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How SATA is being Implemented

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Serial ATA (SATA) is being implemented in three phases:

SATA 1.0: is complete, with products shipping today.

SATA II Phase 1: (also called current extensions) is complete, with products starting to ship in late 2003 or early 2004.

SATA II Phase 2: (also called future extensions) specification is expected to be completed in 2004, with products shipping the following year.

SATA 1.0 was intended primarily as a single-system implementation of the technology, a replacement for legacy ATA products. Its transfer rate is 150MB/sec.

SATA II Phase 1 addressed server requirements by expanding the connectivity to support backplane connections for disk subsystems, as well as supporting enclosure services for monitoring and managing the subsystem environment. In addition, SATA II added support for command queuing--a technique that supports more efficient disk operations in multitasking environments. Most of the points made in this article about SATA assume SATA II Phase 1.

SATA II Phase 2 introduces dual-porting and support for redundant controllers for higher availability. In addition, it increases the scalability of attached devices as well as doubling the transfer rate to 3MB/sec.

One of SATA's advantages for enterprise storage is its planned support for dual porting in SATA II Phase 2. Dual porting is the foundation of multipathing at the disk drive level, which provides the mechanism for a subsystem to failover to a secondary connection with a disk drive, should the primary connection fail.

There's no way the cost difference between similarly featured server and workstation drives can be justified based on interface alone, although that may be the most distinguishing feature between two drives. Server drives are sold for much higher margins as part of large-capacity server systems or subsystems. The extra money paid for server SCSI drives isn't fully realized by the disk drive vendors, but is shared by the system and subsystem companies.

Network-attached storage (NAS) companies such as Network Appliance Inc. (NetApp), Sunnyvale, CA, have been selling NAS systems with desktop-class ATA drives inside. NetApp's NearStore is based almost completely on the aggressive value proposition of ATA drives to achieve a high capacity at a lower cost per gigabyte.

Although there aren't many external ATA-based SAN storage subsystems on the market, that situation is going to change in the next couple years with the introduction of SATA in enterprise-class storage subsystems.

The idea behind enterprise SATA-based subsystems is to use SCSI or FC external interfaces to connect to host systems and to use SATA drives and interfaces inside the subsystem. This is similar to the situation with SAN storage subsystems that have external FC or iSCSI interfaces.

As it turns out, SANs are also contributing to the acceptability of ATA desktop drives for enterprise applications. For starters, SANs make it easy to set up disk mirroring between two subsystems, each with their own redundancy protection. This technique is commonly being used to build higher availability I/O networks.

In previous articles for Storage, I explained how open- systems store-and-forward software would reduce the cost of remote business continuity implementations. By allowing storage subsystems to be located greater distances from each other, SANs increase data availability in response to a local disaster without any dependence on the disk drive interfaces. Data redundancy through remote mirroring or store-and-forward technologies can take advantage of any type of disk drive interface by using subsystems with multiple controllers. The concept of using cheaper desktop drives for remote or second-tier copies of data has broad appeal for many IT organizations that need low-cost disk subsystems to achieve their business-continuity goals.

Another application targeted for SATA drives has been for disk-based backup and recovery. In general, user requirements for backup have little to do with the disk drive technology and more to do with the backup software and metadata systems responsible for restoring individual files and versions of files. Most proponents of disk-based backup overlook the fact that file systems on disk drives don't lend themselves to storing historical versions or snapshots of files--with the possible exception of the write anywhere file layout (WAFL) file system from NetApp. A backup system that can't store and restore multiple versions of data objects is uninteresting as a replacement for backup. The cost advantages of SATA drives aren't enough to obliterate the fundamental requirement for restoring data.

This was first published in October 2003

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