Lustre 1.6 Operations Manual

9.1.5 Known Issues with Quotas
Using quotas in Lustre can be complex and there are several known issues.
9.1.5.1 Granted Cache and Quota Limits
In Lustre, granted cache does not respect quota limits. In this situation, OSTs grant
cache to Lustre client to accelerate I/O. Granting cache causes writes to be successful
in OSTs, even if they exceed the quota limits, and will overwrite them.
The sequence is:
1. A user writes files to Lustre.
2. If the Lustre client has enough granted cache, then it returns ‘success’ to users
and arranges the writes to the OSTs.
3. Because Lustre clients have delivered success to users, the OSTs cannot fail
these writes.
Because of granted cache, writes always overwrite quota limitations. For example, if
you set a 400 GB quota on user A and use IOR to write for userA from a bundle of
clients, you will write much more data than 400 GB, and cause an out-of-quota error
(-EDQUOT).
Note – The effect of granted cache on quota limits can be mitigated, but not
eradicated. Reduce the max_dirty_buffer in the clients (just like echo XXXX >
/proc/fs/lustre/osc/lustre-OST*/max_dirty_mb).
9-10 Lustre 1.6 Operations Manual • September 2008