3PAR: Considerations when mixing sizes of same disk type

Generally it is supported to mix disks of different sizes of the same type within a 3PAR system. For example you can use 900GB and 1.2TB FC-disks – within the same cage and even within the same CPG. When a disk fails, HPE sends an replacement disk. Some time ago, stock of 900GB FC disks seem to be empty. So when a 900GB disk fails, you will probably get a 1.2TB disk instead.

So how to handle different disk sizes? Here are a few points to consider:

1. 1. How do a 3PAR system handle different sizes within the same CPG? The system tries to put he same amount of data on every disk in a CPG – no matter if there a different disk-sizes. When the smaller disk are full, larger disks continue to fill up. So replacing just a few disks within a CPG with larger disks does not matter – as long as smaller disks not running full. When this happens, just larger disks gets new data. This can lead to a serious performance problem.
   2. When talking about SSDs: mixing different sizes will probably be no problem. Even when you think of point 1. But: when your SSDs are near the performance maximum you can also get an performance problem after smaller SSDs are full.
   3. When you have different CPGs for different disk sizes (how this can be done, you can read here), you must check before replacing a failed disk by a disk of a new size. Will the replaced disk be part of the right CPG? If not, your should re-define you CPG disk filter. By the way, this cannot be done in SSMC any more! You need CLI. See point 4.
   4. What about filtering disks for CPGs by cage or position in cage instead of disk size? Since I know, HPE replaces 900GB disks by 1.2TB disks, this is my preferred option, when different CPGs are desired.
      For example you can use this command to change the disk filter for an existing CPG:
      setcpg -sdgs 32g -t r6 -ha mag -ssz 10 -p -devtype NL -cg 2 -saga „-ha -p -devtype NL -cg 2“ NL_r6_cage2
      The meaning of the different parameters, you can find here (Option -devtype is mandatory for option -cg, which is for cage selection. You can list more than one cage by list them separated by comma (1,2), or define as range (1-3). Another option is to define filter as positions of disks. Check 3PAR Command Line Reference for more information – command: setcpg.

Notes

• Read here more about setsize to disk count ratio in 3PAR arrays.