RULES OF THUMB FOR PAGING SPACE
QUESTION:
I have heard different versions of "rules of thumb" for paging space.
I have heard both 2X and 3X real memory, spread evenly over all the
PVs. When you get into very large memory sizes, or very large numbers
of disks, does this still apply? I specifically am looking for a
recommendation for how much paging space I should have for a 64MB
system with 11 PVs. If I use the rules of thumb, I would put
somewhere between 8 and 20MB on each disk - does that sound right? I
realize like most things that this is all very dependent on what my
system is doing, but if you could just help me with what the latest
and greatest rules of thumb for paging space are, I'd appreciate it.t
Comments on large amounts of memory systems and/or large numbers of
disks would help me too.
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A: There is no such thing as the "right" amount of paging space
for a system. The paging space requirements are unique for
each system, depending on such things as which applications
are running and/or the number of active users.
There are several "rules of thumb" used to determine how to
create your paging space:
Rule 1:
The install process will create paging space equal to
two times memory (2X) for systems with less than 64MB
of RAM. For systems with 64MB of RAM or more, the
paging space will be RAM size + 16MB. Please note
that while this may be applicable to some computing
environments, it may not be enough paging space for
others.
Rule 2:
Set paging space equal to two times RAM. However,
systems with large amounts of memory typically do
not need such large amounts of paging space.
Rule 3:
The "twice RAM rule" is a good starting point for
systems with 256MB of RAM or less. Use the
following equation for systems with more than
256MB of RAM:
Page Space = 512 + ( RAM - 256 ) * 1.25
Things to consider when creating or enlarging paging space:
Consideration 1:
Do not put more than one paging space on a physical
volume.
Reason for Consideration 1:
All processes that are started during the boot process
are allocated paging space on hd6. After the additional
paging spaces are activated, paging space is allocated in
a "round robin" manner in 4KB "chunks". If you have
paging space on multiple physical volumes and you put more
than one paging space on a physical volume, you will no
longer be spreading your paging activity over multiple
physical volumes.
Consideration 2:
Do not spread paging space over multiple physical volumes.
Reason for Consideration 2:
If your paging space is spread over multiple drives, you
will not be spreading your paging activity across all of
your physical volumes. Add a new paging space on a
different physical volume.
Consideration 3:
Make paging space a similar size as much as possible.
Reason for Consideration 3:
If you have paging spaces of different sizes, when the
smaller ones becomes from, you will not longer be
spreading your paging activity across your physical
volumes.
Consideration 4:
Avoid putting a paging space on the same volume as a
heavily active logical volume, such as that used by a
database.
Reason for Consideration 4:
It is not necessary to put a paging space on each physical
volume.
How will you know if you need more paging space?
1. A message presented on the console which
says "INIT: Paging space is low".
2. Applications will abort with the message,
"Not enough memory".
3. Use of the AIX command 'lsps -a' shows a
high percent used for paging space. This
percent will typically be over 50 .
4. Other AIX command which can be used for
performance tuning and analysis are:
iostat - Reports CPU and I/O statistics
vmstat - Reports statistics about processes,
virtual memory, disks, faults, and
CPU activity
svmon - Memory and page space used by all
processes
rmss - Simulate the amount of memory
available for paging analysis
Finally, do not put a paging space on a removable disk drive.
The system will crash if the disk is removed.