Lustre 1.6 Operations Manual

20.2.6 Mechanics of Lustre Readahead
Readahead is a method of reading part of a file's contents into memory with the
expectation that a process working with the file will soon want the data. When
readahead works well, a data-consuming process finds that the information it needs
is available when it asks, and waiting for disk I/O is not necessary.
Lustre readahead is triggered when two or more sequential reads by an application
fail to be satisfied by the Linux buffer cache. The size of the initial readahead is 1
MB. Additional readaheads grow linearly and increment until the readahead cache
on the client is full at 40 MB.
/proc/fs/lustre/llite/-/max_read_ahead_mb
This tunable controls the maximum amount of data readahead on a file. Files are
read ahead in RPC-sized chunks (1 MB or the size of read() call, if larger) after the
second sequential read on a file descriptor. Random reads are done at the size of the
read() call only (no readahead). Reads to non-contiguous regions of the file reset the
readahead algorithm, and readahead is not triggered again until there are sequential
reads again. Setting this tunable to 0 disables readahead. The default value is 40 MB.
/proc/fs/lustre/llite/-/max_read_ahead_whole_mb
This tunable controls the maximum size of a file that is read in its entirety, regardless
of the size of the read().
Chapter 20 LustreProc 20-19