Configuring Celerra pass-through behavior for placeholder shortcuts

You can use EMC Celerra's read policy override with placeholder recalls, if required. The Celerra's -read_policy_override parameter determines how a read request is handled for a file in secondary storage. For example, you can opt to pass a file directly through to the client without recalling it to the Celerra. The Celerra Network Server then recalls the file only if a write request is received.

For pass-through, the Celerra uses the same cache on the Enterprise Vault server that you set up for Enterprise Vault to use when retrieving files for the Celerra.

Note:

If you configure Celerra pass-through, do not configure the Enterprise Vault option to delete archived files on placeholder deletion, as this combination can lead to data loss.

To configure the Celerra's pass-through behavior, include the -read_policy_override parameter in one of the following commands:

The fs_dhsm -modify command to configure a Celerra file system. This method sets the pass-through behavior for all the placeholders on the file system.
The fs_dhsm -connection command to define the HTTP connection that the Celerra uses for recall requests. This method sets the pass-through behavior for all the placeholders that are created through the HTTP connection.

The syntax of the -read_policy_override parameter is as follows:

-read_policy_override  [none | full | passthrough | partial]

The effect of the values is as follows:

none (the default value). The setting has no effect.
full. Recall the whole file to the Celerra on read request before the data is returned.
passthrough. Retrieve the data without recalling the data to the Celerra.
partial. Retrieve only the blocks that are required to satisfy the client read request.

Note the following:

If you do not set a read policy override for either the file system or the connection, the Celerra uses a value of passthrough by default.
The Celerra uses a value of passthrough if the Celerra file system is read only.
The Celerra uses a value of passthrough if attempts to recall data produce an error that is due to insufficient space or quotas.

For example, the following command syntax configures pass-through for a file system:

fs_dhsm -modify fs_name -read_policy_override passthrough

where fs_name is the name of the file system on the Celerra.