Here is the concern with Cost mode. You wind up with most of your data in the NL since there is little or no hot data. The issue you then have in a DR scenario is high latency as the array warms up and then potentially days of delays as AO bring the hot blocks back to life. We feel this pain every DR test we do. Also if you run cost mode make sure to set CPG warnings on your lowest tier as to make sure you never fill your NL drives. AO will use the CPG growth warning to prevent AO from going past that number on that CPG when moving data in.
Side note, if you set the warning to a number lower than the current CPG size AO will NOT attempt to move data out to meet the new lower threshold. You still may get the CPG to shrink as AO naturally moves out hot blocks to go back up, but it will not aggressively try and vacate the excess.
Also, keep in mind that if a higher tier fills up and there is no NL space then you can have a deny write situation. So if you fill a FC tier and have NL still available (regardless if it is in the AO configuration for the volume) then the array will not deny the FC write, but will instead defer it to NL. We are still waiting on 3par to confirm what happens in an array that only has SSD and FC drives, if the SSD fills with the write be deferred to FC if available?
For us we set all our DR array AO configs to performance to keep as much data up top as possible even if some of it is un-accessed. With AO set to cost ont he DR side our NL was ruinng over 95% utilized and our FC was under 40% utilized, again very painful when testing, not to mention if we had a real DR. Now our FC runs 90% in FC and less than 85% NL.
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