Server or between two IBM S/390 Integrated Servers.The main reason for
splitting workload is to achieve better performance when accessing data.
Disk I/O Performance and Caching
The disk I/O rate (measured in S/390 I/O operations per second) which the IBM
S/390 Integrated Server is able to sustain will vary greatly from system to system,
depending on workload characteristics and system configuration.Some workloads
may saturate the I/O subsystem at an average of 100 I/Os per second, while other
workloads can successfully run with peaks up to 400 I/Os per second.If the
expected S/390 disk I/O rate falls between 100 and 400 I/Os per second, check
with your certified reseller for additional guidance.
In general, the more I/O intensive the workload, the higher potential for bottlenecks.
By
using data caching techniques, I/O bottlenecks can be reduced.
The IBM S/390 Integrated Server environment throughput can be improved through
I/O caching.Caching writes in either of the methods described below may improve
the ability to drive higher I/O rates through the IBM S/390 Integrated Server
depending on the read/write ratio.However, because the standard integrated
controller's cache is not backed up with a battery, certain system failures can cause
data integrity exposures that can be minimized by using an optional IBF.See
“Power Management” on page9.
Software caching using the IOSP memory to store I/Os before writing to the hard
disk is enabled using the LAZY WRITE option of OS/2.A specific amount of
memory is reserved for caching use.This value is defined in the CONFIG.SYS file.
For
a High Performance File
System (HPFS), use the CACHE command to enable
and disable lazy-writes.(The default is on.)For
a FAT file system use the
DISKCACHE command.
When a write operation is issued from a program, the I/O is written to the cache
and the issuing program is informed that the write I/O is complete before the data is
actually written to the hard disk.Data integrity exposure exists if there is a power
failure (without an IBF), an OS/2 failure that cannot be recovered with a soft boot,
or some hardware caused failure.
Another form of caching is to make use of S/390 Expanded Storage to allow the
S/390 operating system to keep more data in storage and avoid doing I/O
operations. IOSP memory can be used to emulate S/390 Expanded Storage.This
may offer a significant overall performance improvement for systems and
applications which can take advantage of Expanded Storage.This is available only
on the VM/ESA and OS/390 operating systems.If your VSE/ESA operating system
is executing natively, third party software (Opti-Cache) from Barnard Softare Inc. is
available to provide the advantages of S/390 expanded storage.
As with all systems, the performance benefits and applicability of caching is
dependent on the operating system and its workload characteristics.
24 S/390  Integrated Server