Question: The CDSERVER process logs thousands of I/Os in the course of a day. What is it doing?

Answer: Every five seconds, CDSERVER asks the CLNdriver, a pseudo-device driver, to look for newly-created processes and enable fragmentation prevention on their behalf. This request is made via an "I/O request" to the CLNdriver. Although OpenVMS logs this as an "I/O operation" there is no data actually being read from or written to any external devices. (This is somewhat similar to a write to the null device.)

I.2.3 Why are Some Processes Enabled/Disabled

Answer: There are several reasons:

1. Very recently created processes (within the last five seconds) may not have been seen yet by the auto-enable mechanism.
2. Fragmentation prevention is not enabled for OpenVMS's SWAPPER process.
3. The auto-enable mechanism does not enable fragmentation prevention for processes which are outswapped or which are in any of several wait states (such as SUSPEND, MWAIT, etc.). Such processes will not be doing any disk I/O while they are in such states, so enabling fragmentation prevention for them will have no benefit. Fragmentation prevention will be enabled for these processes once they leave these "long-term idle" states.

I.2.4 Working with Existing Files that get Extended

Answer: When the file is extended, if there is free space adjoining the last file extent, fragmentation prevention will concatenate this free space to the last extent. This causes the last extent to be larger...and avoids further file fragmentation. If there is not enough adjoining free space, fragmentation prevention will cause a contiguous-best-try extent to be created. The result is a significant reduction in daily file fragmentation!

I.2.5 Existing Software Products that Intercept File Opens

Answer: No, the fragmentation prevention intercept is probably at a level below where these types of products intercept file opens. Fragmentation prevention intercepts at the XQP level. Even if other software products intercept at this level, fragmentation prevention will not interfere because fragmentation prevention specifically checks for other software products intercepting at the XQP level.

Working Set

The physical pages of memory allocated to a process at any point in time.

Working Set List

The list of pointers to memory pages. Pointers can also be unused, waiting to be attached to a page faulted into the working set.

Process

The entity that is selected for scheduling by the OpenVMS scheduler. A process is the environment within which programs execute on OpenVMS.

Page Fault

The result of a virtual address translation exception. If a virtual address accessed by a process is not in its working set, a page fault is incurred. If the page containing the virtual address is on the modified list or the free list, a soft page fault occurs. Otherwise the page is either in a page file or some other disk file, and a hard page fault occurs. Hard page faults take hundreds of times longer than soft page faults take to be processed by OpenVMS.

Static Parameters

SYSGEN parameters which require the system to be rebooted in order to take effect.

Dynamic Parameters

SYSGEN parameters that can be changed and take effect while OpenVMS is operating. No system reboot is required.