White Papers

BP1028 EqualLogic iSCSI for Fibre Channel Professionals 4
4 Fabric architecture comparison
4.1 Network congestion and prevention
FC utilizes a credit or token method of dealing with congestion, or more accurately of preventing
congestion altogether. When end points (hosts and storage) initiate communication, they exchange
operating parameters to inform each other about their capabilities and decide how much data they
can send to each other. Each device can only send data if it has “credit”. In simpler terms, the
receiving device must have enough buffer space to store the data from the sender. This system
ensures that FC frames are not typically dropped in normal operation.
Ethernet relies on congestion avoidance algorithms built into TCP/IP and Flow Control capabilities
between switches and end points. Without Flow Control enabled, one or more devices could cause
another to start dropping packets when its buffer is filled or near full depending on implementation.
With Flow Control enabled, PAUSE frames are sent to manage packet flow rate in hopes of avoiding
congestion or the overload of a switch port or end device. However, some packets may still be
dropped and when packets are dropped, TCP/IP will attempt to resend or re-transmit those missing
packets or packet segments. Occasional retransmissions are normal for TCP/IP; however continual
retransmitting can have a significant impact on performance.
For Flow Control to function properly, all devices on the network (or SAN) must enable and support it.
Otherwise, packets may be dropped or ignored by that device. EqualLogic PS Series array ports are
always automatically configured to negotiate flow control. Switches and host ports may require
manual configuration depending on the device.
Another recent technology for preventing congestion on Ethernet networks is the use of Data Center
Bridging (DCB). DCB introduces Priority Flow Control (PFC) and Enhanced Transmission Selection
(ETS) to help create a “loss-less” Ethernet environment. DCB does require hardware support so it is
most often found in newer models of 10 Gb Ethernet switches. FC over Ethernet (FCoE) devices
require features found in DCB to function properly. For proper support of iSCSI devices, an iSCSI
Type-Length Value (TLV) is required to allow the device to identify and separate iSCSI packets. While
an in-depth discussion of DCB is out of the scope of this paper, detailed information can be found in
Creating a DCB Compliant EqualLogic iSCSI SAN with Mixed Traffic , which is available on
delltechcenter.com
.
http://www.delltechcenter.com/page/Creating+a+DCB+Compliant+EqualLogic+iSCSI+SAN+with+Mi
xed+Traffic