SAS
What is Serial-Attached SCSI (SAS)?
Serial-Attached SCSI (SAS) is a technique for accessing computer peripherals by transmitting digital data serially (one bit at a time) through thin cables. In enterprise settings, SAS is especially useful for linking mass storage devices such as external hard drives and magnetic tape drives.
The American National Standards Institute defines SAS under the Serial-Attached SCSI standard, also known as ANSI/INCITS 376-2003.
What Does SCSI Mean?
Small Computer System Interface (SCSI) is a set of ANSI electronic interfaces enabling personal computers (PCs) to connect with peripheral devices like tape drives, disk drives, printers, CD-ROMs, and scanners. SCSI communication is faster and more versatile compared to earlier parallel data transfer interfaces.
How does SAS Work？
SAS is a protocol designed for point-to-point serial communication between storage devices and the computers they serve. "Point-to-point" refers to direct data transfers between connected devices, such as a storage device and a computer, through a physical cable. "Serial" means that data is transmitted sequentially, one bit at a time.
The SAS protocol is used in computers to create dedicated connections between the computer and storage devices such as disk drives, tape drives, and other SCSI storage devices via host bus adapters (HBAs) over a serial interface.
A SAS system comprises four main components:
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Initiator: A device that initiates service and task management requests for processing by a target device and receives responses. Initiators may be built into the motherboard, especially in server-oriented systems, or provided as an add-on host bus adapter.
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Target: A device containing logical units and target ports that receive service and task management requests and send responses to the initiator. Examples of target devices include hard disk drives and disk array systems.
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Service Delivery Subsystem: The part of the I/O system responsible for transmitting information between an initiator and a target. This typically includes cables connecting the initiator and target, with or without expanders and backplanes.
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Expanders: Devices within the service delivery subsystem that facilitate communication between SAS devices, allowing multiple SAS end devices to connect to a single initiator port.
The current SAS standard supports up to 255 direct connections, whether they are storage devices, hosts, or expanders connected to each port. Each expander can support up to 255 additional connections, allowing for a maximum of 65,535 SAS devices if all available connections are used for expanders.
SAS History
The Serial-Attached SCSI (SAS) standards are developed by the INCITS/T10-SCSI Storage Interfaces committee. The initial SAS standard, SAS-1, was released in 2003 and offered a data bandwidth of 3 gigabits per second (Gbps). Subsequent versions increased this capacity: SAS-2 supports 6 Gbps, SAS-3 supports 12 Gbps, and the current version, SAS-4, supports 24 Gbps. SAS-5, which is currently in development, is expected to exceed 24 Gbps.
Serial-Attached SCSI vs. Parallel SCSI
Parallel SCSI, which defines protocols for using SCSI devices over parallel connections. Although both SAS and parallel SCSI use the SCSI command set, they differ significantly in how they transfer data.
The table below outlines many of the key differences between these two interfaces.
| Feature | SAS | Parallel SCSI |
| Bus Operation Mode |
Operates in point-to-point mode, each device is
connected to the initiator via a dedicated link,
no contention.
|
Operates in multi-point mode, contention can
occur even if one initiator is connected to one target.
|
| Termination Issues |
No termination issues, does not require terminator
packs.
|
Requires terminator packs. |
| Clock Skew | Eliminates clock skew. | Clock skew is present. |
| Device Connection Count | Supports up to 65,535 devices through expanders. |
Supports a maximum of 8 or 16 devices on a single
channel.
|
| Data Transfer Speed |
Supports higher speeds, SAS-1, SAS-2, SAS-3, and
SAS-4 support data bandwidths of 3,
6, 12, and 24 Gbits/sec respectively.
|
Data bandwidth is shared across the entire
multi-point bus, resulting in lower speeds.
|
| Port Functionality |
Dual-port, allows redundant backplanes or
multipath I/O (dual-domain SAS).
|
Single-port connection. |
| Command Set | Uses SCSI command set. | Uses SCSI command set. |
| Cables and Connectors | Thinner and longer cables, smaller connectors. | Thinner and longer cables, smaller connectors. |
| Hardware Cost | Lower hardware cost for serial interfaces. | Higher hardware cost. |
| Device Support | Supports up to 65,535 devices on a single interface. | Supports up to 16 devices connected to HBA. |
| Standard Updates |
Serial SCSI standard is more advanced and
continuously updated to expand SAS capabilities.
|
Updates more slowly. |
| Crosstalk |
Less likely to encounter crosstalk issues in serial
interfaces due to fewer conductors in the cable.
|
More prone to crosstalk issues due to
more conductors.
|
Parallel interfaces transmit multiple bits simultaneously, which can sometimes make them faster than serial connections that send data one bit at a time. However, the accuracy of parallel interfaces diminishes as cable lengths increase, limiting their effectiveness to short cable runs.
Despite being an older technology, parallel SCSI may still be preferred in legacy applications where there has been substantial investment in parallel infrastructure.
SAS vs. SATA
SATA(Serial Advanced Technology Attachment) serves as a computer bus interface, connecting host bus adapters to mass storage devices like hard drives and solid-state drives, making it the most common interface for consumer storage devices. SAS and SATA are both technologies that utilize thin cables to transfer data serially between the computer motherboard and storage devices. However, there are several key differences between SAS and SATA, including the following:
|
Feature
|
SAS
|
SATA
|
|
Basic Design
|
Four wires divided into two cables, can connect
motherboard to storage device and other
devices.
|
One cable contains four wires, can only connect
motherboard to storage device.
|
|
Data Transfer Rate
|
Offers higher data transfer rates, SAS-3
typically up to 12 Gb/s, with newer versions
supporting even higher speeds.
|
Generally slower, SATA III (SATA 3.0) speed up to 6
Gb/s.
|
|
Reliability
|
Designed for 24/7 operation, higher MTBF,
typically 1.6 million hours or longer.
|
Suitable for regular use, but not as robust as SAS for
continuous heavy workloads, MTBF typically around
1.2 million hours.
|
|
Storage Capacity
|
Typically smaller storage capacity, focused on
performance over storage size.
|
Offers larger storage capacity at a more affordable
price, up to 20TB or more.
|
|
Cost
|
More expensive due to enterprise-grade
components and features.
|
More affordable, ideal for consumers and small
businesses.
|
|
Compatibility
|
Controllers typically can work with both
SAS and SATA drives.
|
Controllers can only work with SATA drives.
|
|
Use Case
|
Ideal for servers, data centers, and
high-performance workstations.
|
Perfect for desktops, laptops, and consumer
storage needs.
|
|
Connector Design
|
Uses more robust connectors with more pins,
supporting additional features.
|
Uses simpler L-shaped connectors, easier to damage
but more user-friendly.
|
|
Dual Port Functionality
|
Supports dual port functionality, allowing
redundant paths for the drive, supports multipath I/O.
|
Usually single-port, but some enterprise SATA drives
offer dual-port functionality. Limited multipath I/O support.
|
|
Error Handling
|
Uses SCSI command set, includes advanced
error detection and correction features.
|
Uses a command set based on Parallel ATA, includes
basic error correction features.
|
|
Queue Depth
|
Supports queue depth of up to 254 commands.
|
Queue depth limited to 32 commands when using NCQ.
|
|
Signal Voltage
|
Signal voltage (800–1,600 mV for transmitting,
275–1,600 mV for receiving).
|
(400–600 mV for transmitting, 325–600 mV for receiving).
|
|
Cable Transmission
Distance
|
Due to higher signal voltage, SAS can use
cables up to 10 meters (33 feet) long.
|
Cable length limited to 1 meter (3.3 feet).
|
The choice between SAS and SATA ultimately depends on your specific requirements and budget. SAS is the preferred option for enterprise-level storage solutions where performance and reliability are paramount. On the other hand, SATA offers an excellent balance of performance and affordability for personal and small business use.
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