Enterprise-class versus Desktop-class Hard Drives

Revision 1.0

1.1.4 Reliability

Reliability is influenced by the factors described below.

1.1.4.1 Bad Sector Recovery

Desktop drives perform heroic efforts to recover data in a bad sector. Usually there is one hard

drive in a desktop system and a bad sector can result in a catastrophic failure of the operating

system or an application failure. Typically desktop systems do not provide an online backup

copy of the sector. To recover data contained in a bad sector, desktop class drives are

designed to attempt to re-read the sector many times before responding with an “un-recoverable

read error”. During this recovery effort, the drive may become unresponsive and may ignore

bus resets. When the drive disappears from the bus for an extended period of time, the

operating system, the application, and the user are required to wait for the system to respond.

A typical desktop drive command timeout can take many minutes and no disk access is allowed

while the system attempts to retry the command.

Long drive recovery timeouts are not acceptable in an enterprise environment because multiple

users can be affected, and because RAID systems, which are prevalent in enterprise systems,

do not tolerate an unresponsive drive. A feature of enterprise-class hard drives is a short

command timeout value. When a drive has a difficult time reading a sector and the short

timeout is exceeded, the drive will respond by attempting to recover missing data from sector

checksum if available. If that attempt fails, the drive will notify the controller and the controller

will attempt to recover the sector from redundant data on adjacent disks and remap bad sectors

related to the error. The short timeout allows these recovery efforts to take place while the

system drives continue to support system disk access requests by the operating system.

Typical timeout for an enterprise class drive is 7 to 15 seconds and retries are limited to a few

attempts.

Desktop-class drives with timeout values exceeding 30 seconds should not be used in an

enterprise-class system. They could cause drives to be marked as offline more often, and may

increase operating system crashes, kernel panics, or blue screens.

1.1.4.2 Rotational Vibration

Rotational vibration is measured in Radians per second per second, which refers to vibration in

the same plane that the drive spins. Vibration in other planes is frequently lumped in with this

term though. Drive manufacturer specifications for vibration are usually limited to

measurements within the rotational plane and high vibration frequency ranges may not be

addressed by manufacturer specifications.

The source of vibration that can affect drive operation can come from moving components

within a system, such as fans and neighboring hard drives. Vibration from fans can be

transmitted through the system chassis to hard drives, and vibration from a drive can be

transferred to another drive, or from the drive to the system and then reflected back to the

originating drive. While data is read from or written to a disk, these vibrations can push the

read/write head out of alignment with the data track. Failure to compensate for vibration

induced misalignment can result in data that is written or read off track. This increases the

possibility that an off-track write could corrupt data in an adjoining track, or that an off-track read

could result in incorrect data or data that cannot be located or read.