When disks are added or removed from a diskgroup, ASM rebalances the blocks on the ASM disks so that the data is evenly spread across all ASM disks.
After blocks have been relocated, ASM enters a compaction phase where blocks are consolidated to the outer part of each disk, any evacuated block space is eliminated and the high water mark of each disk is reset.
This compaction phase is unnecessary on ScaleIO as the physical geometry of the disks is hidden from the application, and the LBA is no indication of the physical placement of a block on the storage media.
ASM Compaction may be disabled by setting the hidden parameter _DISABLE_REBALANCE_COMPACT to true in the ASM instances.
Alternatively, the hidden ASM diskgroup attribute _REBALANCE_COMPACT may be set to FALSE on a diskgroup by diskgroup basis.
Although this step may be disabled by the parameters shown above, Dell EMC does not recommend setting hidden parameters in ASM without the approval of Oracle Support.
Although the compaction phase is redundant on ScaleIO, leaving it enabled does not negatively impact performance once the rebalance operation has completed.