Global File System 2 or GFS2 is a shared disk file system for Linux computer clusters.
GFS2 allows all nodes to have direct concurrent access to the same shared block storage. GFS2 has no disconnected operating-mode, and no client or server roles. All nodes in a GFS2 cluster function as peers.
A GFS2 file system is intended to provide a feature set which is as close as possible to a local file system, while at the same time enforcing full cluster coherency between nodes. In a few cases, the Linux file system API does not allow the clustered nature of GFS2 to be totally transparent; for example, programs using Posix locks in GFS2 should avoid using the GETLK function since, in a clustered environment, the process ID may be for a different node in the cluster. In most cases however, the functionality of a GFS2 file system is identical to that of a local file system.
GFS2 supports up to 32 cluster nodes. Using GFS2 in a cluster requires hardware to allow access to the shared storage, and a lock manager to control access to the storage. GFS2 is based on a 64-bit architecture, which can theoretically accommodate an 8 EB file system.
GFS2 is a journaling filesystem and one journal is required for each node that will mount the filesystem. The one exception to that is spectator mounts which are equivalent to mounting a read-only block device and as such can neither recover a journal or write to the filesystem, so do not require a journal assigned to them.
GFS2 adds a number of new features which are not in GFS. These features include:
- The metadata filesystem.
- GFS2 specific trace points .
- The XFS-style quota interface.
- Caching ACLs.
- Supports the generation of “discard” requests for thin provisioning/SCSI TRIM requests.
- Supports I/O barriers (on by default, assuming underlying device supports it. Configurable from kernel 2.6.33 and up).
- FIEMAP ioctl (to query mappings of inodes on disk).
- Splice (system call) support.
mmap/splice support for journaled files (enabled by using the same on disk format as for regular
- Far fewer tunables (making set-up less complicated).
- Ordered write mode (as per ext3, GFS only has writeback mode).
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