A method and system for allocating storage space on a storage device to a requesting entity. The storage device includes a set of extents. An allocation map is maintained in computer memory. The allocation map has a plurality of map blocks, the map blocks each having a plurality of map entries. The map entries include respective logical extent identifiers, physical extent information and obsolete indicators. An invalid extent logical identifier represents a free extent and a valid extent identifier represents a prior allocated extent. An obsolete indicator marks a map entry as either current or obsolete. In one technique an extent allocation request is received from the requesting entity. A first map block is selected having a first map entry marked as obsolete. A second map block is selected having a second map entry not marked as obsolete. Physical extent information is copied from the second map entry to the first map entry. The logical extent identifier is assigned to the first map entry. The first map entry is marked as current and the second map entry is marked as obsolete. The storage allocation system includes a disk storage device having a set of extents. A requesting entity is configured to request allocation of one of the extents. An allocation map is maintained in computer memory. An allocator is configured to allocate one of the extents on the disk device to the requesting entity.