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Typically, each Ethernet data frame transmitted requires a calculation of all the bytes in the frame to make
sure that the bytes were transmitted and received correctly. At GbE speeds, the processing power required
can slow the host computer down because the main CPU is used to perform the calculations. Several
Ethernet HBAs implement this calculation in hardware so that the main CPU does not have to perform the
calculations, which can improve performance.
The iSCSI protocol implements an additional data integrity check on iSCSI information, called header and
data digests. Both of these require processing power to calculate, and at GbE speeds can consume
significant main CPU cycles. Some iSCSI HBAs embed these digest calculations in hardware on the HBA,
thus offloading the main CPU of the host computer even more, improving performance.
Simple direct connect or switched networks should not require the use of digests.
Simple direct connect or switched networks are the recommended environment
Note
for your SNC 450x.
More complex networks, such as those using routers, bridges, gateways, or WAN can introduce undetected
errors. In these configurations digests are highly recommended to help protect against data corruption.
In general GbE iSCSI requires significant processing power. If you have a multiple processor host with
spare processing power, a standard GbE HBA is sufficient. If your host is heavily loaded, and the CPU runs
at a high utilization load, then you should use a HBA with processing offload. iSCSI GbE HBAs are available
from several vendors with TCP/IP checksum and iSCSI digest processing offload. Standard GbE HBAs,
many with TCP/IP checksum offload, are also available.
GbE Network Switches
Several unmanaged and managed GbE network switches are available from a variety of vendors. The
important specification to check when purchasing these switches is that the switch supports jumbo frames–
frames that are 9000 bytes or larger–and that all network switches in between the iSCSI host and target are
GbE jumbo frame-capable.
GbE Cabling
GbE requires Cat 5e cabling quality or better to sustain the highest data rates. Most current buildings are
only cabled for 100 Base-T (using two pairs of wires), and not 1000 Base-T (using four pairs of wires). If you
plug a GbE HBA into a standard 100 Base-T network jack, the network runs at 100 Base-T speeds. All
HBAs, network switches, and cabling must be GbE-capable to run at GbE speeds.
Host Ethernet Software Configuration
The biggest advantage in Ethernet performance may be gained by properly configuring at least the following
TCP/IP parameters:
•
TCP/IP window size
•
TCP/IP socket read/write buffer size
GbE network traffic processing provides a high load for the host computer system and requires attention to
optimize TCP/IP performance tuning. The large TCP/IP window size allows the system to buffer more data
before it is confirmed to the connection pair and compensates for user process turnaround time while
read/write buffer size allows for user program to use larger I/O operation sizes.
108
iSCSI Performance Tuning
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