Upgrade home server HBA and disk shelf to SAS12 when drives are SAS6?

[nemeth782]

I have a Netapp DS4243 with a Dell Xyratex Compellant HB1235 controller in (basically IOM6).

If I put an IOM12 controller in, I get SAS12 instead of SAS6, but if the drives are all SATA6 or SAS6, do I actually gain anything?
E.g., the bandwidth from HBA to controller is now 12Gb/channel, with a 4 channel link, but does it actually run at that between the HBA and the controller, or does it run at the speed of the drives?

Obviously I'm never going to get >6gb/s from a single drive, but say I'm reading from 8 drives at once. Are they fine because 8x6 = 4x12, or are they throttled because it can't run at 12 and 8x6 > 4x6?

 

[GaPony54]

You don't anything if you HBA is 12Gbs and your drives are 6Gbs. The fastest you can transfer depends on the weakest link, ie. the 6Gbs limit of your drives.

 

[nemeth782]

The backplane limits the drives to SAS6, even the SAS12 ones. All the SATA ones have SAS interposers and are running at SAS6 speeds.

The link between the controller in the shelf and the RAID controller is SAS12. I know I'll not get faster than 6Gbs to any one drive, but obviously in aggregate they can transfer more than 4x6Gbs of data as there are 24 drives!!

That's where I mean though, does the SAS12 link between the controller and HBA slow down to 6Gbs when transferring data from drives linked at 6Gbs, or can 24 drives (8 per port) at 6Gbs saturate the 4 12Gbs lanes.

 

[nexox]

That's where I mean though, does the SAS12 link between the controller and HBA slow down to 6Gbs when transferring data from drives linked at 6Gbs, or can 24 drives (8 per port) at 6Gbs saturate the 4 12Gbs lanes.

As I understand it the answer is: it depends. Some 12G expanders can apparently do the right thing and go full 12G to the HBA with 6G drives, some don't, and it's very difficult to figure it out by reading docs. You might just need to try it and let us know how it goes.

 

[GaPony54]

The backplane limits the drives to SAS6, even the SAS12 ones. All the SATA ones have SAS interposers and are running at SAS6 speeds.

The link between the controller in the shelf and the RAID controller is SAS12. I know I'll not get faster than 6Gbs to any one drive, but obviously in aggregate they can transfer more than 4x6Gbs of data as there are 24 drives!!

That's where I mean though, does the SAS12 link between the controller and HBA slow down to 6Gbs when transferring data from drives linked at 6Gbs, or can 24 drives (8 per port) at 6Gbs saturate the 4 12Gbs lanes.

I can only speak for my own personal experience. Being in the middle of a transaction of replacing my old server, which has a combination of 24 7200rpm drives consisting of 4TB, 8TB, 10TB and 22TB running off 3x Supermicro AOC-SAS2LP-MV8 cards, all connected to a 6Gbs backplane with SFF-8087 connectors. I bought a new server with 12 Gbs backplane and SFF-8843 ports, using the same drives with an LSI 9305-16i HBA (12Gbs) and an Adaptec 82885T 36-port SAS expander (12Gbs). All connections are SFF-8643. My transfer rates on old setup were around 100-115MBs, depending on the load and file sizes. Nothing changed much with the new configuration, except that I can maintain a slightly higher transfer rate for longer.

The ONLY reason for the upgrade is that the old chassis, motherboard and Core i3-2120T CPU were getting long in the tooth and I was having some trouble cooling the newer, larger drives. I wouldn't have bothered if I was just looking to get better transfer rates. I should mention that I'm not running in a fast RAID, I'm using Stablebit Drivepool for the sheer simplicity and built in redundancy.