We will be implementing D910 to some of our IBM i systems, migrating from 520FB format. I'd like to go to 70G LUN on D910. Is there any technical documents out there about performance impact on different LUN sizes for IBM i systems Or Best Practice is selecting LUN sizes.
Thanks and input is highly appreciated.
The D910 device type is an enabler for IBM i to be able to use thin devices (and FAST). Priot to thin, a device was limited from a storage performance perspective by the physical spindle count related to the RAID type used. A small number of disks effectively. Now with thin, the device has access to all of the disk resources in the pool, so really the back-end limitation has gone away. One of the benefits of thin is the ability to use larger devices without the risk or running out of array back-end resources.
It matters in a couple of ways. In theory a D910 could be 2TB, but the maximum devices size on VMAX-2 is around 240GB, so the maximum size is the EMC size. The other consideration is with synchronous SRDF. Will these be SRDF/S devices?
There is a limit on the number of concurrent in-flight writes to an SRDF/S device and in some circumstances a larger number of devices is better. There can be up to one outstanding write per device per FA slice with SRDF/S.