See here for an explanation of how our writes work.
Basically Aerospike writes in 128KB write blocks which may consist of many records. An update to a record will cause it to be rewritten and the space occupied to be deducted from the write block’s use size (similar for deletes and expirations and evictions but without the rewriting of the record). The used % disk is computed from the aggregate wblock usage divided by the size of the disk. The white area of this graph would be the portion of the in use wblocks that are not currently occupied. Eventually some of this space will be reclaimed by defrag when a wblocks usage drops below the defrag-lwm-pct threshold.
The Avail % is basically the wblocks that are on the free queue which are not currently in use.
No id doesn’t cout against the disk high watermark.
You still have 20% 
.