Author: Charlie Kraus, Director, HBA Business Unit
After enjoying an amazing run of longevity and evolution, parallel SCSI is finally seeing the end of its reign, to be replaced by Serial Attached SCSI (SAS). SAS builds on, and improves on, the parallel SCSI foundation and succeeds Ultra320 SCSI on the industry roadmap:
With the decision that a serial interface was required, there were several options. Fibre Channel is an existing serial interface, but too expensive for in-the-box connectivity. Serial ATA (SATA) would not satisfy enterprise metrics for reliability and performance. Even customers in the mid-range required better performance and reliability than SATA would provide. A new standard was needed that included interoperability with SATA technology, higher performance than SCSI, and enterprise-level reliability.
With 3.0 Gb/s (300 MB/s) initial throughput, advancing to 12 Gb/s in successive generations, point-to-point connections with dedicated bandwidth, full-duplex data transfer allowing simultaneous read and write data flow, and wide ports comprising up to four links of aggregated bandwidth, SAS hit a home run in fulfilling its mission.
The following diagram shows the inherent advantage of having dedicated links to each internal disk drive vs. a shared SCSI bus.
Figure 2: SAS Has Dedicated Links to Each Disk Drive.
SAS wide ports are another key feature, which, if combined with dual-port SAS disk drives, provide scalable bandwidth up to 24 Gb/s for full-duplex data transfers and the resiliency of dual-port drive connections. Low-cost serial links and rate matching accommodate 3.0 Gb/s SAS drives and 1.5 Gb/s SATA drives in the same infrastructure (See Figure 3).
Figure 3: A Typical Wide-port Connection to Dual-port Drives.
The SAS Infrastructure
There are many products that make up a complete SAS infrastructure. The first is the SAS controller chip, an important device that handles the interface to the server OS and provides the I/O to the SAS disk drives. For flexibility in system design, LSI makes four-port and eight-port controllers. Each provides 1.5 Gb/s and 3.0 Gb/s data transfer rates per port and enables integrated RAID solutions, such as mirroring and striping, in storage environments including servers, workstations, blade servers and external storage systems. The controllers leverage an electrical and physical interface that is compatible with Serial ATA technology. Each port independently auto-negotiates speed and supports the Serial SCSI Protocol (SSP), SCSI Management Protocol (SMP), SATA Tunneled Protocol (STP) and SATA protocols.
These controllers contain internal processors for high throughput and to offload work from the host processor. OS drivers for Windows 2000, XP and Server 2003, Linux, Solaris, SCO, Unixware and Netware are available. The four-port LSISAS1064 and LSISAS1068 come in EPBGA (Ball Grid Array) packages.
While it is common in blade servers and external storage devices to have the SAS controller as a “chip down” solution, server applications and workstations will usually require a host bus adaptor (HBA) for SAS connections. LSI produces a wide variety of SAS HBAs to meet the needs of server and workstation manufacturers. A popular HBA, the LSISAS3442X provides four internal and four external SAS ports. The internal ports provide connectivity to SAS drives within the server, while the four external ports allow storage expansion to external storage devices.
Figure 4: The LSISAS3442X HBA.
The typical application for this HBA would connect to SAS drives (see Figure 5).
Figure 5: HBA Connection to SAS Drives.
The four-port external connector to external storage will usually be enabled as a wide port, providing 24 Gb/s full duplex throughput. For other applications, LSI produces HBAs with eight ports, all internal, and eight ports, all external. Different connector types allow for connectivity to backplanes instead of directly to drives.
Figure 6: The LSISAS3080X HBA is Ideal for Connecting to a Large Number of Internal Drives.
Besides HBAs, LSI produces a variety of SAS RAID controllers that support full RAID stacks up through RAID 6. With the ability to connect multiple servers and disk arrays, another important SAS device is the RAID-on-chip or ROC. Many RAID implementations today use an external processor to the controller chip, or implement software RAID. To lower the cost of RAID, LSI has introduced the LSISAS1078E ROC. This chip has eight SAS ports, a PCIe bus interface and an integrated processor to run a RAID stack. So far two RAID stacks have been ported to run in the 1078, LSI’s MegaRAID stack and HP’s SmartArray stack, utilizing the advanced RAID 6 capability of the 1078.
Expanders in JBODs
The next important SAS component is the SAS expander. Expanders do as their name implies – they expand the SAS controller ports to a larger number of connectivity points. The simple use of expanders will be to allow a controller to support many more drives than just a single drive per port. 12-, 24-, 28- and 36-port expanders are appearing that will be typically deployed in SAS/SATA JBODs. The SAS controller in a server will be able to interface with many drives in the JBOD:
Figure 7: Example of a JBOD Connected by a 36-port Expander
LSI’s 28- and 36-port expanders contain an integrated processor for enclosure management services and extended SAS SMP functions. These expanders provide large storage environments with the ability to connect multiple targets and initiators through a switched device for scalability and fault-tolerant path redundancy to improve system reliability. Like the LSI SAS Controllers, the 28- and 36-port expanders support the SSP, SMP, STP and SATA protocols. All expander ports support 1.5 Gb/s and 3.0 Gb/s SAS and SATA data transfer rates, and the ports will auto-negotiate speed. All ports support target and initiator modes.
Expanders Enable Clustering
While the use of expanders in JBODs will be a common application, there is another important application. Expanders are essentially switches, which allow them to play a
more complex role. Instead of a single server interfacing to a single JBOD, multiple servers can be connected to multiple JBODs. This is a very powerful capability. It can be thought of as a SAS clustering, or connections switch, not to be confused with Fibre Channel or Ethernet fabrics. SAS has an important difference. SAS is not packet-based like Fibre Channel and Ethernet, but is connection-oriented. SAS also does not have a transport layer. SAS expanders allow switched serial interfaces.
SAS fabrics will likely be relatively small, less than 100 ports, as the early SAS expander chips will be 12-, 24- or 36-ports, limiting how many can be cascaded to build larger port-count switches. Future higher port-count expanders will allow larger switches. What will make this especially compelling, compared to a Fibre Channel fabric, is the much lower cost of a SAS cluster switch and its ability to support both SAS and SATA. SAS switches will likely be one-fourth of the price-per-port, compared to Fibre Channel fabrics. SAS also allows diskless servers to boot from a SAS disk – very easy to do across a SAS link. SAS switches can even support storage applications such as virtualization, replication and RAID.
SAS Disk Drives
The final component of the SAS infrastructure is SAS disk drives. To see the usage of disk drive types over time, the following chart is useful (see Figure 11).
All the SAS infrastructure pieces are available. Server OEMs are already shipping systems with SAS drives installed. SAS JBODs are soon to hit the market. SAS is poised to enjoy a long and successful run for high performance disk drive connectivity and as a cluster connection technology in rack and multi-rack server-to-storage connectivity.
What’s next for SAS? In 2007 SAS links will move to 6.0 Gb/s with controllers and expanders evolving to support the next-generation speed. Further out, 12 Gb/s is on the horizon.