Lustre 1.6 Operations Manual

Number of groups scanned (grps column) should be small. If it reaches a few dozen
often, then either your disk filesystem is pretty fragmented or mballoc is doing
something wrong in the group selection part.
20.2.8 mballoc3 Tunables
Lustre version 1.6.1 and later includes mballoc3, which was built on top of mballoc2.
By default, mballoc3 is enabled, and adds these features:
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■
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Pre-allocation for single files (helps to resist fragmentation)
Pre-allocation for a group of files (helps to pack small files into large, contiguous
chunks)
Stream allocation (helps to decrease the seek rate)
The following mballoc3 tunables are currently available:
Field
stats
max_to_scan
min_to_scan
order2_req
stream_req
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.
Requests smaller or equal to this value are packed together to form large
write I/Os.
20-22 Lustre 1.6 Operations Manual • September 2008