Explain the magic “300” in DPM.

This is a chosen limit that derives from the Logical Disk Manager (LDM) number of volumes that can coexist on a Windows system. We’ll explore some details in a minute but first the following; note that DPM requires 2 volumes to protect a data source, 1 for the replica and 1 for the recovery point volume. This means if we max out a supportable DPM server on this aspect there will be at least 600 volumes on the system.

LDM has a fixed size data structure (the LDM database) with records (to define volumes) that occupy at least 1 ‘slot’ and sometimes 2. To cut short, there are 2960 slots available and each new volume requires 3 or 4 slots and 1 more for each time a volume is extended. Wait a minute, with 300 data sources requiring 600 volumes, which consumes 1800 out of 2960. In other words; you cannot extend all replicas and recovery point volumes twice on a maximum configuration. Not that this is likely to occur but chances increase with DPM2010 as we will see. Obviously if there are less data sources or less need be extended you can do that more often. At some point we need ‘consolidation’ to reduce consumption of slots. Okay, but how? Create a new volume large enough to hold all data of an extended volume, move data and delete the old volume releasing all extent slots.