VERITAS Storage Foundation 4.1 Cluster File System HP Serviceguard Storage Management Suite Extracts, December 2005

Chapter 6, SFCFS Architecture
About CFS
57
SFCFS and the Group Lock Manager
SFCFS 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 reproduce single-host write semantics, system caches must be kept coherent, and each
must instantly reflect updates to cached data, regardless of the node from which they
originate.
Asymmetric Mounts
A VxFS file system mounted with the mount –o cluster 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 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 the file system,
or the options can be altered when doing a remount (mount –o remount). The first
column in the following table shows the mode in which the primary is mounted. The
check marks indicate the mode secondary mounts can use. See the mount_vxfs(1M) man
page for details on the cluster read-write (crw) mount option.
Secondary
ro rw ro, crw
Primary ro
✔
rw
✔✔
ro, crw
✔✔