OpenVMS Cluster Systems - OpenVMS Systems - HP

Building Large OpenVMS Cluster Systems
9.5 System-Disk Throughput
9.5.1 Avoiding Disk Rebuilds
The OpenVMS file system maintains a cache of preallocated file headers and
disk blocks. When a disk is not properly dismounted, such as when a system
fails, this preallocated space becomes temporarily unavailable. When the disk is
mounted again, OpenVMS scans the disk to recover that space. This is called a
disk rebuild.
A large OpenVMS Cluster system must ensure sufficient capacity to boot nodes
in a reasonable amount of time. To minimize the impact of disk rebuilds at boot
time, consider making the following changes:
Action Result
Use the DCL command
MOUNT/NOREBUILD for all user disks,
at least on the satellite nodes. Enter this
command into startup procedures that
mount user disks.
Set the system parameter ACP_
REBLDSYSD to 0, at least for the
satellite nodes.
Avoid a disk rebuild during prime
working hours by using the SET
VOLUME/REBUILD command during
times when the system is not so
heavily used. Once the computer is
running, you can run a batch job or a
command procedure to execute the SET
VOLUME/REBUILD command for each
disk drive.
It is undesirable to have a satellite node rebuild the disk,
yet this is likely to happen if a satellite is the first to
reboot after it or another node fails.
This prevents a rebuild operation on the system disk
when it is mounted implicitly by OpenVMS early in the
boot process.
User response times can be degraded during a disk
rebuild operation because most I/O activity on that
disk is blocked. Because the SET VOLUME/REBUILD
command determines whether a rebuild is needed, the
job can execute the command for every disk. This job can
be run during off hours, preferably on one of the more
powerful nodes.
Caution: In large OpenVMS Cluster systems, large amounts of disk space can
be preallocated to caches. If many nodes abruptly leave the cluster (for example,
during a power failure), this space becomes temporarily unavailable. If your
system usually runs with nearly full disks, do not disable rebuilds on the server
nodes at boot time.
9.5.2 Offloading Work
In addition to the system disk throughput issues during an entire OpenVMS
Cluster boot, access to particular system files even during steady-state operations
(such as logging in, starting up applications, or issuing a PRINT command) can
affect response times.
You can identify hot system files using a performance or monitoring tool (such
as those listed in Section 1.5.2), and use the techniques in the following table to
reduce hot file I/O activity on system disks:
Potential Hot Files Methods to Help
Page and swap files When you run CLUSTER_CONFIG_LAN.COM or CLUSTER_
CONFIG.COM to add computers to specify the sizes and locations
of page and swap files, relocate the files as follows:
9–14 Building Large OpenVMS Cluster Systems
• Move page and swap files for computers off system disks.
• Set up page and swap files for satellites on the satellites’
local disks, if such disks are available.