ONTAP Hardware

Maximum SAS Shelves on a 3170


So we are in a bit of a pickle with our 3170.  We would like to use Flash Cache, 10GB, and maximize our loops (6), but we see no way to do this with only 4 PCI slots.  If we were to use 1 slot for 4 port fiber (2 loops), another for 10GB ethernet, another for Flash Cache, and then the final slot for SAS, how many SAS shelves could we "stack" off the single quad port SAS adapter?  I thought I read the limit was pretty high, and you just needed to be aware of perfomance if you chain too many shelves.  Is this correct?  Would we be able to load our 3170 with SAS shelves in this scenario?





Limit for DS4243 is 10 shelves in a stack (240 drives). So two quad SAS HBA will max out FAS3170 (840 drives max) leaving you one slot for 10G and another one for PAM II.

Unless you plan to build metrocluster I'd use SAS shelves exclusively.


One additional piece which should be factored in is the disk size used (2TB SATA) and whether or not you're running Data ONTAP 8.x or 7.3.x.  If you're not running 8.x and you plan on using 2TB SATA drives you'll need to cut your disk count in half. 


To circle back and pick up the question of throughput/performance......most times SAN admins measure performance in IOPS, which for loops negates the discussion of raw MB/sec of throughput.  If, however, you are needing to plan for throughput as well as IOPS, let's use the currrent specs for SAS and go with 12Gbps per stack connection (assumes non-MPHA here).  The rule of thumb on FC-AL was 4x shelves per 4Gbps non-MPHA, so along those lines 4Gbps would support about 56 drives (or somewhere around 7MB/sec sustained from each disk).  Using some extrapolation that would mean than in a non-MPHA SAS stack we could support about 168 disks for throughput.  Since best practice uses MPHA connections, and the previous author is assuming this by needing 2x quad port SAS cards for 4 stacks, you would double this number and easily exceed the 240 drive limit of a stack.  Hope that helps as well!