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running the vmtune command shown in Example 15-18 (pre AIX V5.2) and the vmo
command in Example 15-19 (AIX V5.2 and later).
Example 15-18 Tuning pre-5.2 AIX file system cache usage
/usr/samples/kernel/vmtune -P50
Example 15-19 Tuning AIX 5.2 and newer file system cache usage
vmo -o maxperm%=50
For a detailed explanation of the vmo tunables, please see VMM page
replacement tuning under:
http://publib.boulder.ibm.com/infocenter/pseries/v5r3/index.jsp?topic=/com.ibm.
aix.doc/aixbman/prftungd/memperf5.htm
If, after a period of time, paging still occurs, this may be lowered further. This
behavior can also be avoided by using raw logical volumes for the disk storage
pools. AIX will not attempt to cache data residing on a raw logical volume and will
then be less likely to need to page out application memory. Raw logical volumes
are also more efficient (in terms of CPU utilization) since AIX does not spend
time attempting to cache this data. If a Tivoli Storage Manager server has high
CPU utilization, better performance might be obtained by using raw logical
volumes.
Often system paging is interpreted as a lack of memory. With Tivoli Storage
Manager, this does not happen often, and paging can (and should) usually be
eliminated with the tuning above. AIX clients can also benefit from altering the
maxperm parameter. During backup, files are read into cache that are not likely
to be read again.
As before, this can cause application memory to be paged out, causing the main
application of that machine to suffer during backup. Lowering maxperm can help
with this. Beware of the effects of tuning maxperm on the client application. The
performance of the main application may depend on file system cache in some
instances (like a file server). In this case it may be desirable to just alter
maxperm before and after a Tivoli Storage Manager backup, or not at all.
Another tunable parameter is maxpgahead, which controls the AIX read-ahead
algorithms. Read-ahead is desirable on the client during backup, and on the
Tivoli Storage Manager server during storage pool migration from disk.
515
Chapter 15. Performance considerations
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