S/390 PartnerWorld for Developers, ITSO/EFS Project EFS Systems on a Linux Base: Additional Topics

20 EFS Systems on a Linux Base: Additional Topics

The key message is that you should adjust your FLEX-ES system parameters (emulated

S/390 memory, disk caches, instruction cache) to avoid Linux swapping. Defining a smaller

emulated S/390 memory size may increase z/OS paging. Of course, it would be nice to avoid

any paging, but z/OS paging is much less damaging than Linux swapping and your trade-offs

should always be in this direction. You can juggle disk cache versus instruction cache versus

S/390 memory allocations for your best performance. Simply be careful not to push Linux into

swapping.

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2.3 Other monitoring tools

There are many Linux monitoring tools available. In addition to vmstat, the top command and

the GUI display available through gnome System Tools->System Monitor can be interesting.

Most of the information displayed by these (such as memory allocated to various processes)

should be fairly static during FLEX-ES operation. However, PC utilization will vary with the

S/390 workload and with Linux background processing. This is nicely displayed by the System

Monitor graphics.

The top command is started from a command line window; the window where you started

resadm may be convenient for this. Enter q to exit from top.

2.4 Disk caches

If you are using simple Linux files for your emulated S/390 volumes (as described in the

Getting Started publication), on a lightly-loaded small system, you can probably ignore disk

cache tuning. Linux will automatically use free memory as a disk cache and (if there is

sufficient free memory) this is often good enough for a small, non-production FLEX-ES

system. (There are negative effects for this mode of disk usage; these are described in

section 2.1.5 of the Getting Started publication.)

If you are using the raw disk interface for emulated disk volumes (described in “Raw disk

devices” on page 57), we

strongly recommend that you work with the disk cache parameters

described here. These can have

major effects on the performance of your system. Any larger

FLEX-ES system, especially those considered as production systems, should use the raw

interfaces.

FLEX-ES automatically caches 15 tracks of data for each emulated 3390 or 3380. You can

adjust this in three ways:

򐂰 Specify a different number of tracks to cache for a particular emulated drive.

򐂰 Specify a different number of tracks to cache at the control unit level; excess tracks (above

those needed for the specified or default cache for each device on the control unit) will

float, as needed, among all the devices on the control unit.

򐂰 Use the writethroughcache parameter to force a different operation of the cache (on a

device level). The default operation uses a writeback cache technique.

Here is an example that uses all three options:

(resource definitions)

....

c3990A: cu 3390

interface local(1)

options ‘trackcachesize=150’

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Again, we stress that “Linux paging” refers to steady-state operation after S/390 emulation is started. Linux

booting or FLEX-ES startup may cause Linux paging and we are not concerned with this temporary effect.