.Q.MAP[]

Keeps partitions mapped to avoid the overhead of repeated file system calls during a select. (Since V3.1.)

For use with partitioned HDBS, used in tandem with \l dir

q)\l .
q).Q.MAP[]

NOT recommended for use with compressed files, as the decompressed maps will be retained, using physical memory|swap.

You may need to increase the number of available file handles, and also the number of available file maps

For Linux see vm.max_map_count.