Lustre 1.6 Operations Manual

25.4.1 Round-Robin Allocator
When OSTs have approximately the same amount of free space (within 20%), an
efficient round-robin allocator is used. The round-robin allocator alternates stripes
between OSTs on different OSSs. Here are several sample round-robin stripe orders
(the same letter represents the different OSTs on a single OSS):
3: AAA one 3-OST OSS
3x3: ABABAB two 3-OST OSSs
3x4: BBABABA one 3-OST OSS (A) and one 4-OST OSS (B)
3x5: BBABBABA
3x5x1: BBABABABC
3x5x2: BABABCBABC
4x6x2: BABABCBABABC
25.4.2 Weighted Allocator
When the free space difference between the OSTs is significant, then a weighting
algorithm is used to influence OST ordering based on size and location. Note that
these are weightings for a random algorithm, so the "emptiest" OST is not,
necessarily, chosen every time. On average, the weighted allocator fills the emptier
OSTs faster.
25.4.3 Adjusting the Weighting Between Free Space and
Location
This priority can be adjusted via the /proc/fs/lustre/lov/lustremdtlov/qos_prio_free
proc file. The default is 90%. Use the following
command to permanently change this weighting on the MGS:
lctl conf_param -MDT0000.lov.qos_prio_free=90
Increasing the value puts more weighting on free space. When the free space priority
is set to 100%, then location is no longer used in stripe-ordering calculations, and
weighting is based entirely on free space.
Note that setting the priority to 100% means that OSS distribution does not count in
the weighting, but the stripe assignment is still done via a weighting—if OST2 has
twice as much free space as OST1, then OST2 is twice as likely to be used, but it is
not guaranteed to be used.
Chapter 25 Striping and I/O Options 25-9