Interpreting Head Health and Status
-----------------------------------
Beginning in 2016, some Seagate nearline drives started to support the
Remanufacture command set which allows a disk drive with a manageable
read/write head problem to remain in service at a reduced capacity by
"depopulating" the head element.  You can check if a drive supports the
Remanufacture command set by running --deviceInfo and looking for 'Storage
Element Depopulation' in the list of supported features.  (Not to be confused
with 'SATA Rebuild Assist' which is a feature used to rebuild RAID systems.)

A drive failure (due to a head problem) that is easily removed from the system
is replaced with a spare drive.  A drive that is nearly inaccessible may make
sense to remain in the system at a reliable lesser capacity by depopulating the
problem head.

If supported by the drive, the --showPhysicalElementStatus command will display
a simple table showing head number, type, health, and status.

The Health level is represented like a percentage, 0-100.  0 is perfectly
healthy, 100 = at manufacturer's limit, above this to 207 is above the
manufacturer's limit. A report of 254 means a depopulate is in progress.

The Status levels are active, in limit, degraded, truncated, truncate failed.

If a drive head has developed isolated, degraded performance, and does not
affect any other reliability in the system, then the head can be removed from
service. A drive with seven platters and fourteen heads would lose 1/14th of
its storage capacity, or ~7%.  Likewise, a four head drive would lose 25%.

If a drive is reported as "degraded" by --showPhysicalElementStatus then it can
be depopulated using the --removePhysicalElement command.  When a head is
depopulated the drive must perform a complete full pack write; this is
obviously data destructive and takes a very long time to complete.

When disk drives are built the post assembly factory process includes media
scanning, any defects mapping, defining sector sizes, and establishing system,
cache and data zones.  This activity is generically known as a low-level
format.  During head depopulation (i.e. removing a physical element) the drive
performs a kind of 'mid-level format' using Sanitize Overwrite which has the
unique behavior that once started it must finish.  If power is interrupted
during a Sanitize Overwrite it will pick up again where it left off when power
returns.

