Veritas Storage Foundation™ 5.0.1 Cluster File System Administrator's Guide Extracts for the HP Serviceguard Storage Management Suite on HP-UX 11i v3

Table Of Contents
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 (page 18).
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 reproduce single-host write semantics, the file system must keep system caches coherent, and
each must instantly reflect updates to cached data, regardless of the node from which the updates
originate.
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 when you use the mount o option. A single clustered file system can be mounted with
different read/write options on different nodes. These are called asymmetric mounts.
Asymmetric mounts allow shared file systems to be mounted with different read/write capabilities.
For example, 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
(page 16) 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.
Table 2-1 Primary and Secondary Mount Options
Secondary: ro, crwSecondary: rwSecondary: ro
XPrimary: ro
XXPrimary: rw
XXPrimary: ro, crw
Mounting the primary node with only the -o cluster,ro option prevents the secondary nodes
from mounting in the read-write mode. Note that mounting the primary node with the rw option
implies read-write capability throughout the cluster.
16 Cluster File System Architecture