Serial Attached SCSI (SAS) is the next evolution of SCSI beyond Ultra320. It utilizes serial, point-to-point, switched architecture and promises to greatly expand upon the existing capabilities of its parallel predecessor. Serial Attached SCSI intends to leverage the 20 years of proven SCSI legacy middleware/software that is entrenched in the enterprise, as well as deliver a powerful small form factor connection scheme, capable of achieving substantially better performance and scalability than parallel SCSI offers today.
Some of the key features and benefits of SAS are:
| Features | Benefits |
- Point to point topology
- Performance starts at 3.0 Gbps, scalable to 6.0 Gbps and beyond
- Fewer signals than parallel buses
- Thinner cables
- Disk/backplane interoperability
| - Dedicated disk connections with scalable throughput
- Future performance and investment protection
- Easier routing, higher density, lower cost connectors
- Improved chassis airflow
- Flexible Serial Attached SCSI and Serial ATA deployment options
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Features like these are accelerating the transition to serial technologies and SAS is expected to replace parallel SCSI in the multi-user storage environment over the next few years. It will be utilized everywhere SCSI is used today and is likely to be a popular choice for internal RAID configurations, particularly those in mid-range server designs. In the SAN industry, vendors are also favoring a migration from 3 1/2" toward 2 1/2" HDDs. Because SAS and Serial ATA better accommodate these small form factor designs, an unprecedented degree of storage density is achievable. This allows system vendors to offer high performance RAID capabilities within the increasingly popular 1U and 2U server chassis and emerging “blade server” enclosures.
SAS defines an external (box-to-box) connection scheme using InfiniBand 4x cables. Known as “Wide” links, these connections include 2 or 4 physical links and are used to aggregate bandwidth and provide a convenient way to attach multiple external end devices.
The SAS specification also introduces a Fan-out PHY architecture that utilizes Expander devices to enable one or more SAS host controller ports to connect to a large number of drives. Each Expander allows connectivity to 64 ports, which may include host connections, other Expanders, or hard disk drives.
Expanders have a profound effect on test and analysis because they have a unique ability to use different physical pathways across a wide link to complete a single SCSI operation. This means the SAS Initiator can open a connection to a SAS target device yet complete the transfer over a different physical connection. These transfers over wide links may change their pathways dynamically within the expander device.
This increases the complexity of SAS test and debug because it requires users to monitor multiple links concurrently to record all dwords associated with a single SCSI transaction. Engineers can’t predict or control which physical pathway is used which means they must have the ability to monitor all relevant paths. Only by monitoring all 4 links in a wide connection can the user be assured they will record all frames associated with the transfer.
Other complexities introduced by wide links:
CATC’s new SASTracer is the first protocol analyzer designed from the ground up to support wide-links. Using a single plug-in module, the system transparently taps 2-wide links. Plus, it is expandable using a second plug-in module to concurrently monitor all frames on a 4-wide link. It also displays multilane traffic interleaved and synchronized to a common reference clock. SASTracer automatically groups frames that are part of a common transaction to better show the logical relationship between SSP initiators and targets. SASTracer provides powerful analysis features in a highly portable form factor that can simplify multi-link verification and accelerate time to market for SAS-based product designs.
