A P P E N D I X  G


Introduction to Serial Attached SCSI

This section provides a basic overview of the main features of Serial Attached SCSI (SAS), introduces some common SAS terms, and explains how SAS differs from parallel SCSI.

Note - This section is not specific to the Sun StorageTek SAS RAID External HBA. Instead, this appendix provides general information. Diagrams are for reference purposes only and do not represent any specific configurations supported by the Sun StorageTek SAS RAID External HBA.

For technical articles and tutorials about SAS, refer to the SCSI Trade Association (STAtm) web site at www.scsita.org.

The appendix contains the following sections:

Terminology Used in This Appendix

For convenience, SAS HBAs and SAS RAID HBAs are referred to generically in this chapter as SAS cards. HBAs, RAID HBAs, disk drives, and external disk drive enclosures are referred to as end devices and expanders are referred to as expander devices.

For convenience, this chapter refers to end devices and expander devices collectively as SAS devices.

About SAS

Legacy parallel SCSI is an interface that lets devices such as computers and disk drives communicate with each other. Parallel SCSI moves multiple bits of data in parallel (at the same time), using the SCSI command set.

SAS is an evolution of parallel SCSI to a point-to-point serial interface. SAS also uses the SCSI command set, but moves multiple bits of data one at a time. SAS links end devices through direct-attach connections, or through expander devices.

SAS cards can typically support up to 128 end devices and can communicate with both SAS and SATA devices. (You can add 128 end devices--or even more--with the use of SAS expanders. See SAS Expander Connections.)

Note - Although you can use both SAS and SATA disk drives in the same SAS domain (see SAS Expander Connections), Sun recommends that you not combine SAS and SATA disk drives within the same array or logical drive. The difference in performance between the two types of disk drives may adversely affect the performance of the array.

Data can move in both directions simultaneously across a SAS connection (called a link--see About SAS Device Communication). Link speed is 300 MB/sec in half-duplex mode. Therefore, a SAS card with eight links has a bandwidth of 2400 MB/sec.

Although they share the SCSI command set, SAS is conceptually different from parallel SCSI physically, and has its own types of connectors, cables, connection options, and terminology, as described in the rest of this chapter.

To compare SAS to parallel SCSI, see Differences Between SAS and Parallel SCSI.

About SAS Device Communication

SAS devices communicate with each other through links. A link is a physical connection between two phys.

As shown in the following figure, SAS devices contain ports (see About SAS Ports), ports contain phys, and each phy contains one transmitter and one receiver--one transceiver. A phy can belong to one port only.

FIGURE G-1 SAS Device Communication

The figure is a block diagram that shows ports on a SAS device transmitter connecting to a receiver.

About Phys

Phys are part of the physical communication connection between SAS devices. Each phy contains a transceiver that sends data back and forth between SAS devices.

When a connection is formed between two end devices, a link is established from a phy in one port to a phy in the other port. As shown in the figure above, a wide port can support multiple independent links simultaneously.

Phys are internal, within SAS connectors (see About SAS Connectors).

SAS cables physically connect one or more phys on one SAS device to one or more phys on another SAS device.

About SAS Ports

Note - Because the physical link between SAS devices is from phy to phy, rather than port to port, a “port” is more of a virtual concept, different from what is normally considered a port on other types of RAID HBAs and storage devices.

A port is one or more phys. A narrow port contains one phy. A wide port typically contains four phys.

Each port has its own unique SAS address (see About Identifying Disk Drives in SAS), and all the phys in a port share that same SAS address.

SAS card port options vary. A SAS card with four phys could be configured with one wide port, with two wide ports that comprise two phys, or with four narrow ports each containing one phy. (A wide port with four phys is referred to as a 4-wide or 4x port.)

About SAS Addresses

Each SAS port is identified with a unique SAS address, which is shared by all phys on that port.

For example, a SAS disk drive might have two narrow ports. Each port has one unique SAS address. The single phy in each port uses its port’s SAS address.

In another example, a SAS device might have one 4-wide port. That port has one SAS address, which is shared by all four phys in the port.

Unlike SCSI devices and SCSI IDs, SAS devices self-configure their SAS addresses. User intervention is not required to set SAS addresses, and SAS addresses cannot be modified.

About SAS Connectors

A SAS or mini-SAS connector is the physical plug or receptacle that you see on a SAS device. It’s what you plug a SAS cable into, or the end of the SAS cable that’s being plugged in.

A connector is what forms physical links between phys. Some SAS connectors can support multiple links. The number of links a SAS connector can support is referred to as its width. Narrow connectors support a single link; wide connectors supports more than 1 link.

A single SAS device may have one or more connectors. A single SAS connector may help form links between more than two SAS devices.

About SAS Cables

Internal standard SAS cables are narrower than internal parallel SCSI cables. The connectors vary in size depending on the number of links they support, from single link connectors to 4-wide (or larger) connectors. Internal fan-out cables let you attach four disk drives to a single 4-wide connector.

Mini-SAS connectors support both internal and external SAS connections. The mini-SAS connectors are smaller than the standard SAS internal and external connectors. Mini-SAS connectors support single and multilinks with the ability to scale to future speed needs.

About Identifying Disk Drives in SAS

In the BIOS and in the management utilities (see To Identify a Disk Drive With the Disk Utilities), disk drives are identified in the following formats:

where X is the count number.

Note - Devices other than disk drives (CDROM, tape drives, and so on) are listed in order after the system disk drives.

In parallel SCSI, XX is the disk drive’s channel number, YY is the target number, and ZZ is the logical unit number (LUN).

About SAS Connection Options

You can connect end devices to each other through direct cable connections and through backplane connections. When you use one or more expander devices (see SAS Expander Connections), you can create large configurations.

Direct-Attach Connections

In a direct-attach connection, SAS or SATA disk drives are connected directly to a SAS card with SAS or mini-SAS cables. One disk drive is attached to one SAS/mini-SAS connector with one SAS/mini-SAS cable (or multiple disk drives are attached to one SAS/mini-SAS connector with one fan-out cable).

The number of direct-attached disk drives is limited to the number of phys supported by the SAS card. (Note that there may be multiple phys within a single connector. See SAS Expander Connections.)

Backplane Connections

In a backplane connection, disk drives and SAS cards are attached to and communicate with each other through a system backplane.

There are two types of backplane connections, passive and active. When connecting to either backplane, it’s important to properly connect the disk drive LEDs in order to identify disk drive conditions. See Component Layout for the RAID HBA Activity LED connections and locations.

When connecting to a backplane, the Sun StorageTek RAID Manager enables you to manage the system disk drives, see the Sun StorageTek RAID Manager User’s Guide.

The number of end devices is limited to the number of slots available on the backplane. For example, the Sun S50 enclosure, which contains an expander, is a backplane connection that supports up to 12 SAS or SATA disk drives.

Some backplanes support daisy-chain expansion to other backplanes. For example, you can daisy-chain (connect one to the next) up to nine Sun S50 enclosures to a single SAS card in a host system.

SAS Expander Connections

A SAS expander device literally expands the number of end devices that you can connect together. Expander devices, typically embedded into a system backplane (see Backplane Connections), support large configurations of SAS end devices, including SAS cards and SAS and SATA disk drives. With expander devices, you can build large and complex storage topologies.

There are two types of SAS expanders: fanout expanders and edge expanders. Each performs a different role in a storage system. (For more information about how SAS expanders work, refer to the STA Web site at www.scsita.org.)

You can connect up to 128 SAS ports to an edge expander. (A single edge expander can therefore support up to 128 SAS addresses.)

You can connect up to 128 edge expanders to a fanout expander.

You can use only one fanout expander in any single SAS domain (a topology of SAS--and possibly SATA--end devices and expander devices). A single SAS domain can therefore comprise up to 16,384 SAS ports (and therefore up to 16,384 SAS addresses including the fanout expander).

The next figure illustrates (in very basic terms) a SAS domain and shows how SAS cards, SAS and SATA disk drives, and expander devices can fit together in a large data storage topology.

FIGURE G-2 SAS Expander Connections

The figure shows a SAS domain and shows how SAS cards, SAS and SATA disk drives, and expander devices can fit together in a large data storage topology

Differences Between SAS and Parallel SCSI

In summary, although SAS and parallel SCSI both use the SCSI command set, how they move data from one place to another is very different. To support point-to-point serial data transport, SAS introduces new types of connectors, cables, connection options, and terminology.

Generally speaking, SAS is faster and more flexible than parallel SCSI, and provides more options for building your storage space. SAS lets you mix SAS and SATA disk drives together, and lets you connect many, many more devices.

This table describes many of the main differences between the two interfaces.

TABLE G-1 Differences between Parallel SCSI and SAS

Parallel SCSI

Serial Attached SCSI

Parallel interface

Serial interface

Maximum speed 320 MB/sec shared by all devices on the bus

Maximum speed 300 MB/sec per phy when in half-duplex mode

Supports SCSI devices only

Supports SATA and SAS disk drives simultaneously

Up to 16 devices per SCSI channel

More than 100 disk drives per SAS card, using an expander (see SAS Expander Connections) or 50 SATAII disk drives.

Supports single-port devices only

Supports single- and dual-port devices

Uses SCSI IDs to differentiate between devices connected to the same adapter

Uses unique SAS addresses to differentiate between devices

User intervention required to set SCSI IDs

SAS addresses self-configured by SAS devices

Requires bus termination

Requires no bus termination

Standard SCSI connectors

SAS connectors