By Dave Simpson
Few doubted that the Serial Attached SCSI (SAS) interface would succeed, but even fewer realized how quickly it would take off. If estimates from market research firm IDC prove accurate, SAS will account for 50% of all enterprise-class disk drive shipments this year (see Figure 1).
The rapid growth of SAS drives comes at the expense of Fibre Channel drives, which are expected to account for only 19% of drive shipments. Enterprise-class SATA drives are expected to make up the remaining 31% of unit shipments this year, according to John Rydning, IDC's research director, hard disk drives (HDDs).
And by 2012, IDC estimates that Fibre Channel will have a mere 1% market share, compared to 25% for SATA and 74% for SAS.
So far, virtually all of those SAS drive shipments have been first-generation, 3Gbps devices. However, second-generation, 6Gbps drives are available now, a variety of 6Gbps SAS components (e.g., adapters, controllers, chips, expanders, and test equipment) are shipping in volume, and 6Gbps SAS-based disk arrays are expected to reach end users late this year.
As reported in more detail in InfoStor's April issue Special Report (see "'Do more for less' with 6Gbps SAS" at infostor.com), in addition to the 2X performance boost, SAS offers a number of other features that systems/storage integrators and end users will find advantageous, including zoning, self-discovering expanders, table-to-table routing, longer cable lengths, multiplexing, and improved diagnostics (see table).
Zoning allows IT administrators to define zones to control access to different disk drives by different hosts in a domain (see Figure 2). For example, administrators can define a security policy that only allows certain hosts to access certain HDDs containing secure information.
Zoning was first implemented in 3Gbps SAS expanders, but different vendors implemented zoning in different ways, leading to incompatibility among products. The SAS-2 (6Gbps) specification standardizes zoning so that multiple vendors' products can interoperate in a 6Gbps SAS zoning fabric.
With self-discovering SAS expanders – another feature defined in the SAS-2 specification – each expander has the ability to discover attached HDDs and other expanders and populate its own routing tables without being managed by the host/controller. This results in improved convergence time during initialization and discovery after topology changes (e.g., adding or removing drives).
With the new generation of SAS, table-to-table routing no longer requires administrators to be aware of special ports when connecting expanders together. This eliminates the need for special keyed cables and connectors. SAS expanders ca be connected with any port; as a result, IT managers can reduce costs because they will not have to buy and stock different keyed cables and connectors for inter-expander connections.
6Gbps SAS also increases the cable length to ten meters (vs. six meters for the previous generation). The benefit is increased flexibility and scalability in data-center cabling infrastructure, with the possibility of longer distances between controllers and disk arrays or between arrays and other devices.
Multiplexing is an optional feature of 6Gbps SAS that allows for a gradual transition from 3Gbps SAS drives, because two 3Gbps links can be interleaved onto one 6Gbps link. This allows the 3Gbps links to take advantage of the increased 6Gbps bandwidth.
Multiplexing can translate into cost savings because it allows more disk drives to be connected to each host controller (see Figure 3).
Lastly, 6Gbps SAS promises improved diagnostics, which will decrease failure debugging time and allow storage administrators to quickly and more accurately detect points of failure in the storage infrastructure.
This month, Finisar released the Xgig 6Gbps SAS Target Emulator, a test and analysis platform that also works with 3Gbps and 1.5Gbps devices (e.g., hosts and expanders). The emulator supports bi-directional SAS target emulation up to level-4 functionality (e.g., SCSI and ATA commands) and has compare circuitry to verify accuracy without, according to the company, imposing performance penalties. Dual wide-link connector blades support four SAS lanes per connector, or up to eight lanes per chassis.
Aimed at storage equipment designers, testers and manufacturers, the Xgig Target Emulator supports up to 32 synchronized ports cascaded across multiple chassis (in a single "sync group"), and supports up to 32 simultaneous users. Other features include support for the SATA Tunneling Protocol (STP), hardware-based encryption, and automated operation.
The Xgig Target Emulator can be used with Finisar’s Xgig Analyzer (to capture and analyze traffic), Xgig Generator (for creating arbitrary traffic), and Xgig Jammer (to modify live traffic). The Target Emulator is part of Finisar's Monitoring and Analysis platform.
Atto Technology is shipping five 6Gbps SAS host adapters in its ExpressSAS family: the H608 (eight internal ports), H680 (eight external ports), H644 (four internal and four external ports) and, most recently, the H60F (16 internal ports) and H6F0 (16 external ports), both of which have dual independent controllers. Other features include x8 PCIe 2.0 support and Atto’s Advanced Data Streaming (ADS) technology for bandwidth-intensive applications.
Atto has also added support for 6Gbps SAS host connections on its FastStream Storage Controller, which supports 3Gbps SAS disk drives. The FastStream SC 8200 RAID controller supports 4KB sector sizes to address partitions larger than 2TB. (In addition to 6Gbps SAS, the controller supports 8Gbps Fibre Channel host interfaces.)
Hitachi Global Storage Technologies (GST) is shipping dual-port 6Gbps, 10,000rpm disk drives. The C10K300 comes in 147GB or 300GB capacities, and has Advanced Power Management technology for lower power consumption (3.4 watts in idle mode, and 6.1 watts in active mode). Average seek time is 3.9 milliseconds, and average latency time is 3 milliseconds.
LSI's latest announcement in the 6Gbps SAS space was a demonstration of more than one million I/Os per second (IOPS) on a single server powered by three LSISAS2108 RAID-On-Chip ICs connected to 12 2.5-inch 6Gbps SAS disk drives.
LSI is shipping 6Gbps SAS MegaRAID adapters and ICs to OEMs, and has announced design wins with vendors such as Dell, Fujitsu Siemens, IBM, Intel, NEC, Sun and Supermicro.
PMC-Sierra's most recent design win was with Hewlett-Packard, which has integrated PMC’s SRC 8x6G 6Gbps SAS RAID-on-Chip IC and SXP 36x6GSec SAS expanders into its ProLiant G6 servers. PMC is also shipping 6Gbps expanders and multipliers in its maxSAS family.
Maxim Integrated Products is shipping to OEMs the NexSAS line of 6Gbps SAS expanders, which include multi-tap DFE-based PHYs with self-calibrating transmitters and SAS-2 features such as zoning and multiplexing.
On the cabling front, Quellan has a portfolio of 6Gbps MiniSAS active cables and modules based on the company's QLx4600 Lane Extender IC. Quellen has tested the cables with a variety of SAS devices at cable lengths of up to 25 meters. The active cables have provisions for internal power supplies, as well as a detection mechanism to avoid short-circuiting.
Amphenol is also shipping 6Gbps SAS-2 cables that are compatible with the keying features and signal performance requirements to support cable lengths of up to 10 meters.