Unified Buffer Cache

To increase file system performance, Digital implemented a Unified Buffer Cache (UBC) fully integrated with the file system that caches file system data and can grow or shrink upon demand. Unlike the conventional Buffer Cache which is configured and allocated at boot-time and which relies on bcopy routines to move data in and out of memory, the UBC references the same physical pages as virtual memory and can use map operations rather than bcopy routines to access data, thereby increasing system performance. In addition, since the UBC contains only file system data, the Buffer Cache only needs to cache metadata, requiring only 3% of physical memory rather than the 25% required by previous versions of the operating system.

By default, the UBC can grow to consume all of physical memory so that the system can determine dynamically the percent of memory that should be allocated to the UBC. However, the maximum percent of memory that the UBC can grow to is configurable and can be set in the system configuration file by defining the ubc-maxpercent variable.