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Dell Unity snapshots are fully unified and use a common redirect-on-write technology between file and block. Dell Unity snapshots are taken, treated, and scheduled the same way between block and file resources, resulting in a fully unified data protection experience. Unlike other file snapshot implementations, Dell Unity file system snapshots do not require a separate volume to be set aside to accommodate snapped data. Instead, changes to snapped file system data are written to free space in the same storage pool. Snapshots can be read-only or read/write.
In the figure below, a snapshot is taken of a source file system containing data blocks A, B, C, and D. Afterward, when new data, D’, is written to block D, the new data is redirected to a new location within the same pool and the data in block D is preserved as part of the snapshot. This works the same way when writing to snapshots that share data with the production file system. Unless the data is unique to the snapshot, attempting to overwrite a block will redirect the new block to a new location in the pool.
File snapshots may be restored through Unisphere on a file system level. You can also copy specific files from a snapshot back to the production file system by using the .ckpt directory in UNIX. However, note that some Operating Systems, such as Windows, may not be able to access the .ckpt due to directory caching. For Windows Operating Systems, the Previous Versions tab can be used instead to access the snapshots contents. The name of the .ckpt folder can also be customized by using the cvfs.virtualDirName parameter. For more information about NAS server parameters and how to configure them, reference the Service Commands document on Dell Technologies Info Hub.
Dell Unity also provides the ability to provide read/write access to file system snapshots to hosts and clients through shares. To do this, the administrator creates a new file system share using an existing snapshot. As a result, the snapshot data is exposed as a new share of the production file system, which may then be accessed as a normal share by file system hosts and clients.
Starting with OE version 5.1, read/write snapshots of file systems and NFS datastores can be refreshed. The refresh operation replaces the contents of the snapshot with the current data in the parent object. This happens nearly instantaneously since only pointer updates are required. Shares based on the snapshot being refreshed can remain during the refresh operation. When a snapshot is refreshed, the original Taken timestamp of the snapshot is preserved.
When performing a refresh operation, it is highly recommended to take a backup snapshot to preserve the current data on the snapshot. In Unisphere, a checkbox (enabled by default) is provided to take the backup snapshot before proceeding with the refresh operation. Also, it is highly recommended to quiesce any applications and flush the host cache before initiating the refresh to prevent potential data corruption. The figure below shows the refresh snapshot window.
For more information about Dell Unity’s unified snapshot technology, reference the Dell Unity: Snapshots and Thin Clones white paper on Dell Technologies Info Hub.
Dell Unity systems support 2-way and 3-way NDMP, allowing administrators to protect file systems by backing up to a tape library or other supported backup device. 3-way NDMP transfers the backup data over the network while 2-way NDMP transfers the data over Fibre Channel. 2-way NDMP eliminates backup data on the network by backing up directly to the backup device, potentially decreasing network congestion and reducing backup times. To use 2-way NDMP, the system must be running OE version 4.4 or later. A 2-way NDMP configuration is shown in the figure below.
When configuring 2-way NDMP, the backup device should be connected to a switch and zoned to the Fibre Channel ports on the Dell Unity system. Directly connecting the backup device to the storage system is not supported. When cabling and zoning the system, using the Synchronous Replication port, which is the first Fibre Channel port on the system, for the backup device is not supported.
Dell Unity supports taking NDMP full backups, incremental backups, restores, and tape cloning. Both dump and tar backups are supported but volume-based backups (VBB) are not supported. The backup application can specify the parameters below when running an NDMP backup. It is recommended to enable all these parameters when running an NDMP backup.
Combining NDMP, local snapshots, and remote protection enable Dell Unity storage systems to be deployed with a wide array of data protection capabilities, including the ability to replicate to or from multiple arrays in a multisite topology. In addition, the NDMP backups can be taken on the destination NAS server, alleviating the backup load from the production system.