Recommended access order (RAO) open function

RAO was first introduced on IBM enterprise tape drives in the 3592-E07 (TS1140). RAO enables tape control applications to accelerate the retrieval of a certain number of files from a single tape thereby reducing the seek time between those files.

A feature of the LTO-9 and later full-height drives is the ability to accept a list of User Data Segments and reorder those User Data Segments into a recommended access order that minimizes the locate portion of the time to read those User Data Segments. This sorted list is called a Recommended Access Order (RAO) list. A User Data Segment (UDS) is defined as a grouping of contiguous logical objects (i.e., logical blocks and filemarks) and is described by partition number, beginning logical object identifier, and ending logical object identifier.

The RAO implementation in LTO produces the best results for performance enhancement when there is little variability in block size or data compression ratio. When the variability in compression ratio or block sizes increase, the accuracy of the locate estimates may be reduced and any potential performance enhancements may be diminished.