sonny@charybdis.harris-atd.com (Bob Davis) (04/05/91)
When I compute drive capacity by the formula: Capacity (in bytes) = Cyls X Hds X Sectors X 512 have I computed "Formatted Capacity" or have I computed "Unformatted Capacity"? Seems like on all the drives I have used, this formula has given me the actual number of bytes available for my use -- which I think must be called "Formatted Capacity". Precisely what additional data is recorded on the media that is "overhead" and chews up some of the "Unformatted Capacity"? I have seen some drives having voice coil head positioning mechanisms that have an odd number of heads. But certainly all drives have an even number of platter surfaces. Is the missing head's platter and its bytes counted as part of the "Unformatted Capacity"? Why is there a missing head on such drives? Is that head *really* there and it is just that the platter surface is used for some overhead function? If so, what? Thanks. _____________________________________________________________________________ Bob Davis, UofALA alum \\ INTERNET: sonny@trantor.harris-atd.com | _ _ | Harris Corporation, ESS \\ UUCP: ...!uunet!x102a!trantor!sonny |_| |_| | | Advanced Technology Dept.\\ AETHER: K4VNO |==============|_/\/\/\|_| PO Box 37, MS 3A/1912 \\ VOICE: (407) 727-5886 | I SPEAK ONLY | |_| |_| | Melbourne, FL 32902 \\ FAX: (407) 729-3363 | FOR MYSELF. |_________|
jcburt@ipsun.larc.nasa.gov (John Burton) (04/05/91)
In article <5994@trantor.harris-atd.com> sonny@trantor.harris-atd.com (Bob Davis) writes: [...] > I have seen some drives having voice coil head positioning >mechanisms that have an odd number of heads. But certainly all drives >have an even number of platter surfaces. Is the missing head's >platter and its bytes counted as part of the "Unformatted Capacity"? > > Why is there a missing head on such drives? Is that head >*really* there and it is just that the platter surface is used for >some overhead function? If so, what? > >Thanks. >_____________________________________________________________________________ >Bob Davis, UofALA alum \\ INTERNET: sonny@trantor.harris-atd.com | _ _ | >Harris Corporation, ESS \\ UUCP: ...!uunet!x102a!trantor!sonny |_| |_| | | >Advanced Technology Dept.\\ AETHER: K4VNO |==============|_/\/\/\|_| >PO Box 37, MS 3A/1912 \\ VOICE: (407) 727-5886 | I SPEAK ONLY | |_| |_| | >Melbourne, FL 32902 \\ FAX: (407) 729-3363 | FOR MYSELF. |_________| Sorry I can't help you with the formatted vs. unformatted capacity question... Concerning voice coil drives...you're right, there *is* an even number of surfaces and there *is* an even number of physical heads. With a voice coil drive, the "missing" head (and therefore surface) is used for track positioning information - during formatting, special info is written to each track on the "extra" surface basically stating "I am track xxx". During normal r/w operations the actuator moves the heads until the extra head reads the proper "I am track xxx" information, at which point the regular r/w heads are positioned correctly. This eliminates (or at least greatly reduces) the problem of thermal expansion, since all the platters have the same expansion characteristics, and the same environment. With stepper motors, the proper track is found by moving the head xxx number of steps from its starting position. This DOES NOT take into account the thermal expansion of the disk media since it is an absolute positioning system (position in space) vs. a relative positioning system (position relative to the disk). I know it sounds like a lot of extra bother, but the voice coil normally has faster seek times (most of your < 25msec seek time drives are voice coil - I don't know of any stepper motor drives with that kind of seek times, but I'm sure there are some) and gets you the freebie of Autoparking heads...when the power goes off, the heads automatically return to their park position, no extra circuits required... hope this helps some... John +--------------------------------------------------------------------+ | John Burton | | G & A Technical Software | | jcburt@gatsibm.larc.nasa.gov | | jcburt@cs.wm.edu | | | | Disclaimer: Hey, what can I say...These are *my* views, not those | | of anyone else, be they employer, school, or government| +--------------------------------------------------------------------+
ntaib@silver.ucs.indiana.edu (Nur Iskandar Taib) (04/09/91)
> I have seen some drives having voice coil head positioning >mechanisms that have an odd number of heads. But certainly all drives >have an even number of platter surfaces. Is the missing head's >platter and its bytes counted as part of the "Unformatted Capacity"? The odd head is used for feedback for the head servo mechanism. That disk surface carries tracks that the head can follow. This allows very precise positioning and narrower tracks. This, incidentally, is the same technology used on those ultra-high capacity floppy drives (not the new 2.88 Meg ones, the older Kodak/Verbatim and Barnowl-li units). -- ------------------------------------------------------------------------------- Iskandar Taib | The only thing worse than Peach ala Internet: NTAIB@AQUA.UCS.INDIANA.EDU | Frog is Frog ala Peach Bitnet: NTAIB@IUBACS ! -------------------------------------------------------------------------------
gerardka@hobbes.ism.isc.com (gerard) (04/10/91)
In article <5994@trantor.harris-atd.com> sonny@trantor.harris-atd.com (Bob Davis) writes: > > When I compute drive capacity by the formula: > > Capacity (in bytes) = Cyls X Hds X Sectors X 512 > >have I computed "Formatted Capacity" or have I computed "Unformatted >Capacity"? Seems like on all the drives I have used, this formula >has given me the actual number of bytes available for my use -- which >I think must be called "Formatted Capacity". Yes, you've calculated the formatted capacity. This is more significant than the unformated capacity, since you cannot redefine the track overhead. > > Precisely what additional data is recorded on the media that >is "overhead" and chews up some of the "Unformatted Capacity"? The "overhead" consists of several "things". The data that you read and write is part of the "data record". The data record also consists of 1. a sync byte, to indicate the start of the record, 2. error correction code (ECC) bytes, to detect and possibly rectify read errors. The ECC may be 5 to 7 bytes depending on the implementation in the disk controller. Preceeding each data record is its identification record. The length will vary depending on controller requirements, but the basic information includes: 1. a sync byte 2. the cylinder number 3. the head number 4. the relative sector number 5. perhaps a "bad record" flag 6. ECC When an ID record is clobbered, you may encounter "Bad Sector ID" messages when reading that track. The ID record (and a blank data record) are written during a "low level" format. Preceeding the ID record is often an Address Mark, which provides sort of an interrupt to the controller to indicate that an ID record is coming up (after skipping over a data record). An Index mark has a similar function, but occurs only once on the track. BTW, if all data records are of equal size, the ID + data is commonly refered to as a "sector". The biggest comsumers of overhead are the gaps between these entities. Gaps provide 1. time for the controller to process the information just read; 2. areas where the write head can glitch the surface when write current is turned on or off; 3. a preamble to ID and data records so that the binary zero state can be established for the incoming data stream. (There are no "hard" 1s and 0s written on disks that use MFM techniques to encode data on magnetic surfaces. The values are relative, and are defined by flux changes. A bad preamble could cause all bits in the record to be inverted.) > > I have seen some drives having voice coil head positioning >mechanisms that have an odd number of heads. But certainly all drives >have an even number of platter surfaces. Is the missing head's >platter and its bytes counted as part of the "Unformatted Capacity"? > > Why is there a missing head on such drives? Is that head >*really* there and it is just that the platter surface is used for >some overhead function? If so, what? > >Thanks. The "missing" surface is the servo surface, which has a corresponding, read-only head. The servo information is written only during manufacture. The servo data, which is never included in capacity numbers, is used to provide cylinder positioning information for the head assembly. (On old, floor-standing 300 MB SMD drives, the heads could be positioned fractionally off-cylinder, to compensate for drive to drive misalignment.) Gerard
forrest@orion.sybase.com (Jon Forrest) (04/13/91)
I have a very simple explaination of this that's based on the metaphor
of a parking lot. If you measure the surface of a newly surfaced
parking lot you'll come up with a measurement of area. In one sense
this is the size of the parking lot but it really isn't a useful
measure. If you tried to park in the lot you wouldn't know where to park
because there are no lines drawn on the lot. Not only couldn't you park but
you couldn't tell anybody where you car was, assuming that you had to
give directions based on locations within the parking lot.
Once the lines have been painted on the surface then things change.
You'd know exactly where you could park your car and you could tell
people where you car is parked, kind of like at airport parking lots.
Since the lines take up space, the amount of usuable space in the
parking lot is less then when it was first built but this overhead
provides something useful.
Think of the unpainted parking lot as an unformatted disk and the
painted parking lot as a formatted disk. This will help you understand
and remember what formatting does.
----
Anything you read here is my opinion and in no way represents Sybase, Inc.
Jon Forrest WB6EDM
forrest@sybase.com
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