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Lustre 1.6 Operations Manual

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20.1.5 Free Space Distribution<br />

The free-space stripe weighting is set to give a priority of "0" to the free space<br />

(versus trying to place the stripes "widely" -- nicely distributed across OSSs and<br />

OSTs to maximize network balancing).<br />

You can adjust this priority via the proc file:<br />

$ cat /proc/fs/lustre/lov/-mdtlov/qos_prio_free<br />

Currently, the default is 90%. You can permanently set this value by running this<br />

command on the MGS:<br />

$ ctl conf_param -MDT0000.lov.qos_prio_free=90<br />

Setting the priority to 100% just means that OSS distribution does not count in the<br />

weighting, but the stripe assignment is still done via weighting. If OST2 has twice as<br />

much free space as OST1, it will be twice as likely to be used, but it is NOT<br />

guaranteed to be used.<br />

Also note that free-space stripe weighting does not activate until two OSTs are<br />

imbalanced by more than 20%. Until then, a faster round-robin stripe allocator is<br />

used. (The new round-robin order also maximizes network balancing.)<br />

Chapter 20 <strong>Lustre</strong>Proc 20-11

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