|
HP OpenVMS Systemsask the wizard |
|
The Question is:
How do you calculate the optimal value for either the ACP_WINDOW Sysgen
Parameter or the set volume/window=xx parameter for disk drives. My
understanding is the followng formula
acp_window =
(largest file size of all disks)/65535
The Answer is : The effect of insufficient ACP_WINDOW is a high window turn rate. The cost of a high window turn rate is often overrated in the opinion of the OpenVMS Wizard, mostly because the windows are 'backfilled' from file headers, which in turn are often cached. The cause of an insufficient ACP_WINDOW setting is often excessive disk fragmentation. This is often avoidable (at no cost!) by using adequate initial file allocation and significant (default) file extent sizes, as well as an occasional pass of BACKUP and restoration when needed. (Needed both due to current fragmentation levels and due to the relative age of the last good disk BACKUP.) Consider SET RMS/EXTENSION [/SYSTEM] and SET VOLUME/EXTENSION, as well as available FDL parameters. If the OpenVMS Wizard recalls correctly, the (older) 65535 limitation was rendered obsolete starting with the OpenVMS V6.0 release. Having written all that, the default of 7 pointers is low. If you are indeed monitoring significant window turning, then consider doubling the value for some busy disk, and monitor further. For example, issue the command SET VOLUME/WINDOW=20 If you choose to significantly increase the window size for a large number of open files, you will want to evaluate (and potentially increase) the values for the process BYTLM quota and the system non-paged pool size.
|