Troubleshooting guide

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Cisco Wide Area Application Services Configuration Guide
OL-26579-01
Chapter 1 Configuring File Services
Overview of File Services Features
Data Concurrency
Concurrency control is important when multiple users access the same cached data to read, or write, or
both. Concurrency control synchronizes this access by establishing and removing file system locks. This
file-locking feature ensures data integrity and provides the following benefits:
• Enables a client to aggressively cache file data so it does not have to rely on retrieving data from the
remote file server.
• Provides a performance boost in many applications running on existing CIFS client
implementations.
• Preserves data integrity because only one user at a time can make changes to a section of a file.
WAAS software supports the CIFS oplocks feature, which allows a user to lock a file so the user can
safely read and write data to its local cache instead of using network bandwidth to perform these
functions over the WAN on the file server. By using oplocks, a user can proactively cache read-ahead
data because it knows that no other user is accessing the file so there is no chance the cached data can
become stale. The user can also write data to its local cache and does not need to update the file server
until it closes the file or until another user requests to open the same file.
Oplocks only applies to files. The file server does not grant oplock requests on directories and named
pipes.
File-Locking Process
When a user opens a file, it sends a lock request to the file server. The Edge WAE intercepts and forwards
all lock requests from the user to the file server as well as all responses from the file server to the user.
If no other user has a lock on the file, the file server grants an exclusive lock request so that the user can
safely cache the file.
If a second user requests to open the same file, the following actions occur:
1. The file server revokes the exclusive file lock obtained by the first user.
2. The first user performs the following actions:
– Flushes any file changes stored in its cache to the file server. This action ensures that the second
user opening the file receives the latest information from the file server.
– Deletes any of its read-ahead buffers for the file because that data is no longer guaranteed to
remain up-to-date now that a second user will open the file.
3. The file server allows the second user to open the file.
Prepositioning
The prepositioning feature allows system administrators to proactively “push” frequently used files from
the central storage into the cache of selected Edge WAEs. This operation provides users with faster
first-time file access, and makes more efficient use of available bandwidth. You create preposition
directives from the WAAS Central Manager GUI.
When an end user attempts to open a file that is not found in the Edge WAE cache, the Edge WAE
retrieves it across the WAN from the file server where it is stored. Prepositioning is a feature that allows
administrators to push large, frequently accessed files from file servers to selected Edge WAE caches
according to a predefined schedule. Through the proper use of prepositioning, administrators can allow