[LUDOC-485] document llite.*.read_ahead_range_kb parameter

Related — Fri, 8 Jan 2021 00:20:01 +0000

Add a description of the llite.*.read_ahead_range_kb parameter to LustreProc.xml. From patch https://review.whamcloud.com/38916 "LU-13669 llite: try to improve mmap performance":

    So the pattern could be something like this:
    
    [1M data] [hole] [0.5M data] [hole] [0.7M data] [1M data]
    
    Every time an application reads mmap data, it may not only
    read a single 4KB page, but also a cluster of nearby pages in
    a range(e.g. 1MB) of the first page after a cache miss.
    
    The readahead engine is modified to track the range size of
    a cluster of mmap reads, so that after a seek and/or cache miss,
    the range size is used to efficiently prefetch multiple pages
    in a single RPC rather than many small RPCs.

Whamcloud Community JIRA

[LUDOC-485] document llite.*.read_ahead_range_kb parameter