06-05-2009
Thanks everyone for the responses.
Actually my doubt is if I am doing 2 swipes over the disk with a pattern and its compliment, using either dd/dcfldd for the purpose in a script, then the first swipe should have actually ended up physically on the sectors on disk before the second swipe.
With the script I have pasted in my first post, is the sync in between 2 swipes sufficient to ensure that?
Can anyone help me with some pointers on the impact of OS/disk caches on the way to disk sectors?
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LEARN ABOUT FREEBSD
disktab
DISKTAB(5) BSD File Formats Manual DISKTAB(5)
NAME
disktab -- disk description file
SYNOPSIS
#include <disklabel.h>
DESCRIPTION
Disktab is a simple database which describes disk geometries and disk partition characteristics. It is used to initialize the disk label on
the disk. The format is patterned after the termcap(5) terminal data base. Entries in disktab consist of a number of `:'-separated fields.
The first field for each entry gives the names by which a disk's entry may be selected, separated by `|' characters. The last name given
should be a long name fully identifying the disk.
The optional fields for each entry are:
ID Type Description
ty str Type of disk (e.g. removable, winchester)
dt str Type of controller (e.g. SMD, ESDI, floppy)
ns num Number of sectors per track
nt num Number of tracks per cylinder
nc num Total number of cylinders on the disk
sc num Number of sectors per cylinder, ns*nt default
su num Number of sectors per unit, sc*nc default
se num Sector size in bytes, DEV_BSIZE default
sf bool Controller supports bad144-style bad sector forwarding
rm num Rotation speed, rpm, 3600 default
sk num Sector skew per track, default 0
cs num Sector skew per cylinder, default 0
hs num Headswitch time, usec, default 0
ts num One-cylinder seek time, usec, default 0
il num Sector interleave (n:1), 1 default
d[0-4] num Drive-type-dependent parameters
bs num Boot block size, default BBSIZE
sb num Superblock size, default 0
ba num Block size for partition `a' (bytes)
bd num Block size for partition `d' (bytes)
be num Block size for partition `e' (bytes)
bf num Block size for partition `f' (bytes)
bg num Block size for partition `g' (bytes)
bh num Block size for partition `h' (bytes)
fa num Fragment size for partition `a' (bytes)
fd num Fragment size for partition `d' (bytes)
fe num Fragment size for partition `e' (bytes)
ff num Fragment size for partition `f' (bytes)
fg num Fragment size for partition `g' (bytes)
fh num Fragment size for partition `h' (bytes)
oa num Offset of partition `a' in sectors
ob num Offset of partition `b' in sectors
oc num Offset of partition `c' in sectors
od num Offset of partition `d' in sectors
oe num Offset of partition `e' in sectors
of num Offset of partition `f' in sectors
og num Offset of partition `g' in sectors
oh num Offset of partition `h' in sectors
pa num Size of partition `a' in sectors
pb num Size of partition `b' in sectors
pc num Size of partition `c' in sectors
pd num Size of partition `d' in sectors
pe num Size of partition `e' in sectors
pf num Size of partition `f' in sectors
pg num Size of partition `g' in sectors
ph num Size of partition `h' in sectors
ta str Partition type of partition `a' (4.2BSD file system, swap, etc)
tb str Partition type of partition `b'
tc str Partition type of partition `c'
td str Partition type of partition `d'
te str Partition type of partition `e'
tf str Partition type of partition `f'
tg str Partition type of partition `g'
th str Partition type of partition `h'
FILES
/etc/disktab
SEE ALSO
getdiskbyname(3), bsdlabel(8), newfs(8)
HISTORY
The disktab description file appeared in 4.2BSD.
BSD
October 14, 2006 BSD