7065DiskAttach.fm Draft Document for Review August 30, 2007 12:59 am
8 IBM System Storage DS3000: Introduction and Implementation Guide
from each controller - so communication can take place directly, with no affects caused by an
individual drive failure.
Figure 1-2 SCSI and SAS disk technology compared
1.3 iSCSI disk attachment
iSCSI stands for Internet SCSI. The basic idea of iSCSI is to use the Ethernet infrastructure
on the physical layer and TCP/IP communication for transferring encapsulated SCSI
commands and data. Using iSCSI,
IP SANs can be implemented. Because an Ethernet
infrastructure is readily available at low cost, iSCSI is a good choice for the SMB and also
departmental SANs of larger companies.
IP SANs are a cheaper alternative to FC SANs; however the lower cost of iSCSI also implies
lower performance and scalability. Encapsulation adds overhead and the transfer rate is
lower. A typical Ethernet network operates at 1 Gbps, while a FC SAN can run at 4 Gbps.
However, there are ways to address iSCSI performance:
While the host servers can almost any Ethernet network interface card for iSCSI traffic,
this does mean the CPUs in the host server have to run the iSCSI stack (to perform
encapsulation of SCSI commands and data). This causes CPU and memory overhead,
which can impact performance.
For increased performance, it is better to use dedicated iSCSI HBAs to process the
TCP/IP stack. This technology is known as
TCP Offload Engine (TOE). TOE technology
relieves the CPUs in the host server from having to process the SCSI encapsulation,
which can lead to better performance.
Ethernet transfer rate is growing. 10 Gbps Ethernet is coming. The standards for 10 GbE
have been defined, however at the time of writing this is a fairly new technology, that has
yet to gain wider commercial acceptance. Initial offerings will also carry significant cost,
which is expected to drop as the user base increases.
1.3.1 iSCSI initiators and targets
iSCSI uses the concept of initiators and targets as shown in Figure 1-3 on page 9
Loop Topology (SCSI) Pt-to-Pt Topology (SAS)
Controller
Controller
Controller
Controller
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