Network Congestion; Performance; Iscsi Initiators - IBM TotalStorage DS300 Best Practices Manual

Fibre channel SAN offers better performance, but is more expensive and requires
a higher skill set to implement. iSCSI and NAS offer better pricing and skills may
already be in place to implement them. However, both Fibre Channel and iSCSI
offer the performance benefit of Block I/O. These are all considerations that must
be taken into account when making the decision to add storage to the current
environment.

2.5.1 Network congestion

In the case of NAS, and backup over IP networks, congestion may cause variable
performance for other applications. This also remains a concern for iSCSI
implementations. This can be alleviated with high speed Gigabit Ethernet, and
largely overcome or masked by higher speed network transmission, such as 10
GBps.

2.5.2 Performance

A performance comparison is difficult to generalize because there are so many
variables. That said, Fibre Channel at 100 MBps (1 Gigabit/second) is generally
more efficient for I/O traffic than TCP/IP over Ethernet at equivalent bandwidth.
iSCSI performs better than NAS (when both are on Ethernet) due to reduced
protocol overhead. This is because it handles SCSI directly, rather than
translating between file-I/O protocols and SCSI.
TCP/IP is a software intensive network design which requires significant
processing overhead. This overhead can consume a substantial proportion of
available processor cycles when handling Ethernet connections. This is a
drawback for performance intensive storage applications.
A performance consideration, therefore, is the impact of the software protocol
stack on processor utilization. Fibre Channel SANs support SCSI commands
mapped directly to Fibre Channel media, and processor overhead for this
mapping is low. In iSCSI, handling of the TCP/IP protocol requires processor
cycles at both ends.

2.6 iSCSI initiators

As stated previously, iSCSI initiators consist of either software drivers or
hardware that initiate the SCSI requests over the IP network to a target device.
See 3.7, "iSCSI software and hardware" on page 58 for information specific to
the DS300. There are three ways that an iSCSI initiator can be implemented in
hardware and software. Each of the three approaches differs in price,
performance and capabilities.
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IBM TotalStorage DS300 and DS400 Best Practices Guide