Lustre 1.6 Operations Manual

The following tunables, providing more control over allocation policy, will be
available in the next version:
Field
stats
max_to_scan
min_to_scan
order2_req
small_req
large_req
prealloc_table
group_prealloc
Description
Enables/disables the collection of statistics. Collected statistics can be
found in /proc/fs/ldiskfs2//mb_history.
Maximum number of free chunks that mballoc finds before a final
decision to avoid livelock.
Minimum number of free chunks that mballoc finds before a final
decision. This is useful for a very small request, to resist fragmentation of
big free chunks.
For requests equal to 2^N (where N >= order2_req), a very fast search via
buddy structures is used.
All requests are divided into 3 categories:
< small_req (packed together to form large, aggregated requests)
< large_req (allocated mostly in linearly)
> large_req (very large requests so the arm seek does not matter)
The idea is that we try to pack small requests to form large requests, and
then place all large requests (including compound from the small ones)
close to one another, causing as few arm seeks as possible.
The amount of space to preallocate depends on the current file size. The
idea is that for small files we do not need 1 MB preallocations and for
large files, 1 MB preallocations are not large enough; it is better to
preallocate 4 MB.
The amount of space preallocated for small requests to be grouped.
Chapter 20 LustreProc 20-23