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Old 15th June 2011
jggimi's Avatar
jggimi jggimi is offline
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A little level setting:

In the Good Old Days™ many decades ago now, multi-platter fixed block architecture disk drives reported their actual number of sectors and platters to the operating system.
A series of sectors in a circle on a platter made up a single track, and as the heads moved together, the combined tracks from all of the platters at one head location made up a cylinder. If the OS wanted to read or write a particular sector, the heads moved with a "seek" operation, the drive selected the head of the platter containing the track of interest, and then waited for the sector to rotate around under the head. The "address" of that sector was noted two ways: by Cylinder/Head/Sector (or, if you prefer to think of it, Cylinder/Track/Sector), or by Logical Block Address.
Around twenty five years ago, drive manufacturers began to have their drive electronics "lie" about the underlying architecture of cylinders and heads. They found that if they mapped the sectors the way they wanted to, they could provide their customers -- computer manufacturers who often also wrote OSes -- with adequate performance, simplified or no-management-required of spare/bad sectors, and either simplified data location tuning, or no data location tuning necessary at all.

Commonly these days, drive electronics report having 255 heads (or platters), and that is an obvious fallacy. The drive manufacturers decide what to do with internal sector placement, and those decisions are proprietary and closed.

Note also that with modern drives managing spare sectors without OS intervention (or knowledge), if the drive has replaced a failed sector with a spare, its LBA or CHS address will have nothing at all in common, physically, with adjacent sector addresses.

Bottom Line:
You cannot know the physical requirements for any particular I/O by LBA or CHS address used by the OS. The time required may or may not include one or more seek operations, there may or may be one or more delays for rotation. While the drive manufacturers may have internal maps that minimize seeking or "Rotational Position Sensing (RPS) miss" platter delays for sequential I/Os -- the OS or the admin have either little or no insight into these occurrences.

Last edited by jggimi; 15th June 2011 at 03:02 PM. Reason: added a "bottom line" paragraph
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