How to enhance and measure iSCSI SAN performance

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How to enhance and measure iSCSI SAN performance

Network optimization is crucial if you wish to achieve maximum iSCSI performance. Ideally, this should be done during the LAN infrastructure and topology design stage. If partially or completely modifying the network’s structure is not cost-effective, you can increase iSCSI SAN performance by configuring network equipment or replacing individual infrastructure components. This article will assume that your LAN is built on Gigabit Ethernet equipment, which is required for implementing most of the following advice:

Rule #1: iSCSI SAN traffic must be separated from user Ethernet traffic. Do this by setting up a virtual network (VLAN) on the existing network or building a separate isolated LAN segment exclusively for iSCSI SAN use. Splitting traffic guarantees more than just increased performance: it guarantees the security of the data transmitted over the network as well.

Use network interfaces with TOE (TCP/IP Offload Engine) support. Such equipment reduces the burden on local machine CPUs by offloading TCP/IP data processing to the chips used in the interface. One efficient solution is to use these interfaces on iSCSI targets, which have to process large numbers of requests from initiators (clients). TOE interfaces cost more than garden-variety Ethernet adapters, but they provide an appreciable increase in performance, particularly under large server loads.

The quickest, easiest way to accelerate equipment performance on Gigabit Ethernet networks is to use Jumbo Frames. By default, some network equipment uses standard 1500-byte Ethernet frames for greater compatibility. Jumbo Frames can hold up to 9000 bytes of data, providing a real boost to network performance. If one or more components of your network are set to use standard Ethernet frames, these components can become a real bottleneck for your network traffic overall.
Set all of your network interfaces to duplex mode. Some components may use half-duplex mode by default, which will slow down storage performance.

Ethernet routers may cause delays in iSCSI traffic as well. Keep things zippy with high-performance, low-latency routers.
Some routers may use oversubscription. This means that they can connect multiple devices to a single port and divide the maximum throughput between those. This is helpful when the bandwidth for clients is not too large: device resources are used efficiently in such a case. If the load on the router increases and the total device demands begin to exceed the port’s capacity, however, the devices begin to compete for bandwidth, which causes delays and reduced network performance. If these overloads can happen on your network, reconfigure it to avoid excess connections.
Naturally, these recommendations are of a general nature and do not account for the topology or specifics of any particular network. Use network monitoring utilities to identity problematic nodes on your LAN. Measure iSCSI performance with Iometer (http://www.iometer.org/), a standard utility for testing I/O subsystem speed. The client is installed on the initiators (running Windows, Linux, Solaris or NetWare), while the server component is installed on the target (only Windows is supported). If the server is overloaded, there may be small errors in the measurement result values, but in any case the utility allows measuring the relative change in iSCSI storage performance as a result of network configuration changes.