Viewing OracleASM attributes (including hidden)
A nice and informative note about how to obtain OracleASM attributes, including “hidden attributes” can be found here. Including the special attribute “_rebalance_compact” which affects the post-rebalance ‘compact’ ASM operation, when altering DiskGroups (per-DiskGroup, of course).
Just a reminder about this specific attribute – when ASM performs rebalance operation (adding or dropping disks) it attempts to ‘defrag’ the data in the existing disks, eliminating empty spaces, and trying to compact the data on the beginning of the disks – to reduce seek time and latency when accessing the data. For SSDs, this is counter-productive, since SSD disks (and in general – all flash arrays) do not suffer from any ill effect of fragmentation, and large block migrations will only reduce their expected time-to-live. You are invited to read more about SSD physics and how they are different from rotational disks.