At 08:42 AM 7/8/97 -0400, Christian Lheureux wrote:
>On July 8, Bill Lancaster wrote :
>
>[Lots of very, very valuable stuff deleted]
>
>> A couple of rules of thumb we followed:
>>
>> 1.  Don't mix unlike-size drives on a single volume set.  This is mostly an
>> operational consideration.  We wanted to avoid having the small volumes in
>> the set fill up first with plenty of space left on the larger volumes.
>
>[More good stuff deleted]
>
>MPE SSM (Secondary Storage Manager) module is supposed to fill volumes in
>terms of percentage, not absolute value, so it should not be a problem, as
>occupation percentages should be roughly similar ... to some extent, that is.
>SSM internals provides for checks to ensure that a given volume always has
>some free space. That is especially true for system disks. Degraded mode is
>expressed as a percentage of system disks occupation, critical mode is in
>absolute value, a very low value indeed.

The algorithm orders the candidate volumes by %full.  This results in the
larger disc, which has move free space, being the most frequent candidate.
As such, all new extent creation would land on this disc and therefore
access queues for this disc would tend to become longer, making it an
I/O bottle neck, affecting I/O performance.  So adding the disc to help
increase storage or improve I/O through-out can indeed result in a
further reduction in performance. This is big reason why there is a
warning of mixing discs of different capacities on the same volume set.

/plug-alert

The Disk Space Manager product from Bradmark helps spread the existing
files across all volumes in a volume set thereby more evenly distribute
I/O load when adding new volumes. For more information and a free demo,
contact Bradmark @ 800/621-2808 or 713/621/2808.

/end-plug