Cross volume links in Data Migrator
Data Migrator allows you to move data from primary to secondary storage based on policies that you define. When a file is migrated, a cross volume link is left, indicating the new location of the file's data. A cross volume link is a special file on the local file system that "points" to the file on a remote file system. The cross volume link stores the migrated file's metadata and is used to construct a handle for the file on the remote file system.
When a read request for a migrated file is received, the storage server reads the cross volume link, constructs the file handle, retrieves the file from the secondary storage, and sends the file to satisfy the read request. In this way, the read request is serviced and the client need not be aware that the file is not actually stored on the local file system.
The original cross volume link format (CVL-1) required that the file systems had to be bound to the same EVS. Original cross volume links are supported by first-generation NAS Platforms, as well as the most recent NAS Platforms. Enhanced cross volume links (CVL-2) are supported by NAS Platform 3080 and NAS Platform 3090. CVL-2 is the default. Enhanced cross volume links offer the following benefits:
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Remote file systems might be on a storage device attached to a remote server (not necessarily another NAS Platform) accessible through the NFSv3 protocol. This capability, called remote migration, allows the storage server to migrate files to a separate storage device, such as content archival or compliance products.
Remote migration to a Hitachi Content Platform (HCP) system, is supported through the HTTP or HTTPS (cloud only) protocol for new remote migration paths, but pre-existing remote migration paths that were created in releases that supported the NFSv3 protocol are still supported. You cannot, however, create paths to an HCP system using the NFSv3 protocol.
Access to files located on the external storage device is a licensed feature, requiring an external volume link (XVL) license and a Data Migrator license. See the Server and Cluster Administration Guide for information on adding a license key.
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Local access to migrated file attributes increases performance for getattr/lookup/readdir+ requests.
For original cross volume links (CVL-1), some file attributes for a migrated file are stored on the local file system and some are stored on the remote file system. When an operation requires the attributes of a migrated file, the storage server combines locally stored attribute information with attributes it retrieves from the remote file system.
For enhanced cross volume links (CVL-2), all file attributes for a migrated file are stored on the local file system. When an operation requires the attributes of a migrated file, the storage server responds with locally stored attribute information, which provides better performance.
In addition, local read caching can be used to improve performance when accessing migrated files. See the Server and Cluster Administration Guide for information on local read caching.
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Enhanced cross volume links provide improved quota management.
With original cross volume links, file length is tracked on the remote file system. As a result, quotas are tracked independently on the local file system and on the remote file system. By storing attributes on the local file system, enhanced cross volume links make it possible to track quotas entirely on the local file system, because file space and file count quotas are managed and calculated using local attributes. This simplifies quota management, but does not allow storage administrators to set up separate quotas for data based on the data's location.
As a result of differences in how quotas are calculated when using original cross volume links or enhanced cross volume links, mixing of cross volume link formats is not supported within the same file system. By default, enhanced cross volume links are created for local and remote (external) migrations, but you can configure the storage server to create original cross volume links for local migrations. Contact customer support for more information about limiting the creation of cross volume links to the original format.