Serial attached SCSI is leading the pack of storage interfaces - Storage Networking

Computer Technology Review, Feb, 2003

Of the storage interfaces available today, as well as those that will soon enter the market, Serial Attached SCSI is clearly in the leadership position as the most flexible, powerful interface.

Serial Attached SCSI will be the first storage interface to offer extensive configuration options for any mission-critical enterprise applications. It will be the first I/O to offer the investment protection advantages of SCSI software and middleware, which have evolved over the years and are the most robust in the industry today. Serial Attached SCSI's advantage is due, in part, to its compatibility with Serial ATA (SATA), which is targeted to the desktop market. This compatibility feature will make it possible for SATA drives to operate in a Serial Attached SCSI system, either alone or along with Serial Attached SCSI drives, providing wider configuration options than any other interface.

SATA Configurations and Flexibility

A desktop PC with a low- to mid-capacity drive and ATA controller integrated onto the motherboard is a starting configuration that everyone is familiar with. This would probably satisfy most household computing requirements in which the driving metric is undoubtedly cost. (See Figure 1.)

Moving up a notch or two with a dual processor, a multiport SATA controller, matching drives and possibly a software RAID package, all of which improve the capacity and performance without too much impact on the savings account. (See Figure 2.)

But if performance is a serious consideration, is SATA still viable?

SATA Compared to Fibre Channel and Serial Attached SCSI

SATA was designed for the desktop and its incorporation on disk drives is consequently motivated by the need to meet the overriding market expectation of low-cost storage. That doesn't mean the drive is lower quality or that its quality is in any way jeopardized by its production cost targets. The table on page 23 compares the differences in the three disk drives.

SATA in Enterprise Applications

A SATA drive is not optimized for fast access to data. The speed of the data coming off (or going to) the media is quite impressive, but it takes the drive longer to find it. However, if the data is stored sequentially, it can be accessed immediately.

In this third configuration, a pair of SATA RAID controllers and their drive sets are hooked up to a server. The controllers and drives may be housed in the server or they may be in separate cabinets, tied to a server HBA. In the latter case, the HBA-to-RATD connection could be any convenient serial interface, but first the drive must be considered. (See Figure 3.)

If the application requires sequential access to data, these SATA RAID sets would provide a raw data rate that is comparable with enterprise drives. Candidates for such a configuration could include video applications, document imaging and backup storage. But before deciding that this configuration is the answer to all applications involving sequentially ordered data, one should look more closely at the pros and cons.

Pros:

* Initial hardware costs are minimized.

* Storage performance is excellent under error-free conditions.

* Sequential access prevents RV from being a limiting factor in performance.

* The RAID redundancy will take care of drive failures and compensate for the lower reliability of ATA vs. SCSI drives. Hot spares will replace failed units.

Cons:

* With no guarantee of data integrity, use in mission critical applications would be inappropriate.

* Error recovery management must be handled at the controller level.

* Lower reliability increases down time due to rebuild operations after failures.

* Each hot spare represents additional hardware cost and rebuild time. The more critical the application, the more significant the downtime and the greater the impact on total cost of ownership.

* Single ported drives do not support fail-over recovery.

To handle large storage requirements, either racks of high capacity disk drives, a forest of moderately sized units, or a combination of both could be used. There are significant advantages and disadvantages to each of these solutions, but the common denominator at the system level is connectivity and addressability.

A Switched Environment

Since SATA and Serial Attached SCSI are point-to-point interfaces, some form of switch is required to allow their controllers to access multiple drives. Fabric switches are synonymous with FC; so one solution would be to integrate FC into the SATA or Serial Attached SCSI topologies. Interconnects at the network (switch) level would then be handled by FC and a converter in the controller (bridge) would handle the SATA/Serial Attached SCSI translation to the drive. Using a bridging topology would allow connectivity of any disk interface to any network interface such as iSCSI or Ethernet. But what if the system could have more limited and inexpensive connectivity at the local level? Serial Attached SCSI has the answer built into its protocol.

Expanders are used in the Serial Attached SCSI environment to permit multiple initiators and targets to be addressed while the cables and connectors are SATA compatible. In addition, the Serial Attached SCSI interface supports a controller-to-expander dialogue known as Serial Tunneling Protocol (STP). This allows the controller to send SATA-formatted data to the expander, which in turn transmits it via SATA protocol to a standard SATA drive. (See Figure 4.)