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Handling of users that belong to many groups

Users can belong to many different (UNIX) groups. These groups are generally used to allow or deny permissions for executing commands or access to files and directories.

The number of groups a user can belong to depends on the operating system, but there are also components that support fewer groups. In Gluster, there are different restrictions on different levels in the stack. The explanations in this document should clarify which restrictions exist, and how these can be handled.


  • if users belong to more than 90 groups, the brick processes need to resolve the secondary/auxiliary groups with the server.manage-gids volume option
  • the linux kernels /proc filesystem provides up to 32 groups of a running process, if this is not sufficient the mount option resolve-gids can be used
  • Gluster/NFS needs nfs.server-aux-gids when users accessing a Gluster volume over NFS belong to more than 16 groups

For all of the above options counts that the system doing the group resolving must be configured (nsswitch, sssd, ..) to be able to get all groups when only a UID is known.

Limit in the GlusterFS protocol

When a Gluster client does some action on a Gluster volume, the operation is sent in an RPC packet. This RPC packet contains an header with the credentials of the user. The server-side receives the RPC packet and uses the credentials from the RPC header to perform ownership operations and allow/deny checks.

The RPC header used by the GlusterFS protocols can contain at most ~93 groups. In order to pass this limit, the server process (brick) receiving the RPC procedure can do the resolving of the groups locally, and ignore the (too few) groups from the RPC header.

This requires that the service process can resolve all of the users groups by the UID of the client process. Most environments that have many groups, already use a configuration where users and groups are maintained in a central location. for enterprises it is common to manage users and their groups in LDAP, Active Directory, NIS or similar.

To have the groups of a user resolved on the server-side (brick), the volume option server.manage-gids needs to be set. Once this option is enabled, the brick processes will not use the groups that the Gluster clients send, but will use the POSIX getgrouplist() function to fetch them.

Because this is a protocol limitation, all clients, including FUSE mounts, Gluster/NFS server and libgfapi applications are affected by this.

Group limit with FUSE

The FUSE client gets the groups of the process that does the I/O by reading the information from /proc/$pid/status. This file only contains up to 32 groups. If client-side xlators rely on all groups of a process/user (like posix-acl), these 32 groups could limit functionality.

For that reason a mount option has been added. With the resolve-gids mount option, the FUSE client calls the POSIX getgrouplist() function instead of reading /proc/$pid/status.

Group limit for NFS

The NFS protocol (actually the AUTH_SYS/AUTH_UNIX RPC header) allows up to 16 groups. These are the groups that the NFS-server receives from NFS-clients. Similar to the way the brick processes can resolve the groups on the server-side, the NFS-server can take the UID passed by the NFS-client and use that to resolve the groups. the volume option for that is nfs.server-aux-gids.

Other NFS-servers offer options like this too. The Linux kernel nfsd server uses rpc.mountd --manage-gids. NFS-Ganesha has the configuration option Manage_Gids.

Implications of these solutions

All of the mentioned options are disabled by default. one of the reasons is that resolving groups is an expensive operation. in many cases there is no need for supporting many groups and there could be a performance hit.

When groups are resolved, the list is cached. the validity of the cache is configurable. the Gluster processes are not the only ones that cache these groups. nscd or sssd also keep a cache when they handle the getgroupslist() requests. When there are many requests, and querying the groups from a centralized management system takes long, caches might benefit from a longer validity.

An other, less obvious difference might be noticed too. Many processes that are written with security in mind reduce the groups that the process can effectively use. This is normally done with the setegids() function. When storage processes do not honour the fewer groups that are effective, and the processes use the UID to resolve all groups of a process, the groups that got dropped with setegids() are added back again. this could lead to permissions that the process should not have.