Bill Lancaster wrote:
> Tom Shaw wrote:
> >  Does anyone know what the appropriate minimum free space percentage
> >  should be on the system volume set in order to maintain system
> >  performance (disregarding other factors)? I have heard of everything
> >  from 20% to 50%.
> >

> There really isn't a pat answer for your question.  It depends on how dynamic
> your disk environment is.  [...good advice snipped...]

"Free" space is somewhat intuitive - have enough so that you don't run
out, bearing in mind transient space, spool space, sort work areas,
temporary files, etc.  If you have user volume sets, monitor the values
on the individual sets, particularly mpexl_system_volume_set (which is
where *all* of your transient and spool space will go).

A less-intuitive factor is disc fragmentation (as Bill mentioned).  It
is a big issue on MPE/iX now that there are no strict limits on the
number of disc extents.  It is a *major* issue on the system volume set
on 5.0/5.5, especially if you are using any posix applications (samba,
httpd, Apache, even "official" products like Open Market and inetd).
You can determine fragmentation from ":discfree a" and looking at the
number of free space regions and the distribution by size, notably the
max contiguous area.  Anything < 4K sectors on the system volume set
can cause trouble (though there's a patch in the works, I'll leave that
soapbox unattended tonight).

Also with 5.0/5.5, there are special considerations for ldev 1 if you
have only a few volumes in the set.  As of 5.0, the system enforces an
implicit limit of 50% capacity on ldev 1 regardless of your percentage
allocations in :volutil.  It will not allocate space on ldev 1 beyond
50% unless there is no other alternative.  This aggravates fragmentation
of the non-ldev-1 system volumes and can drastically imbalance the
allocation of transient space (paging areas) across the volume set.
If you are memory constrained (who isn't, given the right application
mix?) this can bottleneck paging I/O unless you have, say, 4-5 spindles
in the system volume set.  In rare cases this can be a blessing since
system pages typically occur on ldev 1 (allocated during boot-up, plus
swapping from NL/XL/SL.PUB.SYS) and if that drive is full, user stacks
get allocated on other volumes.  But on medium to large systems this is
not the usual case.

User volumes are relatively immune to "percentage" of free space given
that fragmentation is low as only job/session temporary files and of
course permanent files are allocated there.  The system volume set is
much more dynamic (and sensitive).

Jeff Kell <[log in to unmask]>