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New SAS 2.0 Features: Expanders, Performance

Next-Generation SAS: 6 Gb/s Storage Hits The Enterprise
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SAS 1.0/1.1SAS 2.0
Features
Preserves legacy SCSI
SATA compatible
3 Gb/s compatible
Improved signaling
Zoning management
Improved scalability
Storage Features
RAID 6
Small form factors
HPC
High capacity SAS drives
Ultra320 SCSI replacement
Choice: SATA or SAS
Blade servers
RAS (Data Protection)
Security (FDE)
Clustering
Larger Topologies
SSDs
Virtualization
External Storage
4K sector size
Line Rate and Cable Throughput4 x 3 Gb/s (1.2 GB/s)4 x 6 Gb/s (2.4 GB/s)
Cable Type
CopperCopper
Cable Length
8 m10 m


Source: www.scsita.org

Expander Zoning and Self-Configuring Expanders

Edge expanders and fanout expanders are nearly history. This is often attributed to updates in SAS 2.0, but the reality is that SAS zoning, which predates 2.0, was already paving the way to eliminating the edge/fanout distinction. Admittedly, zoning is usually implemented in a vendor-specific, rather than industry-standard fashion.

In a nutshell, various SAS zones can now be deployed across shared delivery subsystem hardware. This means that targets (drives) in a storage appliance can be accessed by various initiators through the same SAS expander. Domain segmentation is taken care of through zoning, and access is managed in an exclusive, non-shared manner.

With device discovery moving from the initiator to the SAS expanders, this allows expanders to self-configure their functions. Discovery time on large topologies can thus be reduced and zoning could be standardized to get rid of proprietary solutions.

Doubled Link Throughput

SAS 2.0 mandates that the bandwidth per port doubles, from 3 Gb/s to 6 Gb/s. Quad links, which are typically used for wide ports, go from 1.2 GB/s to 2.4 GB/s. And since SAS HBAs typically offer eight ports organized into two wide ports, the effective maximum throughput stands at 4.8 GB/s.

PCI Express 1.1 then becomes a bottleneck because the popular x8 link width only offers 2.0 GB/s (250 MB/s per lane each way). As a result, all upcoming SAS 2.0 hardware will also be based on PCI Express 2.0 host interfaces, doubling the available bidirectional bandwith to the host to 4.0 GB/s.

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