HP NonStop G-Series - Manual (Page 71)

Configuring and Managing SMF Processes
HP NonStop Storage Management Foundation User's Guide—523562-008
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Indeterminate Outcomes
The SMF process does not wait until all pending operations have been resolved before
resuming normal operations.
Indeterminate Outcomes
From the viewpoint of the application, the VDP appears to be the sole process doing
certain operations. However, the disk process (DP2) is also involved. Whenever more
than one process is involved in an operation, a failure might result in an indeterminate
outcome of an operation.
For example, a VDP receives a request to rename an ENSCRIBE file and, after
updating its catalog, forwards the information to the disk process. After the disk
process reads the message, but before it can reply, its primary and backup processors
fail. If the VDP is not executing in either of these processors, the VDP receives a
communications error reply. If the processors are down for an extended period of time:
that is, for several seconds, the VDPs retry count becomes exhausted, but it still does
not know whether or not the file was renamed.
In general, the VDP attempts to minimize the impact of indeterminate outcomes. If a
file create request is lost, the VDP operates as if the physical file was not created and
selects another volume. If a file purge request is lost, the VDP operates as if the file
was not purged and returns a FESMSPHYSVOLNOTAVAIL response. However, for
operations in which the file label is updated (file renames and file code changes), the
VDP operates as if the old value is correct and returns a FESMSPHYSVOLNOTAVAIL
or FESMSPHYSVOLINDETERMRESULT response, depending on whether or not the
physical volume actually read the request before the failure.
Note. Because it is likely that the original requestor received an error 201: THE CURRENT
PATH TO THE DEVICE IS DOWN, AN ATTEMPT WAS MADE TO WRITE TO A NON-
EXISTENT PROCESS..., when the process takeover occurred or restarted, the SMF process
issues an EMS event notifying you of the outcome of the PENDOPS recovery operation.
Note. This previous example is not significant for most operations involving SQL files because
most SQL operations that affect file labels are executed through TMF with back out protection.
The VDP is designed to take advantage of TMF transaction protection so that if the disk
process operation is backed out because the transaction aborts, then so are the VDP’s catalog
updates.