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# File migration terminology

This topic defines File migration terminology.

Table 1. File migration terminology
Term | Definition
--- | ---
File cluster | A set of File nodes that operate together. A File cluster can support a maximum of eight nodes. The first node by default operates as the Cluster controller and any node can take over Cluster controller activities in the event of first node failure.
NAS server | Network-attached storage (NAS) is a single point of access to a remote resource. File system data may be protected by remote NDMP software. Data may be stored on remote servers (offline DHSM files). Data may be protected against virus (virusChecker/CAVA (Common anti-virus Agent)). A NAS server runs by default on his primary node. In case of crash of the node, the NAS server is moved to his backup node (failover).
Node | Node is the hosting platform for running PowerMax File in a dockerized container. Each node hosts Base Linux operating system, Docker services, and other necessary software to connect each PowerMax File container to replicated sites. There is one node running per Front-end director.
Primary Node | One of the File cluster nodes on which the NAS Server runs.
Backup Node | One of the PowerMax File cluster nodes, on which the NAS server is backed up for fault tolerance purposes. This means that the NAS server is moved to this node during any failover event.
Client Interface | Network interface attached to the VDM (or NAS server) that is accessible by protocol clients, a.k.a. production interface. The interface(s) that are taken over are all client interfaces.
Migration | As a tech refresh, migration is the transfer of eNAS1.0 VDM (source), both data and configuration, to a NAS server on PowerMax File (destination). After migration, the NAS service provider switches to the migrated one on destination. During the migration, the data layout has been changed from UFS32 in source to UFS64 in destination.
Initial Copy | Initial copy is the migration stage between migration start and cutover. It includes the provision of the migration object,that is, NAS server (VDM) and File Systems, copy the data from source to destination, and move the VDM configuration.
Cutover | The production NAS server is switched from source eNAS1.0 VDM to destination PowerMax NAS server. All user traffic is directed to destination NAS server after cutover.
Incremental Copy (reverse) | After cutover, the migration progress needs to sync the uncopied and changed data during the baseline data copy from source to destination. This is the so-called Incremental Copy stage of migration cycle. During this stage, all writing operations against the migrated NAS on destination are synced back to source so that the user change is not lost in case a migration cancel.
Commit | Commit is the operation to complete a file migration. It can only occur when the data are 100% synced between source VDM and destination NAS server. After commitment, any data change is not synced back to source again. The source VDM is totally offline.
Cold Files | Cold Files are files to be transferred in Initial Copy phase. Considering a Filesystem to be migrated, during Initial Copy phase, the migration session migrates those files which are not supposed to be modified during the whole Initial Copy phase, so that the most data transferred in Initial Copy are valuable. It's possible that some cold files are changed during its transfer; in that case, they are transferred again in Incremental Copy phase. Currently, those file with mtime smaller than "migration connection creation time - one hour" are defined as cold file.
Hot Files | Hot Files are on the opposite position of cold files, it's highly possible that these files are modified during the migration Initial Copy. So, migration session does not transfer those files in Initial Copy phase.
Inode | In a Unix-style file system, the inode is a data structure that is used to represent a filesystem object, which can be one of various things including a file or a directory. Each inode stores the attributes and disk block location(s) of the filesystem object's data.
Migration Interface | Network interface(s) on the source or destination arrays that occur in pair for the migration engine to copy and sync data. Note: This interface is only for file migration purpose. Using it also in host I/O leads to data inconsistency between source and destination.
Non-disruptive Migration | The host (NAS client) should not be aware of the switch of the NAS server. No service interrupt occurs during the whole migration life cycle. To achieve that, the total DU time during cutover should be as small as 30 s (maximum 90 s).
Production Interface | See Client Interface above
Transparent Migration | See Non-Disruptive Migration above.
Active Migration | A migration session whose state is not "Completed" or "Canceled".