HP UX Serviceguard Storage Management Software manual About CFS, Asymmetric Mounts

Cluster File System Architecture

About CFS

About CFS

If the CFS primary node fails, the remaining cluster nodes elect a new primary node. The new primary node reads the file system intent log and completes any metadata updates that were in process at the time of the failure. Application I/O from other nodes may block during this process and cause a delay. When the file system becomes consistent again, application processing resumes.

Failure of a secondary node does not require metadata repair, because nodes using a cluster file system in secondary mode do not update file system metadata directly. The Multiple Transaction Server distributes file locking ownership and metadata updates across all nodes in the cluster, enhancing scalability without requiring unnecessary metadata communication throughout the cluster. CFS recovery from secondary node failure is therefore faster than from primary node failure.

See “Distributing Load on a Cluster” on page 20.

Cluster File System and The Group Lock Manager

CFS uses the Veritas Group Lock Manager (GLM) to reproduce UNIX single-host file system semantics in clusters. UNIX file systems make writes appear atomic. This means when an application writes a stream of data to a file, a subsequent application reading from the same area of the file retrieves the new data, even if it has been cached by the file system and not yet written to disk. Applications cannot retrieve stale data or partial results from a previous write.

To simulate single-host write semantics, system caches are kept coherent and each node’s cache instantly reflects updates to cached data, regardless of the node from which the update originates.

Asymmetric Mounts

A Veritas™ File System (VxFS) mounted with the mount -ocluster option is a cluster or shared mount, as opposed to a non-shared or local mount. A file system mounted in shared mode must be on a Veritas™ Volume Manager (VxVM) shared volume in a cluster environment. A local mount cannot be remounted in shared mode and a shared mount cannot be remounted in local mode. File systems in a cluster can be mounted with different read/write options. These are called asymmetric mounts.

Asymmetric mounts allow shared file systems to be mounted with different read/write capabilities. One node in the cluster can mount read/write, while other nodes mount read-only.

You can specify the cluster read-write (crw) option when you first mount a file system, or the options can be altered when doing a remount (mount -o remount). The first column in Table 2-1 on page 18 shows the mode in which the primary node is mounted. The X marks indicate the modes available to secondary nodes in the cluster.

See the mount_vxfs(1M) manual page for more information.

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