Near-Line Enterprise Disk

Fixed disk has gone through many evolutions. Its reliability, especially in more commodity disk, is always debated. Some users have had excellent experiences with both online and off-line commodity disk reliability, while others have repeatedly run into failure with commodity disks in a 24x7 operating environment or when the drives are left on the shelf too long. Even interface is not a guarantee of reliability, as some lower cost (and higher density) SCSI drives roll off the same mechanical fabrication lines as ATA or SATA.

The good news is that commodity disks are becoming more reliable. Some vendors have introduced new commodity drive models rated for 60-degree Celsius operation, a full 20 degrees higher than recent generations. They can also stay unused on the shelf longer, and improved fabrication techniques have brought vibration down some (thus increasing drive life). Several vendors have returned to the practice of offering 3- or even 5-year warranties on their commodity disks sold in the retail channel, further asserting that reliability has improved on new commodity disk lines.

Unfortunately, the bad news is that even these newer commodity disks are still rated by vendors for only 50,000 restarts and are not designed for 24x7 operation. Their reduced precision in fabrication versus enterprise disk (for clear reasons of fabrication cost), still results in increased vibration. As such, the mean time between failures (MTBF) remains 400,000 hours -- 50,000 starts for 8 hours of operation (see Table 2). Manufacturers will not sell commodity disk products to partner vendors for products designed for use outside of these operational limitations. Integrators are also hesitant to look at initial cost savings when it means they will be visiting clients to replace drives more often than with enterprise disks.

The cost of commodity disk in an enterprise environment has never been due to the lack of warranty coverage. The aggregate failure rate of 1,000 commodity disks is more than 12 per year -- before even considering the increased rate of failure due to a hot, 24x7 environment with increased vibration. The real number is likely much higher, as any desktop support department will attest to in the number of disk replacements required annually. What enterprise environments have been waiting for is a near-commodity priced, leading-edge density fixed disk for a 24x7 network-connected, but managed, environment where the disk is often idle, not operating.

This new "middle ground" between commodity disk and enterprise-rated disk in the network-attached era of IT is the "near-line" enterprise storage. Near-line disk is designed with a MTBF of 1,000,000 hours when operated in a near-line device. The idea is that the device is network attached and operating 24x7, while the disk may not necessarily be. In fact, this solves the other issue with commodity disk -- recoverability in long-term, off-line storage. Such a network device would "exercise" a disk that has been idle or powered-down for an extended period, while the near-line disks themselves are never unplugged (not taken completely off-line) from the system itself.

Although different near-line disk products will vary in their design/selection, most near-line products roll off the same mechanical fabrication lines as commodity disk units -- possibly those that test to better tolerances, vibration, etc. Thus explaining why they are typically offered in the same capacity as commodity disk (for only a slight premium). Although end-users may debate whether the "near-line" enterprise product is of interest, vendor partners and system integrators find it to their advantage to pay the small premium for the "near-line" enterprise products from a support and replacement aspect.