HP CIFS Server 3.0f Administrator's Guide verison A.02.03

11 CIFS File System Module (CFSM) Support
This chapter describes the CIFS File System Module (CFSM) support, it contains the following sections:
• Using the CIFS File System Module (CFSM) for Concurrent NFS Client Access
• Stacking CFSM
• Using CFSM with Other Stackable File System Modules
• CFSM Implemented as Dynamically Loadable Kernel Modules (DLKMs)
• Special Issues When Using CFSM
• CFSM Tracing
Using the CIFS File System Module (CFSM) for Concurrent NFS Client
Access
Due to differences in file locking between CIFS (Windows) environments and POSIX (UNIX) environments,
accessing files from both CIFS clients and NFS clients can create some risks. CIFS file locks are expected to
be enforced, preventing files from being read, written or even renamed by other clients. In a POSIX
environment, file locks typically only prevent other processes from creating a conflicting file lock, as only file
permissions are checked before reading, writing or renaming a file.
The well behaved POSIX programs should always request a lock on a file if the file is to be accessed by
more than one process concurrently. But there is no guarantee that a POSIX program may not read, write
or rename a file, ignoring any file locks owned by other processes. If these cases were to happen, it could
cause file corruption.
Also, the performance enhancing feature of CIFS called “opportunistic locking” (or OpLocks) could
be safely used if the shared files were only accessed by CIFS clients. If other local programs or NFS clients
accessed the same files, it might cause file corruption.
To address these issues, the HP CIFS Server implements the CIFS File System Module (CFSM). CFSM is a
stackable file system module that takes advantage of the Stackable File System architecture introduced in
HP-UX 11i v3 and provides a method that allows CIFS file locks to inter-operate with accesses from NFS
clients and other HP-UX processes.
Enabling the CFSM functionality prevents the possibility of file corruption due to concurrent file accesses
from both CIFS and NFS, and allows for performance enhancing opportunistic locks to be safely used. All
the locks (CIFS record locks, share mode locks and opportunistic locks) that are granted to CIFS clients are
enforced by CFSM. NFS clients and other local users can then concurrently access files with CIFS clients
with no risk of file corruption, even with opportunistic locking turned on.
NOTE: The CIFS File System Module (CFSM) is only available with the HP CIFS Server on HP-UX 11i v3
and later.
Using the CIFS File System Module (CFSM) for Concurrent NFS Client Access 145