RE: subject: iSeries Legends

[James Rich <james@xxxxxxxxxxx>]

On Tue, 24 Jun 2003, Joe Pluta wrote:

> Anyway, I was saying that unless you take into account the actual workload,
> uptime numbers are like any other statistics.  As I said, based purely on

Good point.  However, it is not true that "a web/mail server is not
exactly a tough job" - it all depends on what kind of web/mail traffic you
are dealing with and how you are processing it.  If you are getting the
numbers of hits that my web server does then no, it not a tough job.  But
neither is the job that our AS/400 does (we are a small shop).

You are right that what the machine is doing is important when considering
reliability (after all, if it isn't doing anything then how is that
different than being off?).  But perhaps an even better criteria for
reliability is how important a machine's function is rather than the
average load.  For many of my customers, the unix machine that runs their
network is just as important as their iSeries.  If either goes down they
can't do business.  The average load on the unix machine is not as high
(though a direct comparison is not really possible, unix architecture is
very different from OS/400 and workloads are handled very differently) but
the unix machine is up more than the iSeries.

Again you are right that looking at what the machine does is important
when considering uptime.  One reason the iSeries experiences less uptime
than unix (in our case, not necessarily in general) is that it undergoes
scheduled downtime for things like backups, upgrades, PTFs, etc.  Most of
my customers IPL their iSeries on a weekly basis to do things like clean
up spooled files and recovering disk space.  This isn't strictly a
shortcoming of the OS, rather a decision by the customer to reboot
(however, it seems stupid to me that an IPL is required to do that).  PTFs
require an IPL usually, whereas unix generally does not require rebooting
when applying patches (except when patching the kernel itself).  But the
largest cause of "downtime" is backups.  This isn't true downtime because
the system isn't down.  Rather, it is just unavailable.  It can be argued
successfully that unavailability and downtime are the same thing.  But we
won't do that here.  But we do see the the job of the iSeries creates a
scenario where scheduled unavailability is required for the extensive
backup.  Unless you are doing work that requires such extensive backups
you don't have to schedule that "downtime".

However it should be noted that unix includes methods of backup that do
not require such large amounts of "downtime".  Snapshots and last changed
with hardlinks are two such methods.  These allow almost instantaneous
copy of large amounts of data that can then be transferred to tape without
requiring locks on the objects being copied.

James Rich