Passthrough-read enables the FPolicy server (functioning as the hierarchical storage management (HSM) server) to provide read access to offline files without having to recall the file from the secondary storage system to the primary storage system.

When an FPolicy server is configured to provide HSM to files residing on a SMB server, policy-based file migration occurs where the files are stored offline on secondary storage and only a stub file remains on primary storage. Even though a stub file appears as a normal file to clients, it is actually a sparse file that is the same size of the original file. The sparse file has the SMB offline bit set and points to the actual file that has been migrated to secondary storage.

Typically when a read request for an offline file is received, the requested content must be recalled back to primary storage and then accessed through primary storage. The need to recall data back to primary storage has several undesirable effects. Among the undesirable effects is the increased latency to client requests caused by the need to recall the content before responding to the request and the increased space consumption needed for recalled files on the primary storage.

FPolicy passthrough-read allows the HSM server (the FPolicy server) to provide read access to migrated, offline files without having to recall the file from the secondary storage system to the primary storage system. Instead of recalling the files back to primary storage, read requests can be serviced directly from secondary storage.

Copy Offload (ODX) is not supported with FPolicy passthrough-read operation.

Passthrough-read enhances usability by providing the following benefits:

Read requests can be serviced even if the primary storage does not have sufficient space to recall requested data back to primary storage.
Better capacity and performance management when a surge of data recall might occur, such as if a script or a backup solution needs to access many offline files.
Read requests for offline files in Snapshot copies can be serviced.

Because Snapshot copies are read-only, the FPolicy server cannot restore the original file if the stub file is located in a Snapshot copy. Using passthrough-read eliminates this problem.
Policies can be set up that control when read requests are serviced through access to the file on secondary storage and when the offline file should be recalled to primary storage.

For example, a policy can be created on the HSM server that specifies the number of times the offline file can be accessed in a specified period of time before the file is migrated back to primary storage. This type of policy avoids recalling files that are rarely accessed.

How read requests are managed when FPolicy passthrough-read is enabled

You should understand how read requests are managed when FPolicy passthrough-read is enabled so that you can optimally configure connectivity between the storage virtual machine (SVM) and the FPolicy servers.

When FPolicy passthrough-read is enabled and the SVM receives a request for an offline file, FPolicy sends a notification to the FPolicy server (HSM server) through the standard connection channel.

After receiving the notification, the FPolicy server reads the data from the file path sent in the notification and sends the requested data to the SVM through the passthrough-read privileged data connection that is established between the SVM and the FPolicy server.

After the data is sent, the FPolicy server then responds to the read request as an ALLOW or DENY. Based on whether the read request is allowed or denied, ONTAP either sends the requested information or sends an error message to the client.