Choose SCSI over SATA for enterprise servers

SATA's lower cost and new features make it attractive, but SCSI remains a standard for enterprise server hardware.

Administrators are must choose between a newer, more exciting technology or an existing, proven one. When it comes

to servers, many admins now face the choice of going with the new serial ATA (SATA) hard drive and the existing SCSI standard.

Its lower cost and new features make SATA attractive, but SCSI continues to be the gold standard for server hardware for a reason.

SATA is a new version of the ATA/Integrated Drive Electronics (IDE) drive standard used for years in desktop hard drives and removable drives such as CD/DVD drives. ATA drives have always been inexpensive and easy to work with, but they have lagged behind SCSI drives in terms of performance. SATA goes a long way toward closing that gap.

SATA: The good news

  • SATA is less expensive than SCSI. Both the drives and the hardware itself -- the controllers and cables -- are much less expensive than SCSI. For those building a server on a budget, this is appealing. A SATA RAID array will usually be much cheaper to build than a SCSI array of the same capacity. For instance, for $175 you can get either a 36 GB, 10,000 rpm enterprise-class SCSI drive or a 200 GB, 7,200 rpm SATA drive.
  • There's no significant speed difference. Under ideal conditions, SATA RAID arrays have been clocked at 90-95% of the speed of a comparable SCSI array. The standalone drives also perform almost nearly as well.
  • SATA is easier to manage, physically. Both the data and power connectors to SATA drives consist of much thinner cables that are easier to manage than what SCSI drives use. The thinner cable mean fewer airflow problems. In addition, SATA cables can run up to one meter in length without signal problems.

However, there are downsides to SATA -- or upsides to SCSI drives.

  • SATA drives are still not built to the same standards as enterprise SCSI drives. They aren't made for the kind of heavy use that an enterprise-class SCSI drive is built to take, which includes the amount of data shunted through it and mechanical failures and surface defects. In general, SATA drives are considered desktop-class drives and do not have warranties for more than one to three years.
  • SATA's command standard isn't enterprise-class. SCSI uses a method of optimizing data called "command queuing," which allows the controller to execute requests for data from the drive in the best possible order. On a server, where dozens if not hundreds of users may be placing requests at once, the disk controller can turn into a serious bottleneck if the hardware doesn't have some low-level method of dealing with multiple simultaneous requests (as does SCSI). SATA is at best a poor man's implementation of this technology.
  • SATA uses the CPU for managing data flow; SCSI doesn't. SATA's reliance on the CPU for managing its data flow is a legacy inheritance from the ATA/IDE standard. SCSI controllers offload the management of data flow to the controller's own dedicated hardware, which means faster overall throughput. While the amount of CPU load that SATA imposes on the system is nowhere near what it used to be in the older ATA/IDE standards, there is still the overhead required to go to and from the CPU -- and that's CPU power and bus bandwidth best devoted to other things.
  • SATA drives require dedicated power connectors. This may be nitpicking, but in some cases it's a key consideration: SATA drives won't work with conventional drive power connectors. The power supply for the system needs to have dedicated SATA power connectors or a converter for same (they run about $10 each).

In short, SATA is best for simple, single-disk servers and desktop and workstation configurations that can benefit from disk-striping setups, such as multimedia editing stations. At this point, it's really not suited for use in enterprise-class servers.

SCSI is the best choice for high availability and durability rather than performance per se. The performance is definitely there; it's just that SCSI is engineered more to be fault-tolerant.

Spend your money on SCSI and rest easy. SATA does make an appealing alternative to low-end (i.e., desktop) SCSI setups, which have been more or less eclipsed by ATA/IDE as a whole for some time now and should flourish nicely there.

Serial ATA (SATA) is a drive interface designed to replace the Parallel ATA physical storage interface. The storage world has been buzzing about SATA drives for years, debating how it stacks up against other technologies.

Users of the SATA interface are benefiting from greater speed, simpler upgradeable storage devices and easier configuration. While SATA drives don't match the performance of Fibre Channel (FC) hard drives, they provide the low cost per gigabyte and high storage densities crucial for "near-line" storage tasks such as performing backups and archiving.

This Fast Guide is a compilation of SATA-related tips that have appeared on SearchWinComputing.com. As our site devotes more coverage to SATA, expect to see more tips related to upgrades and configuration.


Fast Guide: Managing SATA drives
  Introduction
  Balancing SATA and SCSI
  Fixing conflicts between older and newer SATA drives
  SATA technology advances and expands in the enterprise
  Plugging into external SATA
  Choose SCSI over SATA for enterprise servers
  SATA can fill storage upgrade for older computers

 

Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter. Check it out for his latest advice and musings on the world of Windows network administrators -- please share your thoughts as well!

This was first published in February 2005

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