Sharing Bandwidth With Multiple Sas Hbas - Dell PowerVault MD3000i Tuning Manual

Dell™ PowerVault MD3000 and MD3000i Array Tuning Best Practices
Figure 11: MD3000i Advanced IPv4 Settings: VLAN, QOS, Jumbo Frames Support
5.1.3

Sharing Bandwidth with Multiple SAS HBAs

Each SAS wide port includes four full duplex serial links within a single
connector. The individual SAS 1.1 links run a maximum speed of 3Gb/s. A single
path is used as the primary path to a connected device — the second, third, and
fourth paths are used as overflow, when concurrent I/Os overload the primary
channel. For example, if the first link is transmitting data at 3Gb/s, SAS uses
10-bit encoding versus 8-bit for byte transmission, which makes a single 3Gb/s
capped at 300MiB/s. If another block of data then needs to be written to disk, for
example, and link 1 is still busy, link 2 will manage the overflow of data that
cannot be transmitted by link 1. If link 1 finishes its transmission of data, the next
block of data will be transmitted on link 1 again, otherwise another link will be
used. In this way, for heavy I/O workloads, it is possible that all links are being
December 2008 – Revision A01 Page 24