VMM

From Wikistix

VMM 

Virtual Memory Manager. The part of an AIX kernel reponsible for managing memory allocation, and on modern kernels, file I/O. All reads and writes go via the VMM, except if options like O_DIRECT or dio are used.

Under topas, file I/O requiring disk access is also seen in the pagein and pageout counters. The pgspin and pgspout counters are specifically related to I/O to and from paging spaces.