Users Guide

Performance Considerations for Tape Drives and Libraries 13
SAN Configurations
Fibre Channel (FC) offers many advantages over SCSI. First, it overcomes the distance limitations
of SCSI (12 meters for LVD SCSI versus 300 meters for a short-wave 2-Gb FC link)
and allows for
the transmission of data at higher speeds. As a serial network protocol rather than a bus-based
architecture like SCSI, FC has also become the protocol of choice for the implementation of SANs,
allowing for the consolidation of data storage resources. In addition, each FC connection is made
up of a transmit link and a receive link, allowing for full-duplex operation. This means that data can
be transmitted in two directions simultaneously. Therefore, during a backup operation across a
single FC connection, data can be read from a source and written to tape without taking turns in
communication, effectively doubling the bandwidth of a connection. See Figure 1-5.
Figure 1-5. Fibre Channel Link Diagram
When setting up tape libraries in a SAN, performance can still be affected by various factors. These
factors include FC link speeds, data flow between the source and tape library, and performance
limitations of external storage arrays. With an understanding of the overall setup and management
of the solution, many of these factors can be avoided.
Even with the high data bandwidth offered by the FC protocol in SANs, proper considerations
must be made for tape drives in order to avoid a situation in which the FC link may limit
performance. The data rate of a 2-gigabit (Gb) FC link is 200 MB/sec (that is, 200 MB/sec on the
transmit link and 200 MB/sec on the receive link). Therefore, attempting to operate multiple tape
drives across the same link can potentially exceed the full bandwidth of a link. If the host is
operating with a legacy 1-Gb adapter, backing up data to two drives may be sufficient to reveal
significant performance limitations.
Therefore, when using three or more tape drives simultaneously on a 2-Gb link, you may need to
distribute the backups across a number of connections, rather than relying on a single link. This is
where understanding the SAN solution's topology is beneficial. Following the data path during a
backup operation as it is read from the source and then written out to tape will help administrators
recognize any potential bottlenecks. If any bottlenecks are identified, measures may be taken
depending on the configuration. For example, if the backup solution requires multiple drives to be
in operation at once, splitting the tape hardware across separate fabrics may improve performance
by splitting the connections. See Figure 1-6.
HOST
Fibre
Channel
Device
Transmit
Receive
Receive
Transmit
2 Gb = 200 MB/sec
2 Gb = 200 MB/sec