The Logical Volume Manager (LVM) uses a construct called a "pbuf" to control a pending disk I/O. In AIX Version 3, one pbuf is required for each page being read or written. In systems that do large amounts of sequential I/O, this can result in depletion of the pool of pbufs. The vmtune command can be used to increase the number of pbufs to compensate for this effect.
In AIX Version 4, a single pbuf is used for each sequential I/O request, regardless of the number of pages involved. This greatly decreases the probability of running out of pbufs.