low level format details

www.pcguide.com covers disk formatting pretty well. See inline answers.

"Arnab Lahiri" wrote in message
m...
|
| Each surface of a hard disk or of a floppy disk is initially just a
| disk with some magnetic material coated onto it. For data storage /
| retrieval the read / write head uses a (relatively weaker / stronger)
| magnetic field to sense the alignment / align the molecular magnets on
| the disk in either of the two directions signifying a "zero" or a
| "one".
|
| The read / write head can access one "block" or "track sector" (is
| it?) at a time. One block consists of several "clusters" or
| "allocation units". Whenever a particular file is to be accessed, the
| corresponding "directory entry" is checked where the "FBN" or "File
| Block Number" is found. The FBN is the starting BLOCK (not "cluster"?)
| number for the file in the disk. The subsequent cluster numbers are
| noted down in the FAT. The FBN is the starting for the corresponding
| "file chain" that terminates with the "EOF" (End Of File) entry in the
| FAT.
The struct of DOS FAT is completely unrelated to low-level track and sector
format.
|
| Q1. How are the sectors and tracks marked on a (previously
| unformatted) magnetic disk surface?
Servoes are written in the clean room, then sectors are written and verified
after assembly.

| Q2. In magnetic disks (unlike CD's) the "track sectors" (I am not sure
| of this term - I mean here the intersection of a track with a sector)
| are smaller in area near the centre and larger near the periphery.
| Must the read / write head be larger than the largest track sector?
Modern disks have MR heads, which write thick (over 1/2 a track) and read
thin. Tracks are all the same width, but outer tracks have more bits.

| Q3. Does repeated "low-level" format reduce the life of a disk? If
| yes, how?
Servoes are written once. LLF just writes sectors.

Arnab Lahiri wrote in message
m...

I want to verify my existing knowledge on
low level formatting and know more details.

What I know so far is this (relevant for DOS only?):

Its relevant to whatever OS is used, and even for
data only drives that just have data on them too.

Each surface of a hard disk or of a floppy disk is initially just a
disk with some magnetic material coated onto it. For data storage /
retrieval the read / write head uses a (relatively weaker / stronger)
magnetic field to sense the alignment / align the molecular magnets
on the disk in either of the two directions signifying a "zero" or a "one".

Thats a bit simplistic, but is the general idea.

The read / write head can access one
"block" or "track sector" (is it?) at a time.

The magnetic surface is broken into a series of
concentric tracks. Each track has a number of sectors
in it. With modern drives the number of sectors per
track varys in bands across the platter with hard drives.

With modern hard drives each sector is also a logical
block, with its own unique block number. Thats what
LBA is. That allows the detail of the tracks and sectors
to be ignored by everything outside the drive. Access
is purely by logical block number for reads and writes.

One block consists of several "clusters" or "allocation units".

This is an OS level organisation. It has nothing to do with how
the tracks and sectors are organised on the magnetic surface.

Whenever a particular file is to be accessed, the corresponding
"directory entry" is checked where the "FBN" or "File Block Number"
is found. The FBN is the starting BLOCK (not "cluster"?) number
for the file in the disk. The subsequent cluster numbers are
noted down in the FAT.

Only some formats even use a FAT.

The FBN is the starting for the corresponding "file chain" that
terminates with the "EOF" (End Of File) entry in the FAT.

Thats true for a FAT system. Not all formats even have a FAT tho.

Q1. How are the sectors and tracks marked on
a (previously unformatted) magnetic disk surface?

That gets quite complicated. There needs to be some mechanism
to put the heads over a particular track. With modern hard drives
thats done with embedded servo information thats written to the
magnetic disk surface in the factory at manufacturing time.

Floppys use a different system which doesnt have servo info.

Q2. In magnetic disks (unlike CD's) the "track sectors" (I am not sure
of this term - I mean here the intersection of a track with a sector)

See above on tracks and sectors.

are smaller in area near the centre and larger near the periphery.

Each sector on the inner tracks certainly occupys less physical
space on the track than a sector on the outer tracks. But that is
complicated by the fact that with modern hard drives the number
of sectors per track varys in bands across the platter surface, with
more sectors per track in the outer tracks than the inner tracks.

Must the read / write head be larger than the largest track sector?

A sector moves under the head as the platter
rotates. So that question of the size of the head
isnt relevant to the size of the sector in the track.

Q3. Does repeated "low-level" format reduce the life of a disk?

You cant do a true low level format with modern hard
drives. They mostly just write zeros thru every sector
on the drive. That has no effect on the life of the drive,
its just different data written to the sectors.

If yes, how?

On Thu, 7 Aug 2003 09:09:53 -0700, "Eric Gisin" wrote:

LLF just writes sectors.

That is vendor specific with various levels of documentation quality.
I am thinking of the IBM 60 GXP "format unit" command.
--
Svend Olaf

Svend Olaf Mikkelsen wrote:

On Thu, 7 Aug 2003 09:09:53 -0700, "Eric Gisin" wrote:


LLF just writes sectors.


That is vendor specific with various levels of documentation quality.
I am thinking of the IBM 60 GXP "format unit" command.

Is that like the famous "Halt and catch fire" command?

"CJT" wrote in message ...
Svend Olaf Mikkelsen wrote:

On Thu, 7 Aug 2003 09:09:53 -0700, "Eric Gisin" wrote:


LLF just writes sectors.


That is vendor specific with various levels of documentation quality.
I am thinking of the IBM 60 GXP "format unit" command.

Is that like the famous "Halt and catch fire" command?

Nope, thats the FireBalls, stupid.

Rod Speed wrote:

"CJT" wrote in message ...

Svend Olaf Mikkelsen wrote:


On Thu, 7 Aug 2003 09:09:53 -0700, "Eric Gisin" wrote:



LLF just writes sectors.


That is vendor specific with various levels of documentation quality.
I am thinking of the IBM 60 GXP "format unit" command.

Is that like the famous "Halt and catch fire" command?


Nope, thats the FireBalls, stupid.


I see I elicited a typical response from you.

"Svend Olaf Mikkelsen" wrote in message ...
On Thu, 7 Aug 2003 09:09:53 -0700, "Eric Gisin" wrote:

LLF just writes sectors.

That is

What is?
Low Level Format is just a term that stands for the sub-
division (formatting) of all unformatted tracks into sectors,
not necessarily the Format Track or Format Unit command.

vendor specific with various levels of documentation quality.
I am thinking of the IBM 60 GXP "format unit" command.

Why the Format Unit command, particularly?
(Format Unit was actually introduced far before the 60GXP)

--
Svend Olaf

"CJT" wrote in message ...
Rod Speed wrote:

"CJT" wrote in message ...

Svend Olaf Mikkelsen wrote:


On Thu, 7 Aug 2003 09:09:53 -0700, "Eric Gisin" wrote:



LLF just writes sectors.


That is vendor specific with various levels of documentation quality.
I am thinking of the IBM 60 GXP "format unit" command.

Is that like the famous "Halt and catch fire" command?


Nope, thats the FireBalls, stupid.


I see I elicited a typical response from you.

You sound surprised.

"Eric Gisin" wrote in message ...
www.pcguide.com covers disk formatting pretty well.

Except Low Level Formatting.

See inline answers.

"Arnab Lahiri" wrote in message m...
|
| Each surface of a hard disk or of a floppy disk is initially just a
| disk with some magnetic material coated onto it. For data storage /
| retrieval the read / write head uses a (relatively weaker / stronger)
| magnetic field to sense the alignment / align the molecular magnets on
| the disk in either of the two directions signifying a "zero" or a "one".
|
| The read / write head can access one "block" or "track sector"
| (is it?) at a time. One block consists of several "clusters" or
| "allocation units". Whenever a particular file is to be accessed, the
| corresponding "directory entry" is checked where the "FBN" or "File
| Block Number" is found. The FBN is the starting BLOCK (not "cluster"?)
| number for the file in the disk. The subsequent cluster numbers are
| noted down in the FAT. The FBN is the starting for the corresponding
| "file chain" that terminates with the "EOF" (End Of File) entry in the
| FAT.

The struct of DOS FAT is completely unrelated to low-level track and sector
format.

|
| Q1. How are the sectors and tracks marked on a (previously
| unformatted) magnetic disk surface?

Servoes

the 'unformatted' tracks

are written in the clean room, then sectors are written and verified

to the 'unformatted' tracks

after assembly.

| Q2. In magnetic disks (unlike CD's) the "track sectors" (I am not sure
| of this term - I mean here the intersection of a track with a sector)
| are smaller in area near the centre and larger near the periphery.
| Must the read / write head be larger than the largest track sector?
Modern disks have MR heads, which write thick (over 1/2 a track) and read
thin.

Tracks are all the same width,

Basically, since the arm is pivoted and the slider travels in a circumferencial
(if that's a word) motion.

but outer tracks have more bits.

| Q3. Does repeated "low-level" format reduce the life of a disk? If
| yes, how?

Servoes are written once.

Servoes are basically the virtual notches that with the help of the servo
system make a voice coil actuator into an extremely precise and fast stepper
motor equivalent.

LLF just writes sectors.
(Never tracks)

And once the sector structures have been written there is no particular need for
them to be overwritten again so a drive LLF in the field isn't necessarily as elaborate
as the one in the factory. A LLF in the field may involve minor changes s.a. sorting
sector IDs and bad sector mapping (which is just another form of s.ID management).

"CJT" wrote in message ...
Rod Speed wrote:

"CJT" wrote in message ...

Svend Olaf Mikkelsen wrote:


On Thu, 7 Aug 2003 09:09:53 -0700, "Eric Gisin" wrote:



LLF just writes sectors.


That is vendor specific with various levels of documentation quality.
I am thinking of the IBM 60 GXP "format unit" command.

Is that like the famous "Halt and catch fire" command?


Nope, thats the FireBalls, stupid.


I see I elicited a typical response from you.

I see I elicited a typical response from you.