Disks typically reside in enclosures that provide power, cooling, and protection for the devices. Intelligent controllers can programmatically provide external indicators about the health of the enclosure and components within it to a software package designed to poll for, and utilize this information.
The indicators may identify the enclosure, may identify proper operation of the devices and enclosure elements, may provide indications of the state of RAID devices in the enclosure, and may provide failure and maintenance information. Many of the individual elements of an enclosure may be removable and replaceable while the enclosure continues to operate. An enclosure services processor typically manages all these enclosure elements and communicates with the application client.
The mechanism for doing this is an ANSI standard, and part of the SCSI specification. The methodology is called SES, if using an enclosure which communicates with a host via fibre channel. The equivalent for a SCSI-attached subsystem is called SAF-TE.
S.M.A.R.T. Disk Monitor supports both SES and SAF-TE enclosures. Not all enclosures support SES or SAF-TE, as the additional electronics required to communicate with these devices typically cost the end-user a few hundred additional dollars. If you do not know if your enclosure supports SES or SAF-TE, then just select the Poll SES/SAF-TE-Compliant Enclosure option on the Alert functions menu. If your disks are compliant, then the software will identify them as in the pictures below.
In the image above, The disks at SCSI ID 5 and SCSI ID 8 are installed in a SES enclosure, attached to fibre channel controller #9. The SAF-TE processors are addressed as ID 0, LUN#6 and LUN #7. Note also that SES enclosures specify one or more disk drives in the enclosure as a gateway to communicate with the SES processor. In the case of SAF-TE enclosures, dedicated LUNs or SCSI ID's are utilized to communicate with the enclosure.
When the enclosure is polled, then additional commands will be sent to these two disks to request enclosure status information.
S.M.A.R.T. Disk monitor has the capability to report information for the following types of SES devices:
Disks (called devices in the spec); power supplies; cooling elements (fans); temperature sensors; Door locks; audible alarms; services electronics; SCC electronics; non volatile cache; uninterruptible power supplies; displays; key pad entry devices; SCSI port/transceivers; language defined; communication port; voltage sensors; current sensors; SCSI target ports; and SCSI initiator ports. Your enclosure would rarely support all of these features, but S.M.A.R.T. Disk Monitor will report details if it can.
For SAF-TE devices, it supports power, cooling, door locks, temperature and device status.
Of course, if you enable SES/SAF-TE polling, and a critical problem is detected, such as a redundant power supply failure, then the event will run through the event logger which will send you email, pop-up messages, or even suppress the information.
Note also that even if you have a compliant enclosure, this does not mean that it will report all values defined in the ANSI specification. Most enclosures don't monitor all of the defined elements.
Finally, if you are only using IDE devices, then none of this is applicable to you. SES and SAF-TE don't support an enclosure unless it is has either a fibre channel or SCSI host attachment.