zPDT home page
uPDT Technical Information
uPDT Reference Information
So how does the zPDT compare to other systems developers have used in the past? Will former
FLEX-ES users feel at home? Will Hercules users (yes, we know you are out there) have a
steep learning cure? What are the differences and advantages of zPDT?
ITC + HITS is uniquely positioned to answer these questions.
- Host Platforms:
All three systems (zPDT, FLEX-ES, and Hercules) will run on a Linux base system. Hercules will
also run in a Windows environment. (I consider this a disadvantage but will be kind and ignore
this option.) Both zPDT and Hercules seem to be less sensitive to the specific Linux
distribution used than FLEX-ES appears to be. I believe this is because FLEX-ES has more low-level
code that is likely to be affected by changing Linux distributions. We'll give a slight advantage
zPDT and Hercules for this category.
- Ease of Use:
There are many facets to "ease of use" and the three systems exhibit different characteristics.
If someone was given copies of the three systems and their documentation and told to install
and make them work I believe they would get zPDT operational first. FLEX-ES has the steepest
initial learning curve while both zPDT and Hercules are initially simpler. zPDT is simpler than
Hercules because it has fewer options to choose from and basic networking is simpler to implement.
As the system configuration gets more complex, with more networking, multiple instances, and
shared DASD and other devices, the picture changes a bit. FLEX-ES does a much better job of
simulating a LPAR environment and this makes a multiple instance configuration easier to
implement and operate than either zPDT or Hercules. I give zPDT the overall advantage in ease
of use while FLEX-ES comes in a close second because of the relative ease of implementing
more complex multi-instance configurations. Hercules suffers some because the documentation is
not as complete or clear, especially when compared to zPDT.
Ease of use frequently comes at the expense of flexibility and we see that in play here.
Both FLEX-ES and Hercules offer more options for things like architectural mode
(S/370, ESA, 64 bit, etc.) and other processor options. FLEX-ES also offers very
powerful configuration options, like multiple instances (very similar to LPARS), shared
DASD and other devices, network access to remote devices and possibly most important,
true channel attach capability with parallel and ESCON channel hardware. The ability to
share processors across multiple instances also provides significant added flexibility
In terms of a complete implementation of the System Z (of System/390) environment, each system
has some advantages. Both Hercules and FLEX-ES offer more architectural modes than zPDT.
FLEX-ES offers a much more complete simulation of the LPAR environment, including processors
shared across multiple instances. However, zPDT offers more of the current System z
functions, such as providing full QDIO capabilities. Network adapters emulate OSA Express
and can be configured accordingly. For example, with zVM we can make full use of a VSwitch
interconnecting multiple guests and then connecting to the outside network via OSA/QDIO
emulation on the network adapter. The availability of parallel and ESCON channels also makes
for a more complete implementation for FLEX-ES, although the lack of Fiber/FICON
channels negate the channel capability somewhat. Overall I believe FLEX-ES is more complete for users
who need to run older operating systems while zPDT may be better for those running current
level operating systems.
Performance is not just pure CPU speed in MIPS, but also I/O performance, and general eficiency.
For example I believe that zPDT is now slightly faster than the most current FLEX-ES (I say
that "I believe" because I have not been able to completely benchmark the most current FLEX-ES
Version 8 system.) Running in 64 bit mode on 64 bit hardware, zPDT can get a few (perhaps 5%)
more MIPS out of a given processor or core. However, FLEX-ES does I/O much more efficiently.
Heavy I/O workloads can easily consume 50% of a processor on zPDT while FLEX-ES rarely uses more
than 5 - 10% of a processor for I/O. I also believe that FLEX-ES's disk caching is more
effective than zPDT's or Hercules' use of the Linux filesystem caching. Both FLEX-ES and
zPDT will provide about 30 - 60% better performance than Hercules. So the advantage goes to
zPDT tor pure CPU performance or to FLEX-ES for better I/O performance/efficiency.
As you might have guessed by now, there is no clear overall "winner". Both FLEX-ES and
zPDT excel in some areas and not in others. In most categories Hercules comes in last, but
if we include a category of cost or code availability, it would certainly come in first there.
Unfortunately, we no longer have a choice of using FLEX-ES, so the logical choice,
for those who have the choice, will be zPDT.