IBM System Storage DS3500 Introduction And Implementation Manual page 31

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Draft Document for Review March 28, 2011 12:24 pm
SAS wide ports
lanes ,
Each SAS port includes four full duplex links or within a single connector as shown in
Figure 1-4, with each lane running a speed of 6 Gbps. A single lane is used as the path to the
drives; the second, third, and fourth lanes are used as overflow when concurrent I/Os
overload the channel. For example, suppose the first link is transmitting data at 6 gigabits per
second. If another block of data then needs to be written to disk, while the link one is still busy,
then link two will manage the overflow of data that cannot be transmitted by link one. If link
one finishes its transmission of data, then the next block of data will be transmitted on link one
again; otherwise, another link will be used. In this way, for heavy I/O workloads, it is possible
that all links are being used at certain times, providing a simultaneous data speed of 24 Gbps.
Figure 1-4 SAS wide ports
SAS drive technology
Figure 1-5 on page 8 shows how SAS drives are attached to the controllers. The
point-to-point topology used in SAS configurations, as shown below, means that there is a
direct path to each drive from each controller, so communication can take place directly, with
no effects caused by an individual drive failure.
7
Chapter 1. Disk attachment technology