FW 05.01.00 and SW 07.01.00 HP StorageWorks SAN High Availability Planning Guide (AA-RS2DC-TE, June 2003)

Planning Considerations for Fibre Channel Topologies
99SAN High Availability Planning Guide
Prior to fabric design, application I/O profiles should be estimated or established
that classify the application bandwidth requirements. Bandwidth consumption is
classified as light, medium, or heavy. These classifications must be considered
when planning ISL and device connectivity. For information about application I/O
(in Gbps) and fabric performance problems due to ISL connectivity, refer to “ISL
Oversubscription.” For information about application I/O (in IOPS) and fabric
performance problems due to port contention, refer to “Device Fan-Out Ratio”
on page 101.
ISL Oversubscription
ISL oversubscription (or congestion) occurs when multiplexed traffic from several
devices is transmitted across a single ISL. When an ISL is oversubscribed, fabric
elements use fairness algorithms to interleave data frames from multiple devices,
thus giving fractional bandwidth to the affected devices. Although all devices are
serviced, ISL and fabric performance is reduced.
Figure 41 illustrates ISL oversubscription. Two NT servers, each with maximum
I/O of 100 MBps, are contending for the bandwidth of a single ISL operating at
1.0625 Gbps. In addition to data, the ISL must also transmit Class F traffic
internal to the fabric. When operating at peak load, each NT server receives less
than half the available ISL bandwidth.
Figure 41: ISL oversubscription
NT Server 1
(100 MBps Max)
1 Gbps ISL
Storage
NT Server 2
(100 MBps Max)
TM
T
M
1
5
10 15
20
25
0
100
150
200
Time (Sec)
Bandwidth (MBps)
50
1 Gbps ISL
NT Server 1
NT Server 2