Disk Staging Storage Unit Considerations

Disk staging provides a method for administrators to create images on disk initially, then later copy the images to another media type (as determined by the disk staging schedule). The later media type would typically be tape, but could be disk or disk staging.

This two-stage process allows the NetBackup administrator to leverage the advantages of disk-based backups in the near term, while preserving the advantages of tape-based backups for long term.

Disk staging may be appropriate for your NetBackup environment to meet the following objectives:

• To allow backups when tape drives are scarce.
• To allow for faster restores from disk.
• To facilitate streaming to tape without image multiplexing.

Disk Staging is conducted in two separate operations:

• Stage I: A backup creates an image on the disk staging storage unit.
• Stage II: A relocation schedule determines when the image from the disk staging storage unit should be relocated to the destination storage unit.

  

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The image continues to exist on both the disk staging storage unit and the destination storage unit. File restores are done from the disk staging storage unit copy, while the destination storage unit copy can be considered the long term copy.

The image copy continues to exist on the disk staging storage unit until either the copy expires based on the copy's retention period, or until another Stage I process needs space on the disk staging storage unit.

When a Stage I process detects a full disk staging storage unit, it pauses the backup, finds the oldest image that has been successfully copied to the destination storage unit, and expires this image copy.

Disk Staging Storage Unit Size and Capacity Considerations

Leveraging the advantages of disk staging requires that the NetBackup administrator understand the life expectancy of the disk-based image. After the disk-based image is copied to the destination storage unit, management of the disk-based copy's retention is handed over to the disk staging disk full logic.

Therefore, the size and usage of the file system containing the disk staging storage unit directly impacts the life expectancy of the disk-based image. This is why it is strongly recommended to have a dedicated file system for each disk staging storage unit.

Example: The NetBackup administrator wants incremental backups to be available on disk for one week:

Incremental backups are done Monday through Saturday, with full backups done on Sunday. The full backups are sent directly to tape, and do not utilize the disk staging feature. Each night's total incremental backups average from 300 to 500MB. Occasionally a backup contains 700MB. Each following day the disk staging schedule runs and copies the previous night's incrementals to the destination storage unit (tape).

Minimum Disk Staging Storage Unit Size

The minimum disk staging storage unit size represents the minimum size needed for the successful operation of the disk staging logic. The minimum size will not accommodate the desired level of service (as disk images remain on the disk for one week in our example).

The minimum size for the disk staging storage unit must be greater than or equal to the maximum size of backups placed on the storage unit between runs of the disk staging schedule.

In this example, the disk staging schedule runs nightly, and the largest nightly backup is 700MB. NetBackup recommends doubling this value to allow for unanticipated problems running a disk staging schedule. Doubling the value gives the administrator an extra schedule cycle (one day) to correct any problems.

The following formula was used to arrive at the minimum disk staging storage unit size in our example:

Minimum disk staging storage unit size = Max data per cycle * (1 cycle + 1 cycle for safety)

For example: 1.4GB = 700MB * (1+1)

Average Disk Staging Storage Unit Size

The average disk staging storage unit size represents a good compromise between the minimum and maximum sizes.

For example, if the average nightly backup is 400GB and the desire is for the images to be kept for one week, the recommended average size is calculated based on the following formula:

Average Size of disk staging storage unit =
Average data per cycle * (number of cycles to keep data + 1 cycle for safety)

2.8GB = 400MB * (6 + 1)

Maximum Disk Staging Storage Unit Size

The maximum disk staging storage unit size is the recommended size needed to accommodate the level of service desired. In this example, the level of service is that disk images remain on disk for one week.

To determine the size, use the following formula:

Maximum Size = Max data per cycle * (# of cycles to keep data + 1 cycle for safety)

For example: 4.9 GB = 700MB * (6 + 1)