VxFS allows an application to set cache advisories for use when accessing files. VxFS cache advisories enable applications to help monitor the buffer cache and provide information on how better to tune the buffer cache to improve performance gain.
The basic function of the cache advisory is to let you know whether you could have avoided a later re-read of block X if the buffer cache had been a little larger. Conversely, the cache advisory can also let you know that you could safely reduce the buffer cache size without putting block X into jeopardy.
These advisories are in memory only and do not persist across reboots. Some advisories are currently maintained on a per-file, not a per-file-descriptor, basis. Only one set of advisories can be in effect for all accesses to the file. If two conflicting applications set different advisories, both must use the advisories that were last set.