Hard Drives: Does 6Gbps Make a Difference?

I am looking into getting a new server.  One thing I'm not sure about is transfer speed vs rotational speed.  I was looking at doing a four-drive RAID 10.  I would like 15k rpm SAS drives because this server will have a lot of reads/writes.  On the particular configuratoin I was looking at, they offered 146GB 15k drives that were 3Gbps and 300GB 15k drives that were 6Gbps.  Obviously there's a significant price jump from one to the next (over $200 per drive).  Would RAIDed 15k rpm SAS drives even warrant the need for the jump from 3Gbps to 6Gbps?
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Mike KlineCommented:
RAID 10 will give you good write performance and will be fine for AD.  How big is your AD?  I'd personally save the money

...but not to blow smoke I've never done any benchmarks against AD between 3Gbps and 6 Gbps drives.


First, there are only a few SAS-2 controllers out there.  I've got both the Atto and one of the LSI SAS-2 adapters here, both work quite nicely, and are downward compatible with SAS-1 and SATA disks.   Also if disks are externally mounted, you need a SAS-2 enclosure if you want the disks to communicate at that bit rate.  SAS-2 has benefits beyond transfer rate .. but you can look that stuff up yourself.    Bottom line, buy SAS-2 now to future proof, but don't expect to see any performance benefits unless you upgrade your controller.

(Supermicro has the LSI2008 SAS-2 controller embedded on one of their server motherboards, when you factor in the cost of a SAS-2 adapter, you come out way ahead if you buy the supermicro mobo)
silver1386Author Commented:
@mkline71 - Mike, I apologize, this won't be an AD server.  The question automatically defaulted to that zone.  I had to go back and add the Hard Drive zone.  This will actually be a Remote Desktop Services / RemoteApp server.  Sorry about that.

@dlethe - We buy all of our hardware (and most of our software for that matter) from Dell, so motherboards aren't even an option.  It sounds like you're using the terms "controller" and "adapter" interchangeably (sorry, this isn't my strongest area).  The one that I had select was their PERC H700 (http://www.dell.com/content/topics/topic.aspx/global/products/pvaul/topics/en/us/raid_controller?c=us&l=en&cs=555).  It says it's 6Gbps.  Would that do what you're saying it needs to do?  I've also attached a picture showing some of the controller/hard drive options that are available to us.
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The enclosure, or backplane, if you will, requires a thing called a SAS expander.  This is a chip and circuitry that lets you plug multiple SAS/SATA disks into the controller (also called adapter, also called HBA).   The expander needs to support 6Gbit SAS-2 also.   Now if the disks are connected w/o an expander, i.e., something that looks like an octopus cable where each disk has it's own cable between the controller and the disk itself, then clearly you are not using an expander, so you will get the 6Gbit.

Since this is a hot-swap system, you have a high probability a backplane is involved, which almost always means you have an expander.  You could very well be getting scr*wed and be limited to 3Gbit/sec.  I am not saying you are getting mislead, I am saying you need to find out about the enclosure architecture, and if it uses SAS-2 or the first-generatation SAS expanders/backplane.

If you have an external JBOD (or called EBOD for SAS sometimes), and it is more than 6 months old, then you can bet it is NOT 6Gbit.  The SAS-2 stuff has not been shipping for very long, and lagged the HBAs & Disks.  
silver1386Author Commented:
@dlethe - Thanks, I'll definitely probe them about that.  Let's just assume, for now, that it truly is a full 6Gbps setup.  Would would using 6Gbps drives offer me any advantage over 3Gbps (at 15k rpm)?  I have read mixed results.  Like that it would only possibly matter in certain quick bursts.  Maybe there's more going on, especially with the RAID array stuff, than I'm aware of that would warrant the 6Gbps transfer speed (even if the HDD's don't read/write that quick).  Thoughts?
The SAS-2 benefits include:
 * Significantly better signaling (fewer errors, fewer retries -> greater performance IF you don't have 100% clean I/O .. which never happens, so you will get some performance gain, even if you just have one disk)
 * Reduced EMI  (with disks next to each other in an enclosure this helps throughput als)
 * Zoning, faster, better topology discovery (faster), can have more sophisticated topologies like dual-porting and sharing disks on more than one host
 * built-in diagnostic & channel tuning

And a bunch more that I didn't want to get into.  In your situation, cleaner signals, fewer errors and self-correcting will translate into marginally better performance, fewer grown defects & ECC errors.  

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silver1386Author Commented:
Last follow-up question: are those benefits (which you suggest would be marginal) that you are talking about only there if all the SAS-2 components are in place, or are they built into the architecture of the drives themselves?
Yes, unless everything is running SAS-2, then you get none of these benefits ... because, everything will be run the lowest common denominator spec, SAS-1.  So there really is more than just "speed".
Something else to remember here:
The next generation 6gb/s standard (SATA III/SAS 2) has typically been implemented in response to faster drive transfer rates.  These are not drives that still identify the rotational rate (even 15k).  It was really meant for solid state drives.  You will not see a significant increase in performance 6gb/s 15k drives - imo not even enough to justify the $800+ increase.  
I wouldn't buy the 6gb/s 15k drives...but I would ensure I could upgrade to 6gb/s ssd's when they're more affordable.  My 2 cents?  Buy the perc h700, stick with the 3gb/s drives.
As a storage architect of 20+ years, I take exception.  Sure, there is a bandwidth improvement, but the improved signaling/topology/diagnostics is the big advancement. One can always add more channels if you are throughput bound, but the rest of the benefits mean a world of difference behind a RAID controller, and even in JBOD environment when it comes to data integrity.

You have no idea how noisy some SAS-1 implementations are, and what that does to throughput (and IOPs) unless you actually measure it.

As for $800 difference, at the smaller capacity price is a wash, but as the larger capacities are backlogged, you will pay more money.  That is the nature of the beast.
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