IBM Hub/Switch Installation Manual page 95

cannot be greater than half the number of drives available. Also, doing multiple copies from disk
to two tape storage classes with the same media type will perform very poorly if the stripe width
in either class is greater than half the number of drives available. The recover utility also requires a
number of drives equivalent to 2 times the stripe width to be available to recover data from a
damaged virtual volume if invoked with the repack option.
Guideline 2: Select a stripe width that results in data transmission rates from the drives matching
or being less than what is available through rates from the network.
Explanation: Having data transmission off the devices that is faster than the network will waste
device resources, since more hardware and memory (for Mover data buffers) will be allocated to
the transfer, without achieving any performance improvement over a smaller stripe width. Also, if
a large number of concurrent transfers are frequently expected, it may be better (from an overall
system throughput point of view) to use stripe widths that provide something less than the
throughput supported by the network - as the aggregate throughput of multiple concurrent
requests will saturate the network and overall throughput will be improved by requiring less
device and memory resources.
Guideline 3: For smaller files, use a small stripe width or no striping at all.
Explanation: For tape, the situation is complex. If writing to tape directly, rather than via disk
migration, writing a file will usually result in all the tape volumes having to be mounted and
positioned before data transmission can begin. This latency will be driven by how many mounts
can be done in parallel, plus the mount time for each physical volume. If the file being transmitted
is small, all of this latency could cause performance to be worse than if a smaller stripe or no
striping were used at all.
As an example of how to determine stripe width based on file size and drive performance, imagine
a tape drive that can transmit data at about 10 MB/second and it takes about 20 seconds on average
to mount and position a tape. For a one-way stripe, the time to transmit a file would be:
<File Size in MB> / 10 + 20
Now consider a 2-way stripe for this storage class which has only one robot. Also assume that this
robot has no capability to do parallel mounts. In this case, the transmission time would be:
<File Size in MB> / 20 + 2 * 20
An algebraic calculation indicates that the single stripe would generally perform better for files that
are less than 400 MB in size.
Guideline 4: Migration can use larger stripe widths.
Explanation: For migration operations from disk, the tape virtual volume usually is mounted and
positioned only once. In this case, larger stripe widths can perform much better than smaller. The
number of drives available for media in this storage class also should be a multiple of the stripe
width. If not, less than optimal use of the drives is likely unless the drives are shared across storage
classes.
HPSS Installation Guide
Release 4.5, Revision 2
September 2002
Chapter 2 HPSS Planning
95