (1.8 Gpbs), so 4 HDDs could be supported (almost) by a one of the 4 channels in the mini-SAS connection.
Further, remember that SAS expanders work on the basis on taking a 4-channel connection from the main RAID adapter and allowing multiple devices to to share that connection, so your are ultimately going to run into a limit of the speed of 4 channels (4 x 6Gpbs = 24 Gbps = 3 MBs). So, using expanders will not explicitly increase your overall performance.
If you want to still pursue a 4-channel connection to 4 HDDs approach, you should think in terms of increasing the number of expanders (or using multiple controllers -- although that has it's own issue).
Personally, for a near fully populated chassis (48 HDDs) I perfer to see 4 expanders -- each connected to 3 backplane channels.
Sean
P.S. You might want to have a look at SuperMicro they have a couple of some 4U chassis (vs. 9U in the Chenbro chassis) which support 36 or 45 3.5" HDDs! So, you could have a lot of rack space.
Appreciate the quick replies. Glad to know that controller chip is supported. SeanLeyne, I understand what you are saying about the data transfer rates of current hard drives. Thanks for the detail. As opposed to thinking that I was gaining performance with the expanders, I was worried about losing it by trying to carry too much traffic down a single 4-lane. I'm having a bit of a tough time visualizing exactly what you are saying about connecting the "4 expanders -- each connected to 3 backplane channels." To take a stab at it, for a 48-drive solution, you would generally recommend one big controller with say 4 mini-SAS 4-lane outputs. Each of the mini-SAS connections would go to a 4-port expander (1-in 3-out) and each of the 3 expander outputs would then connect to 3 backplane channels (each channel commonly correlates to a row of four 3.5" drives). That means that at it's heaviest load, the traffic from 12 drives are getting hauled between the expander and the controller (the smallest pipe) in one 4-lane at up to (4x6Gbps) 24Gbps. Since any one drive isn't likely to exceed 1.8Gbps, the total of their traffic wouldn't exceed (12x1.8Gbps) 21.6Gbps. Am I understanding that correctly? May I ask you what controllers/expanders/chassis you are using? I am pretty set on Intel, but, it would sure be nice to look at what else it out there. I did see SuperMicro had some pretty slick chassis designs. I will definately take a closer look. Thanks again!
Your understanding of the expander usage, as I tried to convey, is correct.
Regarding the use of a single controller:
My own experience is only using a single controller (largest HDD array is 12 HDDs), and I would be concerned about the ability to manage all of the HDDs effectively (allowing me to create RAID groups using the optimal number of HDDs) without having to predetermine which HDDs would be connect to which controller (in a multi-controller scenario).
By the same token, there are valid reasons to justify multiple controllers (depending on number of HDDs);
- some controllers have a limit to the number of RAID sets/groups which can be created (I know that Adaptec controllers do).
- you may want a larger amount of cache memory than available from a given controller
- you may not need a cache controller for some HDDs
- you may want a fancy controller which does hybrid SSD caching, but only for some HDDs
- you may have exceptional throughput requirements which are more than the controller can handle (is only possible in large sequential transfers, or if you are building an array of SSDs)
Regarding Intel:
- Over the years Intel has created RAID controllers using their own chips or have simply rebranded/reused controller chips. Just because it says "Intel" doesn't mean that it is Intel"
- The most of the current Intel controllers (and the expander which you pointed out) are in fact based on LSI chips, so any controller based the same chip would perform the same and be as reliable
- There are other good/excellent RAID controllers available, all off which have good/excellent support. There are plenty of features which need to be considered beyond just the brand name of the controller (hybrid SSD RAID support is an important one -- could allow for lower SATA speed HDDs to be used instead of expensive high speed SAS drives -- for high read/low write applications)
- If you are concerned about warranty support than I would suggest that you simply buy a backup controller. If you run into a real problem, controller failure, the only way your are going to get up-and-running quickly is by simply swapping out the card, and worrying about the warranty return/swap on the card separately. That is what we have done for our 2 SAN/storage systems, waiting even 24 hours for a replacement is much too long for an outage.
Genius
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