Your understanding of the behavior upon expansion pertains to VNX OE for Block v31.  However, if you are running VNX OE for Block v32+:  1) With *or* without the FAST (VP) enabler, a pool expansion triggers a "capacity rebalance"  Without regards to relative performance, it simply balances the existing slices across the newly introduced drives leveling off the pool from a capacity perspective.  I will mention it though, if you expand the pool via CLI you have the ability to override this, and it will behave just as you described (as it was with VNX OE for Block v31).  Then...   2a) If you do *not* have the FAST (VP) enabler loaded, then once the initial capacity rebalance completes there isn't any more relocation of slices once they have been allocated.  Well... until you expand the pool again.   2b) If you do have the FAST (VP) enabler loaded *and* an active tiering policy, you are eligible for "performance rebalancing" within a tier  The system tracks active slices, and will make an effort to remove hot spots also within a tier; and therefore, applies even if the pool has just a single tier.  You can actually see this in the Tiering tab of the pool properties under the column "Move within (GB)".  It should also make sense that in order to shuffle the slices amongst the private RAID Groups, you should leave some unallocated space and not fully provision all the available capacity in the pool.    While the system will make an effort to maintain 10% free per tier (really only relates to pools with multiple tiers), generally around 5 - 10% is a recommended value so there is space to shuffle the slices.  However, arguably even with at least 1GB free and the system has just enough to shuffle the slices but not as efficiently.
Hello,  We have a VNX on which a storage pool with all SAS drives, some of the RAID groups are experiencing high load due to the I/Os while the others seems to be doing fairely fine. Since this impacts on performance of our application, EMC recmmended adding more drives to the pool. But i am confused/ not sure as to how this will help in resolving the perfromance hit. Per my understanding adding more drives will go to new RAID group, since i already see other RAID groups doing fine and that doesn't nowhere effect/impact on performance hit RAID groups, how will having a new RAID group help?Advice/Inputs...???
Help with VNX 5700 (block)