HP NonStop G-Series - Manual (Page 150)

Balancing and Tuning a System
Measure User’s Guide — 520560-003
7- 14
Checking and Tuning Problem Areas
I/Os) and compare the result against the REQUESTS counter (logical I/Os). Averaging
more than two physical I/Os per logical I/O indicates a possible performance problem.
The main causes of unnecessary physical I/O operations can be divided into two
classes: file structure (or access method) and cache configuration.
Structuring a File
Each index level in a key-sequenced file can cause an additional physical I/O
operation. Keeping a minimal number of index levels (no more than two) reduces the
number of physical I/O operations required for each logical I/O operation on the file.
Enter the FUP INFO command to determine the current number of index levels in a
key-sequenced file. To reduce the number of index levels, reload the file with larger
IBLOCK and BLOCK sizes or partition the file. The IBLOCK and BLOCK sizes must be
the same.
Adding and deleting records eventually causes files to fragment. Enter the FUP INFO
command to determine whether the file has a reasonable amount of slack. Too much
slack can cause unnecessary disk seek time and waste disk space. Too little slack can
cause block splits.
If a file is heavily used, partitioning it across multiple volumes spreads out the
workload. Partitions alleviate disk hot spots and spread data over multiple CPUs and
controllers. Partitioning also promotes parallel processing.
Other file attributes can cause unnecessary CPU overhead:
•
Using an alternate key requires two key-sequenced searches rather than one. You
can use the DISCOPEN entity to determine how frequently the application
accesses the alternate-key file:
6+ LIST DISCOPEN alternate key file name
A poorly designed application can use alternate keys as frequently as primary keys. To
correct the problem, you must redesign the application with these considerations:
•
Updating alternate-key files during peak hours can hurt performance. If possible,
turn off automatic alternate-key updates during periods of heavy usage and update
the alternate keys during periods of low use. (See the ALTKEY
key NOUPDATE
option of the FUP SET command in the File Utility Program (FUP) Reference
Manual.)
•
Using data or index compression on a key-sequenced file saves disk space at the
expense of CPU time. With compression off, the file system uses a binary search
algorithm when accessing records. With compression on, the file system must use
a sequential search. Thus saving disk space is rarely worth the CPU cost.
Enter the FUP INFO command to determine whether a file is compressed. If so,
you might want to reload the file with compression off. (For more information, see
the File Utility Program (FUP) Reference Manual.)