By default, Samba as a Domain Controller with an LDAP backend needs to use the Unix-style NSS subsystem to access user and group information. Due to the way Unix stores user information in /etc/passwd and /etc/group this inevitably leads to inefficiencies. One important question a user needs to know is the list of groups he is member of. The plain UNIX model involves a complete enumeration of the file /etc/group and its NSS counterparts in LDAP. UNIX has optimized functions to enumerate group membership. Sadly, other functions that are used to deal with user and group attributes lack such optimization.
To make Samba scale well in large environments, the yes option assumes that the complete user and group database that is relevant to Samba is stored in LDAP with the standard posixAccount/posixGroup attributes. It further assumes that the Samba auxiliary object classes are stored together with the POSIX data in the same LDAP object. If these assumptions are met, yes can be activated and Samba can bypass the NSS system to query user group memberships. Optimized LDAP queries can greatly speed up domain logon and administration tasks. Depending on the size of the LDAP database a factor of 100 or more for common queries is easily achieved.