S.M.A.R.T (Self-Monitoring, Analysis and Reporting Technology)

S.M.A.R.T is designed by IBM. It was created to monitor the disk status by using various methods and devices (sensors). A Single ATA disk may have have up to 30 such measured value, which are called attributes. Some of them directly or indirectly affect hard disk health status and others give statistical information. Nowadays all modern IDE/Serial ATA/SCSI hard disks have S.M.A.R.T feature. it is not a standard so due to this attributes may be different from manufacturer to manufacturer. ESXi supports disk drives that are enabled with Self-Monitoring, Analysis and Reporting Technology (SMART)

A single S.M.A.R.T. attribute has the following fields:

Identifier (byte): the meaning of the attribute. Many attributes have standard meanings (for example, 5 = number of reallocated sectors, 194 = temperature, etc). Most applications provide name and textual description about the attributes.

Data (6 bytes): raw measured values are stored in this field, provided by a sensor or a counter. This data is then processed by an algorythm designed by the hard disk manufacturer. Sometimes different parts (for example, low, middle, high 16 bits) of this value contain different kind of information.

Threshold (byte): the (failure) limit value for the attribute.

Value (byte): the current relative "health" of the attribute. This number is calculated by the algorythm, using the raw data (see above). On a new hard disk, this number is high (a theoretical maximum, for example 100, 200 or 253) and it is decreasing during the lifetime of the disk.

Worst (byte): the worst (smallest) value ever found in the previous lifetime of the hard disk.

Status flags: indicate the main purpose of the attribute. An attribute can be for example critical (able to predict failure) or statistical one (does not directly affect condition).

If you want to know more about S.M.A.R.T refer these links:-
https://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2040405

http://www.hdsentinel.com/smart/index.php 

http://www.hdsentinel.com/smart/smartattr.php