Windows® Internals, Sixth Edition, Part 2

Like local FSDs, client-side remote FSDs usually use cache manager services to locally cache file data belonging to remote files and directories, and in such cases both must implement a distributed locking mechanism on the client as well as the server. SMB client-side remote FSDs implement a distributed cache coherency protocol, called oplock (opportunistic locking), so that the data an application sees when it accesses a remote file is the same as the data applications running on other computers that are accessing the same file see. Third-party file systems may choose to use the oplock protocol, or they may implement their own protocol. Although server-side remote FSDs participate in maintaining cache coherency across their clients, they don’t cache data from the local FSDs because local FSDs cache their own data.

Locking

It is fundamental that whenever a resource can be shared between multiple, simultaneous accessors, a serialization mechanism must be provided to arbitrate writes to that resource to ensure that only one accessor is writing to the resource at any given time. Without this mechanism, the resource may be corrupted. The locking mechanisms used by all file servers implementing the SMB protocol are the oplock and the lease. Which mechanism is used depends on the capabilities of both the server and the client, with the lease being the preferred mechanism.

Oplocks The oplock functionality is implemented in the file system run-time library (FsRtlXxx functions) and may be used by any file system driver. The client of a remote file server uses an oplock to dynamically determine which client-side caching strategy to use to minimize network traffic. An oplock is requested on a file residing on a share, by the file system driver or redirector, on behalf of an application when it attempts to open a file. The granting of an oplock allows the client to cache the file rather than send every read or write to the file server across the network. For example, a client could open a file for exclusive access, allowing the client to cache all reads and writes to the file, and then copy the updates to the file server when the file is closed. In contrast, if the server does not grant an oplock to a client, all reads and writes must be sent to the server.

Once an oplock has been granted, a client may then start caching the file, with the type of oplock determining what type of caching is allowed. An oplock is not necessarily held until a client is finished with the file, and it may be broken at any time if the server receives an operation that is incompatible with the existing granted locks. This implies that the client must be able to quickly react to the break of the oplock and change its caching strategy dynamically.

Prior to SMB 2.1, there were four types of oplocks:

Level 1, exclusive access This lock allows a client to open a file for exclusive access. The client may perform read-ahead buffering and read or write caching.

Level 2, shared access This lock allows multiple, simultaneous readers of a file and no writers. The client may perform read-ahead buffering and read caching of file data and attributes. A write to the file will cause the holders of the lock to be notified that the lock has been broken.

Batch, exclusive access This lock takes its name from the locking used when processing batch (.bat) files, which are opened and closed to process each line within the file. The client may keep a file open on the server, even though the application has (perhaps temporarily) closed the file. This lock supports read, write, and handle caching.

Filter, exclusive access This lock provides applications and file system filters with a mechanism to give up the lock when other clients try to access the same file, but unlike a Level 2 lock, the file cannot be opened for delete access, and the other client will not receive a sharing violation. This lock supports read and write caching.

In the simplest terms, if multiple client systems are all caching the same file shared by a server, then as long as every application accessing the file (from any client or the server) tries only to read the file, those reads can be satisfied from each system’s local cache. This drastically reduces the network traffic because the contents of the file are not sent to each system from the server. Locking information must still be exchanged between the client systems and the server, but this requires very low network bandwidth. However, if even one of the clients opens the file for read and write access (or exclusive write), then none of the clients can use their local caches and all I/O to the file must go immediately to the server, even if the file is never written. (Lock modes are based upon how the file is opened, not individual I/O requests.)