The release-behind mechanism

Release-behind is a mechanism for JFS and Enhanced JFS under which pages are freed as soon as they are either committed to permanent storage by writes or delivered to an application by reads. This solution addresses a scaling problem when performing sequential I/O on very large files whose pages will not be re-accessed in the near future.

When writing a large file without using release-behind, writes will go very fast whenever there are available pages on the free list. When the number of pages drops to the value of the minfree parameter, VMM uses its Least Recently Used (LRU) algorithm to find candidate pages for eviction. As part of this process, VMM needs to acquire a lock that is also being used for writing. This lock contention might cause a sharp performance degradation.

You can enable release-behind by specifying either the release-behind sequential read (rbr) flag, the release-behind sequential write (rbw) flag, or the release-behind sequential read and write (rbrw) flag when issuing the mount command.

A side effect of using the release-behind mechanism is an increase in CPU utilization for the same read or write throughput rate compared to without using release-behind. This is due to the work of freeing pages, which would normally be handled at a later time by the LRU daemon. Also note that all file page accesses result in disk I/O since file data is not cached by VMM.

You can use the mount -o rbr command to use release-behind for NFS.