DCE LFS aggregates are similar to the disk partitions that are found in the UNIX and other operating systems. However, a DCE LFS aggregate also supports specialized fileset-level operations, such as quota-checking and cloning, and low-level operations, such as logging of metadata.
Each DCE LFS aggregate exported to the DCE namespace can house multiple filesets; filesets stored on DCE LFS aggregates are referred to as DCE LFS filesets. Each exported non-LFS disk partition can house only a single fileset; file systems stored on non-LFS partitions are referred to as non-LFS filesets. Most of the specialized features supported for DCE LFS filesets are not supported for non-LFS filesets.
In DFS, only a single type of each non-LFS fileset is available: the version that was made available when the partition on which it resides was exported. However, three types of each DCE LFS fileset are available:
· A read/write version of a DCE LFS fileset contains modifiable versions of the files and directories in that fileset. Every DCE LFS fileset begins as a single read/write fileset. Other fileset types are derived from the read/write version by creating an exact copy, or replica, of all of the data in the read/write, source fileset. There can be only one read/write version of a fileset.
· A read-only fileset is a copy of a read/write, source DCE LFS fileset. Read-write filesets can be replicated and placed at various sites in the file system. Every read-only copy of a fileset shares the name of the source fileset, with the addition of a .readonly extension. If a read/write source changes, its read-only replicas can be updated to match. Every read-only copy of a read/write fileset is generally the same; however, replicas at different sites can differ in controlled ways according to the replication parameters associated with the fileset.
· A backup fileset is a clone of a read/write, source DCE LFS fileset. It is stored at the same site and with the same name as the source, with the addition of a .backup extension.
For both DCE LFS and non-LFS filesets, the Fileset Location Server (FL Server) maintains the Fileset Location Database (FLDB). The database records information about the location of all filesets in a cell. Every read/write fileset has an entry in the FLDB. Each entry for a DCE LFS fileset also includes information about the fileset's read-only and backup versions. For each fileset, the information in the entry includes fileset names, ID numbers, site definitions, and status flags for the sites.
Information about DCE LFS filesets is also stored in fileset headers at each site that contains a copy of the fileset. Fileset headers are part of the data structure that records the disk addresses on the aggregate of the files in the fileset. Fileset headers also record the fileset's name, ID number, size, status flags, and the ID numbers of its copies. Because the header records some of the same information that appears in the FLDB, the fts program can access the information in the header if the FLDB becomes unavailable. The FLDB entry and the fileset header must always be synchronized; any fts commands that affect fileset status automatically record the change in the appropriate FLDB entry.
(See Making Filesets and Aggregates Available for detailed information about DCE LFS filesets, non-LFS filesets, and how the two types of filesets are created.)