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Chapter 3 System Preparation
198 September 2002 HPSS Installation Guide
Release 4.5, Revision 2
❖ Storage Class Sharing - Are any storage classes shared amongst more than one
hierarchy? If so, is that intentional and if so, discuss ramification of sharing vs.
not sharing (usually boils down to avoiding “pockets” of unused storage – one
SC full while another is nearly empty – vs. not being able to separate one set
of users or accesses from another).
➢ Disk Storage Class List
❖ Minimum & Maximum Storage Segment Sizes / Average Number of
Segments - Verify that the values specified, reasonably balance the trade-off of
causing HPSS to track too many segments (segment sizes too small or average
number of segments too large) vs. wasting too much space on average per file
(segment sizes too large or average number of segments too small). If a good
value can not be found for the expected file size distribution to balance these
two extremes, maybe we need more storage classes?
❖ Virtual Volume Block Size & Stripe Width - When striping is used, you need to
be a little careful that these values fit well with those of any other storage class
which is in the same hierarchy, as we will potentially be moving data between
the levels and need to be sure that when the two sets of Movers are chatting
that we don’t wind up with excessive overhead, nor do we wind up with extra
tape segments.
A general rule of thumb is to have the Virtual Volume Block Size values be the
same, so that with good power of two stripe widths, we wind up with a decent
set of Mover to Mover connections (i.e., each Mover on one side does not have
to connect to every Mover on the other). This also usually guarantees that you
don’t wind up with more than one Mover on one side waiting for one on the
other.
The other consideration is to be sure that the minimum disk storage segment
size times 32 (this is the maximum number of source/sink descriptors per IOD
– if this maximum changes, please change these guidelines) is a multiple of the
tape Virtual Volume Block Size times the stripe width. If this is not true, we
could wind up with extra tape storage segments for a file due to the fact that
the disk side can only send 32 segments in an IOD.
❖ Thresholds - Verify that the threshold values seem reasonable. Compare them
to the thresholds for the purge policy – will the SC thresholds be reached
before purging is kicked off? If so, then at least the warning level will likely be
ignored after a period of time.
❖ Migration & Purge Policy Ids - One thing to consider is how many tape drives
will be allocated should more than one of these storage classes be migrating at
once. If a large percentage of the drives – or even more drives than are
physically available – are reserved for migration at a given point in time, will
the retrieval response be acceptable?
➢ Tape Storage Class List
❖ Media block size
❖ Blocks Between Tape Marks - Verify that these two match the admin guide
(which should actually be the default values for 4.2). If not, why not?