Configuring safeguarded virtual capacity

You must configure safeguarded virtual capacity to provision and accommodate safeguarded backups on your storage system.

Safeguarded virtual capacity is the amount of volume (virtual) capacity that is configured to store safeguarded backups for a safeguarded source. The capacity that is required depends on the size of the source volume, the number of backups, and the predicted destage rate of the source volume’s data.

To configure the amount of virtual capacity that is required for safeguarded backups, you must first determine an estimate for the safeguarded capacity multiplier. The safeguarded capacity multiplier is multiplied with the capacity of the data you want to back up to create the safeguarded virtual capacity.

Creating virtual capacity
To create safeguarded virtual capacity with the safeguarded capacity multiplier, use the Safeguarded > Configure Capacity action on the Volumes page.

To create safeguarded virtual capacity with the safeguarded capacity multiplier from the DS CLI, use the -action mksafeguardedcap parameter with either the manageckdvol command (for count key (CKD) data) or the managefbvol command (for fixed block (FB) data).

Expanding virtual capacity
To expand safeguarded virtual capacity with the safeguarded capacity multiplier, use the Safeguarded > Expand Capacity action on the Volumes page.

The new capacity multiplier value is based on the overall virtual capacity that you require. For example, if the original capacity multiplier value was 12 and your capacity calculation determines that you need to expand capacity by one third, the new capacity multiplier value would be 16.

To expand safeguarded virtual capacity with the safeguarded capacity multiplier from the DS CLI, use the -action expandsafeguardedcap parameter with either the manageckdvol command (for CKD data) or the managefbvol command (for FB data).

Determining an estimate for the safeguarded capacity multiplier

  1. Decide the number and retention period of the backups that are maintained by your safeguard schedule.

    For example, if your safeguard schedule creates one backup per hour and holds each backup for up to 3 days, the system will need to maintain 72 backups per volume.

  2. Determine the destage rate of the data that you want to back up by using the cachetrans metric, which shows a count of destaged tracks.

    The cachetrans metric is available with the showckdvol command for CKD data and the showfbvol command for FB data.

    To obtain a clear history of the destage rate on your system, query the cachetrans metric periodically, such as every 30 minutes, and gather about a week's worth of destage data. To ensure that the virtual capacity will accommodate your backups, use the number of destaged tracks from a peak period that matches the period of your safeguard schedule.

    For example, if your safeguard schedule period is 72-hours, then within a peak 72-hour period, you might find 15,000,000 destaged tracks.

    Note: The cachetrans metric is also available with Spectrum Control (Cache to Disk Transfers) and RMF (R745DCTD in the Type 74-5 records).

  3. Convert the destage rate to extents by dividing the destaged tracks by the number of tracks per extent.

    For example, if the destage rate is 15,000,000 tracks, and the number of tracks per extent is 256, then 58,594 extents (15,000,000/256), or approximately 890 GiB, would be required for the safeguarded backups.

    The number of tracks per extent depends on both the extent and volume type:
    • For CKD small extents (21 cylinders), the number of tracks per extent is 315.
    • For CKD large extents (1113 cylinders), the number of tracks per extent is 16695.
    • For FB small extents (16 MiB), the number or tracks per extent is 256.
    • For FB large extents (1 GiB), the number or tracks per extent is 16384.

  4. Determine the safeguarded capacity multiplier by dividing the number of extents required for safeguarded backups by the number of extents in the volume.

    For example, if the source volume size is 500 GiB (32000 extents), and the number of extents required is 58,594, then the safeguarded capacity multiplier would be 1.83 (58594/32000).

Notes:
  • This method might over-estimate the safeguarded capacity multiplier, as it does not account for tracks that are destaged multiple times within one backup time period.
  • This method tends to be most accurate for configurations where there is a shorter time period between backups.
  • The maximum required value of the safeguarded capacity multiplier is the number of backups. If you are using a small number of backups with a high retention period, then use the number of backups as the multiplier if the calculated multiplier is a higher value.
  • The maximum safeguarded capacity for a volume is 14.6 TiB for CKD data and 16 TiB for FB data. If the multiplier results in a virtual capacity that is higher than the maximum capacity, then the retention period might need to be reduced for the workload.
  • The size of the volume will be rounded to the nearest large extent. For CKD volumes, the size can be a multiple of 1113 cylinder granularity.