SATA II Is the Key The key to this challenge is the SATA II specification, which doubles the throughput of the original version of SATA to 300MB/s. The new specification also supports command queuing, which allows the controller to issue a command to the drive before the previous command has completed. Command queuing requires changes to the controller’s firmware, but once these changes are made, SATA II drives can deliver throughput similar to that of more expensive SCSI or Fibre Channel drives. Although SATA drives have been shipping since 2003, SATA II drives began shipping in late 2004. In mid-2005, Pentium 4 PCs using Intel’s next-generation Glenwood and Lakeport chip sets will begin shipping, offering full SATA II support. Because SATA II will eventually be available in most PCs, these drives will attain commodity status, ensuring the lowest possible cost. SATA should not be confused with serial attached SCSI (SAS), another serial storage standard. SAS is an upgrade to the original SCSI standard, much like SATA upgrades ATA. SAS is a dual-port interface that uses a superset of the SATA connector, meaning that the same connector and cabling can support both SAS and SATA drives. SAS drives are designed for high performance and high availability. For example, a commodity SATA drive is typically rated at 600,000 hours of MTBF (mean time between failures), whereas a SAS drive may be rated for twice as many hours. SAS drives also have a faster spindle speed, reducing latency. These extra performance and reliability features, along with their much lower volumes, make SAS drives significantly more expensive than SATA drives. SATA Versus FC Like SAS drives, Fibre Channel drives are designed for the greater reliability and performance required by enterprise applications. But these drives are even more expensive than SAS drives due to low volumes and more complex controllers. FC drives also tend to lag commodity drives in capacity, due to longer design and verification cycles. As a result, SATA drives are increasingly used in enterprise applications. Initially, SATA drives are being used where cost is more important than reliability. For example, many enterprises are backing up their primary drives onto a second set of hard drives, supplementing or even replacing traditional tape backups. This model is attractive because the cost and capacity of a commodity SATA drive is similar to the cost and capacity of a single tape cartridge. Drive performance is not critical because the backup drives are rarely accessed. The lower reliability of SATA drives is masked by the use of redundancy (RAID) techniques. Commodity drives are also used in other applications that are not “mission critical,” such as in print and file servers. Commodity drives in a RAID configuration are becoming popular in IP storage arrays for medium-size companies. These companies want the benefit of networked storage but can’t afford the cost and complexity of a Fibre Channel implementation. In large enterprises, Fibre Channel drives remain the primary choice for mission-critical tasks, such as on-line transaction processing (OLTP) and back-office applications. But even for these tasks, some enterprises are turning to SAS drives, which offer the same performance and reliability as FC drives at a lower cost. These enterprises are building an infrastructure (switches and cabling) for SAS. Because SATA drives can plug into SAS switches and connectors, this SAS infrastructure sets the stage for a move to even less expensive SATA drives. Some vendors now offer a two-to-one mux chip that allows two SATA drives to plug into a single SAS connector, mimicking the dual-port redundancy of SAS. SATA drives generally offer shorter MTBF than SAS drives, but redundancy and RAID can make up for this shortcoming. Longer latency can still be a problem. Some drive vendors, however, are now offering enterprise-class SATA drives that match the spindle speed and latency of SAS and FC drives. Although these drives are more expensive than commodity drives, they leverage much of the SATA design and thus cost less than SAS or FC drives. This approach may ultimately offer the best of all worlds. In summary, replacing Fibre Channel with SAS opens the door for enterprises to mix SAS and SATA drives, trading off performance, cost, and capacity as needed for each application. Over time, more of these drives are likely to use SATA, creating new opportunities for SATA drive vendors and reducing cost for large storage subsystems.
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