Lustre 1.6 Operations Manual

29.1.2 Description
The group upcall file contains the path to an executable that, when properly
installed, is invoked to resolve a numeric UID to a group membership list. This
utility should complete the mds_grp_downcall_data data structure (see Data
structures) and write it to the /proc/fs/lustre/mds/mdsservice/group_info
pseudo-file.
For a sample upcall program, see lustre/utils/l_getgroups.c in the Lustre
source distribution.
29.1.2.1 Primary and Secondary Groups
The mechanism for the primary/supplementary group is as follows:
■ The MDS issues an upcall (set per MDS) to map the numeric UID to the
supplementary group(s).
■ If there is no upcall or if there is an upcall and it fails, supplementary groups will
be added as supplied by the client (as they are now).
■ The default upcall is "NONE", which means the current behavior is used. This can
be set to /usr/sbin/l_getgroups to use the Lustre group-supplied upcall,
which looks up the UID in /etc/passwd, and if it finds the UID, it looks for
supplementary groups in /etc/group for that username. You are free to enhance
the l_getgroups to look at an external database for supplementary groups
information.
■ To set the upcall, echo {path} >
/proc/fs/lustre/mds/{mdsname}/group_upcall or add the --
group_upcall {path} line to the --add mds entry in the lmc script. You can
also override this at startup time with a --group_upcall {path} option to
lconf.
■ To avoid repeated upcalls, the supplementary group information is cached by the
MDS. The default cache time is 300 seconds, but can be changed via
/proc/fs/lustre/mds/{mdsname}/group_expire. The kernel will wait, at
most, 5 seconds (by default,
/proc/fs/lustre/mds/{mdsname}/group_acquire_expire changes) for
the upcall to complete and will take the "failure" behavior as described above. It is
possible to flush cached entries by writing to the
/proc/fs/lustre/mds/{mdsname}/group_flush file.
29-2 Lustre 1.6 Operations Manual • September 2008