Storage Foundation and High Availability Solutions 7.4.2 Solutions Guide - Windows

Managing unallocated space for free space savings

Any policy for maintaining a minimum percentage of a disk group's capacity as unallocated space should include a cap to avoid maintaining wastefully large amounts of free space.

How much unallocated capacity to maintain depends strongly on application characteristics. In most cases, however, there are lower and upper bounds beyond which less or more unallocated storage would be of little use.

For example, an installation may observe a policy of maintaining a level of 8% to 10% of a disk group's total capacity as unallocated space. As the capacity of the disk group grows, however, the amount of unallocated space maintained by this policy can grow beyond any reasonable expectation of exploiting it effectively. If unallocated space is typically used in quantities of around 1 to 10 GB to relocate subdisks or to accommodate data processing peaks, then growing the disk group to 1 TB total capacity would mean that 100 GB are reserved for this purpose. If the typical number of subdisk moves or volume adds is one or two, a significant amount of storage capacity would never be used.