1.2.1 Transparent File Access to End Users

Because shadow volumes present a merged view of the file trees, the end user’s files appear to be in the same logical place regardless of their physical location. This allows the administrator to manage the data without disrupting the end user’s view of the files.

1.2.2 Policy-Based Migration between Primary and Secondary Storage Areas

DST provides policy-based control of the files to move and the direction that you want to move data between devices. You can set up policies that migrate data by file extension, file size, and the date last accessed or modified. You can also specify a list of files to move in either direction for a one-time move. Policies can be scheduled to run and run on demand. You can set policies so that data stored on the secondary storage volume can be accessed without de-migrating it.

1.2.3 Faster and Smaller Backups of Important Data

Backup policies can differ for the primary storage volume and the secondary storage volume. For example, the DST administrator can allocate the data between the two volumes in a way that supports separate backup schedules:

Analyzing the inventory of a volume’s data shows that a large portion of its data is seldom used. Having a shadow volume allows the administrator to spend more on the most important data and spend less on the less important data. The important data, which is stored on the primary area, can be backed up nightly. The less important data, which is stored in the secondary area, can be backed up weekly or even monthly. Getting the less important data out of the way enables the backups of your important data to run more quickly and efficiently. Allocating data in this way can significantly lower the cost of backups by reducing both labor and tape requirements.

For information about backing up data on DST volumes, see:

1.2.4 Faster Disaster Recovery

Start by locating your most important files on the primary area. Then, during a disaster recovery, the server administrator can restore the primary area first. This restores the critical files first, and leaves the recovery of the less important secondary area until later. The users can continue working while files they probably do not need immediately are being restored. Also, other fault-tolerant replication solutions like snapshots can be used for the primary area without wasting money on files that do not require the same level of fault tolerance.

1.2.5 More Efficient Use of Expensive Storage

Policies can be used to partition files based on file age, owner, type, size, and so on. You can move the less important files to from a higher quality storage array to a lower quality storage, thus reserving the higher-cost storage for your most important files. For example, you can configure the primary area on block-based SCSI storage devices in a Fibre Channel SAN-based hardware RAID array or storage array, and configure the secondary area on a lower quality storage array using slower devices like SATA. This allows you to get more use out of your Fibre Channel storage solution, and keep it from filling up with unimportant files. You can store more data on your server with a lower overall cost per gigabyte.

1.2.6 Fast Storage for Active Data and Slower, Less Expensive Storage for Old Data

Storage media can differ for the primary storage volume and the secondary storage volume. Storage costs can be reduced by allowing data that is used infrequently to be stored on lower-cost storage. Locate the primary area on storage drives that are faster and higher quality. Then locate the secondary area on less expensive storage drives. Files that the users are currently working on can be located on the high-performance drives. The files that have not been modified for a long time can be moved to the lower-performance drives to free up space on the high-performance drives. In this way, you can locate a large amount of your data on less expensive, lower-performance storage drives, while your users still get high-quality performance because their active files are located on the high-performance storage drives.

1.2.7 Moving Files from an Existing Secondary Volume

You can start with an empty primary NSS volume, and have the shadow area be an existing volume. The combined view initially presented by the NCP Engine is equivalent to the secondary volume. You can define a policy to move files to the primary as they are modified or accessed. As users access their data through the new primary volume, the files they use are automatically migrated to the new server. This migration-on-demand approach migrates the data gradually, freeing the IT department from spending off-hours time migrating the data with the server offline.

1.2.8 Access to the Secondary Storage Area without the Performance Penalties of HSM Solutions

With HSM (hierarchical storage management) solutions, files are migrated from the primary storage to a secondary storage device, and a copy of the file’s metadata (stub file) is left behind in the volume’s directory tree. If the file is ever accessed again, it needs to be migrated back to the primary storage before it is available.

In contrast, DST shadow volumes can access files directly regardless of which area (primary or secondary) they are in, and without de-migrating them. If a user searches through all the files on a shadow volume, the files are searched without needing to move them to the primary area. Also, shadow volume backups are faster because there are no HSM metadata stub files for the backup software to scan. The backup software does not need to be HSM aware.