Administrator's Guide

Estimating Space Needs for Storage Pools

This section provides guidelines for estimating the initial storage space required for your installation. It assumes the use of the following default random access (disk) storage pools provided by ADSM:

• BACKUPPOOL for backed-up files
• ARCHIVEPOOL for archived files
• SPACEMGPOOL for files migrated from client nodes (space-managed files)

As your storage environment grows, you may want to consider how policy and storage pool definitions affect where workstation files are stored. Then you can define and maintain multiple storage pools in a hierarchy that allows you to contain storage costs by using sequential access storage pools in addition to disk storage pools, and still provide appropriate levels of service to users.

To help you determine how to adjust your policies and storage pools, get information about how much storage is being used (by client node) and for what purposes in your existing storage pools. For more information on how to do this, see "Requesting Information on the Use of Storage Space".

Estimating Space Needs in Random Access Storage Pools

To estimate the amount of storage space required for each random access (disk) storage pool:

• Determine the amount of disk space needed for different purposes:

  • For backup storage pools, provide enough disk space to support efficient daily incremental backups.

  • For archive storage pools, provide sufficient space for a user to archive a moderate size file system without causing migration from the disk pool to occur.

  • For storage pools for space-managed files, provide enough disk space to support the daily space-management load from HSM clients, without causing migration from the disk pool to occur.

• Decide what percentage of this data you want to keep on disk storage space and establish migration thresholds to have the server migrate the remainder of the data to less expensive storage media in sequential access storage pools.

  See "Choosing Appropriate Migration Threshold Values" for recommendations on setting migration thresholds.

Estimating Space for Backed-Up Files in a Random Access Storage Pool

To compute the total amount of space needed for all backed-up files stored in a single random access (disk) storage pool, such as BACKUPPOOL, use the following formula:

Backup space = AvgWkstSize * Utilization * VersionExpansion * NumWkst

Backup Space

The total amount of storage pool disk space needed.

AvgWkstSize

The average data storage capacity of a workstation. For example, if the typical workstation at your installation has a 4GB hard drive, then the average workstation storage capacity is 4GB.

Utilization

An estimate of the fraction of each workstation disk space used, in the range 0 to 1. For example, if you expect that workstations are 75% full, then use 0.75.

VersionExpansion

An expansion factor (greater than 1) that takes into account the additional backup versions, as defined in the copy group. A rough estimate allows 5% additional files for each backup copy. For example, for a version limit of 2, use 1.05, and for a version limit of 3, use 1.10.

NumWkst

The estimated total number of workstations ADSM supports.

If clients use compression, the amount of space required may be less than the amount calculated, depending on whether the data is compressible.

Estimating Space for Archived Files in a Random Access Storage Pool

Computing the amount of storage space for archived files is more difficult, because the number of archived files generated by users is not necessarily proportional to the amount of data stored on their workstations.

To estimate the total amount of space needed for all archived files in a single random access (disk) storage pool, such as ARCHIVEPOOL, determine what percentage of user files are typically archived.

Work with policy administrators to calculate this percentage based on the number and type of archive copy groups defined. For example, if policy administrators have defined archive copy groups for only half of the policy domains in your enterprise, then estimate that you need less then 50% of the amount of space you have defined for backed-up files.

Because additional storage space can be added at any time, you can start with a modest amount of storage space and increase the space by adding storage volumes to the archive storage pool, as required.

Estimating Space Needs in Sequential Access Storage Pools

To estimate the amount of space required for sequential access storage pools, consider:

• The amount of data being migrated from disk storage pools

• The length of time backed-up files are retained, as defined in backup copy groups

• The length of time archived files are retained, as defined in archive copy groups

• How frequently you reclaim unused space on sequential volumes

  See "Reclaiming Space in Sequential Access Storage Pools" for information about setting a reclamation threshold.

• Whether or not you use collocation to reduce the number of volume mounts required when restoring or retrieving large numbers of files from sequential volumes

  If you use collocation, you may need additional tape drives and volumes.

  See "Collocation on Sequential Access Storage Pools" for information about using collocation for your storage pools.

• The type of storage devices and sequential volumes supported at your installation