iSCSI switch selection and configuration

There’s no such thing as a dedicated iSCSI switch because iSCSI storage traffic runs on Ethernet networks. 

That doesn’t mean, however, that we can’t have a discussion about iSCSI switches as there are products with features that are suited to iSCSI network traffic optimisation. 

In this article we will examine the features available in off-the-shelf Ethernet switches suitable for iSCSI traffic optimisation. 

 iSCSI: converged or non-converged networks?

iSCSI switching is primarily employed in two main deployment scenarios; converged and non-converged networks.

In a converged network deployment the key switching feature required is the ability to create and manage VLANs, thereby segregating storage traffic from general network traffic. 

In a non-converged storage network deployment scenario, where iSCSI traffic is physically separated from main network traffic, this feature loses its value and port throughput becomes paramount to storage network speed. 

So, in a converged network the requirement would be for a layer 2/3 managed switch with VLAN capabilities.

In a non-converged storage network a layer 2 unmanaged switch with a high backplane/port throughput value is ideally suited to the role, especially where budget is an issue. A managed switch can be equally effective, but would incur a higher cost.

There are further switching features available that can help optimise iSCSI traffic beyond traffic segregation and port throughput. These include:

Flow control: Enabling flow control on a switch prevents a host being overwhelmed by network traffic levels. It grants the host control over the rate at which it receives data from NICs it is connected to.

Flow control relies on both the host NIC and the switch having this feature available. Flow control can help prevent packet loss and thus avoid any associated network latency, which is preferable in iSCSI storage networks.

Jumbo frames: This feature enables data packets with a payload size of more than 1,500 bytes. The theory is that a larger frame reduces the risk of packet loss and retransmits and puts less load on host and target device NICs.

This can translate into better iSCSI performance but there are processing costs associated with jumbo frame use and benefits have been found to be marginal under testing in some deployment scenarios.

Unicast storm control: This switching feature avoids traffic being disrupted by high levels of packet flow on a network and begins discarding packets when a port is loaded to, for example, more than 80% capacity. Usually it’d be switched on to avoid disruption to the LAN.

For iSCSI switching, however -- a unicast protocol -- it’s preferable to switch off the feature so that high levels of traffic are not identified as a storm and discarded. This allows ports in the storage network to operate at full capacity with no risk of induced packet loss.

Link aggregation control protocol (LACP): Also known a port trunking, this feature enables the aggregation of switch ports into higher bandwidth trunks to provide higher throughput and link redundancy.

Choosing iSCSI switches

With the increasing availability of 10Gbps Ethernet switches at competitive prices iSCSI traffic is not now the bottleneck to storage access. Rather, when selecting a switch for its ability to optimise iSCSI traffic the features detailed above should be viewed in the context of how the rest of the network is constructed and what other traffic types may coexist on the same hardware. 

Alternatively the approach could be to isolate iSCSI traffic physically from outside influence and concentrate on port throughput with the lowest cost overhead, or to add management abilities to the hardware at a greater cost but with possibly beneficial features. The key is to effectively plan an iSCSI deployment and then base the choice of switch on the requirements that emerge from this process.

Entry level iSCSI switch products

At the entry level and well featured for deployment in a dedicated iSCSI traffic-only network for an SME is Netgear’s JGS500 Gigabit unmanaged switch series. This offers up to 48Gbps of bandwidth at 2,000Mbps per port with all ports subject to IEEE 802.3x flow control, though jumbo frames are not supported.

The GreenNet TEG-S24g unmanaged switches offer a similar specification to the Netgear, with 48Gbps of bandwidth and flow control on board. Here, however, support for jumbo frames is included.

Both these switches come with price tags in the low hundreds and meet the high port throughput requirements of an iSCSI non-converged SME storage deployment where switch management features are not needed.

Converged iSCSI switches

Brocade

Brocade offers numerous switches with layer 2 and 3 capability that range from 1Gbps to 40Gbps and from entry-level to fully-featured. Some include Fibre Channel connectivity.

The various series include: the small-midrange or edge deployment FCX series that offers 24 or 48 10/100/1000Mbps Ethernet ports; the midrange TurboIron 24x 10Gbps series; BigIron RX Series switches are suited to for mid-large 1Gbps and 10Gbps Ethernet enterprise-class deployments, and; VDX layer 2 switches are aimed at very large enterprise 1Gbps and 10Gbps Ethernet deployments.

Cisco

Cisco’s Catalyst range are its low to midrange family of switches and come as several different series reflecting their heritage as acquisitions by Cisco.

The Catalyst 2360, for example is a midrange switch suited for medium-sized organisation deployment where VLANs are required on a converged LAN with iSCSI storage traffic. It has Link Aggregation Control Protocol (LACP) and VLAN Trunking Protocol (VTP) which supports dynamic VLANs and dynamic trunks. It has 48 ports with Gigabit Ethernet 10/100/1000-Gbps connectivity and four 10 Gigabit SFP uplinks plus support for 128 VLANs.  

Higher up the Cisco range are the Nexus series switches. These are designed for the fully converged datacentre and are multi-role switches capable of both Fibre Channel and Ethernet connectivity. With a cost to match their specification, the Nexus series switches are usually deployed in enterprise datacentres. They provide a fully-converged network capable of FCoE, iSCSI, Fibre Channel and Ethernet traffic with 32 10Gb Ethernet or Fibre Channel ports and VSAN capability.

Dell

Dell has a range of Ethernet switches available that start at the entry-level PowerConnect 2800 series with between 8 and 48 port and copper 1Gbps Ethernet connectivity and go up to the PowerConnect 8100 series with 10Gbps Ethernet port and 40Gbps connectivity on the 8164. VLAN capability is available throughout the range.

Dell Ethernet switches have an “iSCSI auto-configuration” feature that enables jumbo frames, disables unicast storm control and enables flow control.

HP

HP’s answer to the midrange Cisco Catalyst series are the Procurve 2910AL series switches. Four 10 Gigabit ports are included plus a 176Gbps switching fabric with up to 131Mbps available on the 48-port models. VLAN support is included as well as LACP and jumbo frames. The layer 3 capabilities of this switch series make it the top datacentre converged network switch that HP currently has on offer.

Sanrad

Sanrad markets a pair of switch products aimed at iSCSI SAN deployments; the entry-level V-Switch 2000 and the midrange V-Switch 3440. The 2000 has two 1Gbps Ethernet ports and two Fibre Channel, while the 3440 has three and four of each.