Patent ID: 7454445
Filing Date: 2008-11-18
Classification: G06F,Y10S

Abstract:
1. A method comprising: maintaining an active map of information indicating in-use blocks and free blocks of an active file system in a storage system; maintaining a set of snapshots in the storage system, each snapshot representing a state of said active file system at a particular point in time, each snapshot having a corresponding active map indicating in-use blocks and free blocks of the active file system for a point in time at which said snapshot was generated; computing a summary map as a logical OR of the active maps of at least two of said snapshots, wherein said computing includes performing a bitwise logical operation on at least two copies of earlier active maps included in said set of snapshots; identifying a set of snapshots created since a recent update of a selected portion of the summary map; updating said selected portion of the summary map based on only a most recent one of said identified snapshots, and using the summary map to make write allocation decisions in the storage system based on both a current active map of the active file system and said summary map, including using the summary map to avoid overwriting blocks used by a snapshot.