IBM ZVM - FOR LINUX V6 RELEASE 1 Getting Started page 40

with 200 MB for the size of the Java
Application Server requires 60 MB of memory.
application would be 260 MB. If you have an existing WebSphere application you
know requires 250 MB of memory, the total with WebSphere Application Server
would be 310 MB.
The total memory requirement for your applications, plus memory required for
each Linux operating system and z/VM itself, give you an estimate of the memory
required for a given LPAR. Do not pad the total memory figure.
Many customers prefer to isolate their applications by running each one in a
separate virtual machine. Another strategy is to combine more than one application
in a virtual machine, keeping the number of Linux guests to a minimum rather
than creating one Linux guest for each application. The reason is that each Linux
guest brings with it some overhead: each Linux operating system itself requires
additional memory, and even an idle Linux guest uses some CPU resources. Also,
applications sometimes can share middleware, which conserves memory. If it
conforms to your installation's policy, you might be able to combine more than one
application in a virtual machine, thereby saving memory. For example, several
WebSphere applications can share the same WebSphere Application Server and the
JVM in one virtual machine rather than each application having its own
WebSphere Application Server and JVM in its own virtual machine, which would
multiply the number of virtual machines and require more total memory.
Of the total memory requirement, start by allocating 75% to central storage and
25% to expanded storage. (Though z/OS no longer supports expanded storage,
z/VM and the hardware do.) There are performance advantages to allocating
expanded storage. Because z/VM is a 64-bit system, it seems as though allocating
all memory to central storage makes sense. However, z/VM manages a paging
hierarchy that uses expanded storage first, then slower DASD. Pages move to the
slower paging DASD when not referenced within a given time limit. But, if pages
are referenced again within the time limit, they can be brought back into central
storage rapidly. This paging design gives a more consistent response to users.
During system operations, measure actual memory usage to test the initial memory
allocation, which assumes all your guests need the estimated amount of memory
all the time. Just as CPU demand has peaks and valleys, so does memory usage.
Memory for the virtual machines
For the general case of server consolidation, keep the virtual machine size small.
How small you can define the virtual machine depends on the applications and
workloads running in those virtual machines. Various Linux distributions might
have minimum requirements as low as 64 MB. Some applications run fine in those
minimum configurations. Other applications and workloads might require larger
virtual machines. Avoid defining a virtual machine larger than it needs to be,
because Linux uses excess memory for file and buffer caches. On a standalone
system, these buffers can be very helpful for certain workloads to avoid I/O.
However, in a virtual environment, the extra memory consumed adds to overall
system memory contention. Such cases could cause a negative impact greater than
the positive impact of I/O avoidance, which is especially true in configurations in
which data is shared heavily between guests and is mostly read. In those
configurations, z/VM minidisk caching can help avoid I/O. As a general guideline,
define the virtual machine memory size to keep Linux on the verge of swapping.
1. Current requirements. For more information about Java and WebSphere memory requirements, consult Java and WebSphere
documentation.
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z/VM: Getting Started with Linux on System z
™
Virtual Machine. Additionally, WebSphere
1
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