HP Integrity NonStop H-Series - Manual (Page 59)

Configuring Automatic Transaction Processing
HP NonStop AutoTMF Software User’s Guide—429952-013
4-15
Unstructured Access
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AutoTMF does not return file system errors 27 and 28 when a program performs
an I/O operation without calling AWAITIO on the previous operation. Instead,
AutoTMF signals an assertion failure which commits all outstanding transactions
and terminates the program.
•
Error 26 occurs in the normal manner.
•
When debugging a program, Inspect shows the completed operation state and
error code as soon as the read or write operation has been executed.
•
A FILEINFO, or a similar operation called between READ and AWAITIO can return
updated key values that are normally available only after the call to AWAITIO.
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If executed between the READ/WRITE and AWAITIO, FILE_GETINFOLIST_ can
return the wrong value for the last error code.
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Configuring WAITEDIO normally has a minor or negligible effect on performance.
Overlapped NOWAIT I/O is very useful for non-audited files, where every update
requires a physical disk operation. Since audited file updates are buffered,
NOWAIT I/O is far less useful for audited files.
Unstructured Access
The OPEN and FILE_OPEN_ procedures have an optional parameter to request
unstructured access.
Unstructured access to a structured file (key-sequenced, entry-sequenced, or relative)
allows a program to position to a relative byte address and to read or write a string of
bytes at that location. Interpreting and updating the internal Enscribe block structures
are the responsibility of the programmer.
Unstructured access to either unstructured files or structured files also permits large
transfers to be used if the appropriate BULK I/O interfaces are used.
When a file is audited, some the following rules apply to the use of unstructured
access:
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A structured audited file cannot be opened for write access. An attempt to perform
such an open results in an error 80. TMF and the file system do not permit
unstructured updates to an audited structured file. TMF auditing is based on the
logical records of the file; unstructured access changes the physical structure of
the file, so the changes cannot be properly reflected in the TMF audit.
•
Unstructured read access to an audited file, structured or unstructured, is not
restricted.
Caution. Not all programs that perform multiple NOWAIT operations will encounter this
problem. Using this feature has some side effects. Users should not use Waited I/O
except in consultation with product support.