SAS/SATA Compatibility Simplifies Storage by Design
 


Cameron T. Brett, Manager, Product Marketing
PMC-Sierra, Inc.

Serial Attached SCSI (SAS) was designed from the ground up for compatibility with Serial ATA (SATA) disk drives. The benefits of this compatibility include lower total cost of ownership (TCO), increased flexibility and usability, as well as allowing for larger storage capacities in enterprise and small business environments. To understand SAS and SATA compatibility, the enabling components are a good point of departure.

Components for SAS and SATA Compatibility
SAS Connectivity
SAS connectivity devices are either chips or add-in cards that create the SAS capability in a system. Following are the most common devices:

  • SAS I/O controller (IOC) — typically a PCI-X or PCI Express host interface with 4- or 8-ports of SAS connections on a chip or card.
  • SAS RAID-on-Chip controller (ROC) — typically a PCI-X or PCI Express host interface, hardware RAID 5/6 and four or eight SAS connections on a chip or card.
  • SAS expander — a target device that allows a single SAS connection (either x1 or x4 links) to connect to 12, 18, 24 or 36 devices.
  • SAS/SATA Mux — allows dual-porting of a SATA drive to enable it to connect to two different SAS RAID controllers, typically for redundancy in high-availability systems.

Serial ATA Tunneling Protocol (STP)
STP is a protocol in the SAS specification that allows native SATA devices to talk on a SAS connection. A SAS controller uses STP to wrap the SATA protocol and data packets into SAS packets. SAS devices are able to identify when a SATA device is attached to their ports and will use STP to remove the SAS wrapper and provide the SATA disk drive with the native SATA protocol.

SAS and SATA Disk Drives
SAS disk drives are highly reliable drives designed for enterprise IT applications, such as 24x7 operation or other applications with very high duty cycles. They are also higher performing with RPM speeds of up to 15K, but capacities are typically limited to 300GB per disk drive today. Their high reliability and performance come at a high price as well, typically three to five times the price of a SATA disk drive.

SATA disk drives derive from the low-cost parallel ATA line of disk drives. SATA drives have a lower duty cycle and a much lower mean time between failures (MTBF) than SAS disk drives. Additionally, they have lower performance with typical 7200 RPM platter rotational speed. On the plus side, they come in very high capacities of up to 1TB today.

Serial Cables and Connectors
The way SAS and SATA devices connect are through new serial connectors and cables that are much smaller and thinner, which promotes better airflow within a system. The connectors for SAS and SATA look very similar and both include power and data connections in a single connector. The noticeable difference between a SAS and SATA connector is that the SATA connector has a small notch, or "key," between the data and power connections. This enables a SATA drive to connect to a SAS connection, but prevents a SAS drive from being plugged into a SATA connection.


Figure 1: SATA and SAS connectors. Note: SATA connector is "keyed"

SAS Compatibility by Design
When the first SAS specification was developed, SAS and SATA connections were designed to be compatible. The data and electrical connections are identical, and both reside within a single connector, although SATA also allows discrete data and power cabling, similar to a parallel ATA drive.

With connectors so alike, SAS and SATA drives can connect to the same SAS connector whether on a cable or backplane. The SAS receiving ("female") connector has a space between the power and data connections. This enables either a SAS or SATA connection. In the case of a SATA female connector, there is no gap between the power and data connections, which prevents a SAS drive from being inadvertently connected to a SATA connection. SATA does not support SAS, but SAS supports SATA.


Figure 2: Female SAS connector

Enabling SATA drives to connect to SAS is more than just similar connectors. The T10 technical committee included STP in the SAS specification to allow SATA drives to communicate across a SAS link. Running STP on a SAS link, an STP connection is opened and the STP initiator and STP target communicate as if they were SATA host and SATA device directly attached on a physical link. This differs from a typical SAS or SCSI connection, where an initiator and device connection is established but the connection can be temporarily discontinued to enable other traffic across the same link. The link would then be reestablished to continue communication.

The other protocols included in the SAS specification are Serial SCSI Protocol (SSP) and Serial Management Protocol (SMP). SSP is the SCSI protocol running on a serial connection and SMP is the management protocol that helps manage devices in a SAS system.


Figure 3: SAS architecture model

Applications Using SAS and SATA Disk Drives
In the enterprise, compatibility between SAS and SATA drives enables an economical implementation of information lifecycle management (ILM). As data ages, ILM migrates data from primary 24x7 continual operation storage (SAS), to secondary/nearline storage (SATA) where data is accessed less frequently. When data has effectively completed its useful life, it is moved to tape for archiving.


Figure 4: Tiered storage in an ILM application

SATA drives have a lower duty cycle and are not designed to run 24 hours a day, such as a SAS drive, but are more cost effective and will operate adequately if accessed for several hours per day, rather than all day.

Connecting SAS-based enclosures and a SAS tape library allows a single technology to accommodate the elements of an ILM implementation.

In a small business, SAS and SATA can be combined in a single JBOD enclosure or server to create a simple tiered-storage environment, or if connected to a RAID controller in the server, a simple lower cost RAID 1 mirror. Because SATA drives have much higher capacities, an equivalent storage volume can be created with SATA drives at one-half to one-quarter the cost of a similar SAS volume. Additionally, the performance of the mirror is fairly high. Not quite as high in performance as SAS, but should be adequate as a failover volume.


Figure 5: SAS and SATA in a single JBOD

Designing SATA capability into SAS enables this simpler configuration that lowers the cost of implementing a highly reliable storage system. Some major server and storage vendors offer both SAS and SATA in a single system today.

Conclusion
Compatibility of SAS with SATA systems has paid off for the server and storage industry. It simplifies storage architectures, lowers total cost of ownership, enables flexible usage models and reduces implementation costs with SATA drives. The T10 technical committee's foresight to design SATA compatibility into SAS with the STP protocol, and to create a common connector design, has enabled architectures that benefit enterprise IT as well as small businesses. Look for more innovations in next generation 6Gb/s SAS.

http://www.pmc-sierra.com/sas6q
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