If your id mappings are meaningful and correct, there will be only one on-disk identity for files per 'real' user, either Windows-stye (AD) or UNIX-style (LDAP),  depending on your actual set-up.  To check what applies to your Isilon cluster, this should be helpful: Multiprotocol concepts series part 3: On-disk identity  For quotas it will be easy then: You need to set only one quota definition, either for the Windows or for the UNIX user.  hth  -- Peter
I have a Isilon cluster joined to AD domain and LDAP domain simultaneously. Now I want use quota feature to give each user 1TB space, each user also joined AD and LDAP simultaneously and established mapping relations between AD user and LDAP user. I want each two related user in AD and LDAP(one in fact) share the 1TB  quota, How can I do it? 
Set Quota for both AD and LDAP users?