Disconnected operations

About disconnected operations

The distributed NFS appliance normally communicates with the origin server to

Serve initial MOUNT requests
Ensure that the cached content is current and consistent with that on the origin server (configured using the maximum-age and verification schedule options)
Retrieve data that is cached (request misses)
Write data that is not cacheable by definition

Disconnected operations is a mode that enables the distributed NFS appliance to serve as much data to clients as possible when a connection to the origin server is unavailable. Server unavailability might be due to any of the following conditions:

The origin server is inoperable.
The origin server is unreachable.
Services on the origin server, such as NFS or exportfs permissions, are disabled.

If the previous conditions persist for more than a second or two, the distributed NFS cache switches to disconnected operations mode automatically.

Caching behavior during disconnected operations mode

Your distributed NFS appliance has limited file-serving capabilities when it operates in disconnected operations mode.

Some functions, such as write and write-like operations and read operations of missing files or missing blocks of files, can be blocked or can return an error message to the client.
All volume export operations are treated as read-only (all file modification requests are rejected by the appliance).
The appliance cannot verify object attributes prior to serving a request based on maximum age of content or verification schedules previously configured.

To monitor server health and connectivity, the appliance pings the origin server with a NULL NFS operation.

Troubleshooting inaccessible cached files

In addition to an unavailable or inoperable origin server, there are several reasons why files that had been cached might no longer exist on the appliance.

Because of a client request, a file might be evicted from the cache to be replaced by the updated content on the origin server. During this window, the file becomes inaccessible.
A file might be evicted from the cache because of low disk space.
Write-type operations might make files obsolete and, therefore, inaccessible.
Directories that include files that are deleted and created might also be evicted from the cache.

To reduce client opportunities that can flush content from a cache, you can make content available with a read-only export setting. The read-only export setting directs the appliance to reject all operations that would modify the file system, such as writing a file or creating directory. You can assign this export setting in the NFS - Exports page (Setup tab > NFS > Exports).