Calculating Capacity Requirements
Published : April 8, 2005 | Updated : August 17, 2005
Capacity requirements for the storage pool are variable, and depend primarily on the size of the protected data, the daily shadow copy size, and recovery range objectives.
Daily shadow copy size refers to the total size of changes made to protected data during a single day. It is roughly equivalent to the size of an incremental backup. Recovery range refers to the number of days for which you want to store shadow copies of protected data on disk. DPM can store a maximum of 64 shadow copies for each volume included in a protection group, and can create a maximum of eight scheduled shadow copies for each protection group each day.
To estimate the minimum capacity needed for the storage pool, you can use this formula:
Storage pool capacity = Protected data size + (Daily shadow copy size × Recovery range)
In general, we recommend making the storage pool two to three times the size of the protected data. This recommendation is based on an assumed daily shadow copy size of approximately 10 percent of the protected data size, and a recovery range of 10 days (two weeks, excluding weekends).
If your daily shadow copy size is larger or smaller than 10 percent of your protected data size, or if your recovery range objectives are longer or shorter than 10 days, you can adjust the capacity requirements for your storage pool accordingly.
Regardless of how much capacity you decide to allow for the storage pool in your initial deployment, we recommend that you use extensible hardware so that you have the option of adding capacity should the need arise.
The sections that follow provide guidelines for determining your daily shadow copy size and recovery range objectives.
Estimating Daily Shadow Copy Size
Our recommendation to make the storage pool two to three times the size of the protected data assumes a daily shadow copy size of 10 percent of the protected data size. Daily shadow copy size is related to data change rate, and refers to the total size of all shadow copies created during a single day. To get an estimate of the daily shadow copy size for your protected data, you can review an incremental backup for a recent, average day. The size of the incremental backup is usually indicative of the daily shadow copy size. For example, if the incremental backup for 100 GB of data includes 10 GB of data, your daily shadow copy size will probably be approximately 10 GB.
Determining Recovery Range Objectives
Our recommendation to make the storage pool two to three times the size of the protected data assumes a recovery range objective of 10 days (two weeks, excluding weekends). For the typical enterprise, requests for recovery of data are concentrated within two to four weeks after data loss events. A recovery range of 10 days provides for recovery of data up to two weeks after a data loss event.
To determine your recovery range needs, consider the pattern of recovery requests you experience in your enterprise. If requests are concentrated within two weeks of data loss, 10 days may be an appropriate recovery range for you. If requests are concentrated at a later time, you may need a longer recovery range.
Assuming sufficient capacity on the storage pool, DPM can store a maximum of 64 shadow copies for each volume included in a protection group. After DPM reaches the 64 shadow copy limit, or if DPM runs out of space to store shadow copies, DPM begins deleting the oldest shadow copies to make room for the new shadow copies.
The longer your recovery range objective, the fewer shadow copies you can create each day. For example, if your recovery range objective is 64 days, you can create just one shadow copy each day. If your recovery range objective is eight days, you can create eight shadow copies each day. With a recovery range objective of 10 days, you can create approximately 6 shadow copies each day.