fdisk - PC slice table maintenance utility
WARNING when -f is used, you are not asked if you really want to write the slices table (as you are in the interactive mode). Use with caution!
The final disk name can be provided as a ``bare'' disk name only, e.g. da0 or as a full pathname. If omitted, tries to figure out the default disk device name from the mounted root device.
When called with no arguments, it prints the sector 0 slice table. An example follows:
******* Working on device /dev/ad0 ******* parameters extracted from in-core disklabel are: cylinders=769 heads=15 sectors/track=33 (495 blks/cyl) parameters to be used for BIOS calculations are: cylinders=769 heads=15 sectors/track=33 (495 blks/cyl) Warning: BIOS sector numbering starts with sector 1 Information from DOS bootblock is: The data for partition 1 is: sysid 165,(FreeBSD/NetBSD/386BSD) start 495, size 380160 (185 Meg), flag 0 beg: cyl 1/ sector 1/ head 0; end: cyl 768/ sector 33/ head 14 The data for partition 2 is: sysid 164,(unknown) start 378180, size 2475 (1 Meg), flag 0 beg: cyl 764/ sector 1/ head 0; end: cyl 768/ sector 33/ head 14 The data for partition 3 is: <UNUSED> The data for partition 4 is: sysid 99,(ISC UNIX, other System V/386, GNU HURD or Mach) start 380656, size 224234 (109 Meg), flag 80 beg: cyl 769/ sector 2/ head 0; end: cyl 197/ sector 33/ head 14
The disk is divided into three slices that happen to fill the disk. The second slice overlaps the end of the first. (Used for debugging purposes.)
Note these numbers are calculated using BIOS's understanding of the disk geometry and saved in the bootblock.
The -i and -u flags are used to indicate that the slice data is to be updated. Unless the -f option is also given, will enter a conversational mode. In this mode, no changes will be written to disk unless you explicitly tell to.
The utility will display each slice and ask whether you want to edit it. If you say yes, will step through each field, show you the old value, and ask you for a new one. When you are done with the slice, will display it and ask you whether it is correct. It will then proceed to the next entry.
Getting the cyl , sector and head fields correct is tricky, so by default, they will be calculated for you; you can specify them if you choose to though.
After all the slices are processed, you are given the option to change the ``active'' slice. Finally, when all the new data for sector 0 has been accumulated, you are asked to confirm whether you really want to rewrite it.
The difference between the -u and -i flags is that the -u flag edits (updates) the existing slice parameters while the -i flag is used to ``initialize'' them (old values will be ignored); if you edit the first slice, -i will also set it up to use the whole disk for Fx and make it active.
If you hand craft your disk layout, please make sure that the Fx slice starts on a cylinder boundary.
Editing an existing slice will most likely result in the loss of all data in that slice.
You should run interactively once or twice to see how it works. This is completely safe as long as you answer the last question in the negative. There are subtleties that detects that are not fully explained in this manual page.
These specs can occur in any order, as the leading letter determines which value is which; however, all three must be specified.
This line must occur before any lines that specify slice information.
It is an error if the following is not true:
1 <= number of cylinders 1 <= number of heads <= 256 1 <= number of sectors/track < 64
The number of cylinders should be less than or equal to 1024, but this is not enforced, although a warning will be printed. Note that bootable Fx slices (the ``/ '' file system) must lie completely within the first 1024 cylinders; if this is not true, booting may fail. Non-bootable slices do not have this restriction.
Example (all of these are equivalent), for a disk with 1019 cylinders, 39 heads, and 63 sectors:
g c1019 h39 s63 g h39 c1019 s63 g s63 h39 c1019
Only those slices explicitly mentioned by these lines are modified; any slice not referenced by a p line will not be modified. However, if an invalid slice table is present, or the -i option is specified, all existing slice entries will be cleared (marked as unused), and these p lines will have to be used to explicitly set slice information. If multiple slices need to be set, multiple p lines must be specified; one for each slice.
These slice lines must occur after any geometry specification lines, if one is present.
The type is 165 for Fx slices. Specifying a slice type of zero is the same as clearing the slice and marking it as unused; however, dummy values (such as ``0'' must still be specified for start and length
Note: the start offset will be rounded upwards to a head boundary if necessary, and the end offset will be rounded downwards to a cylinder boundary if necessary.
Example: to clear slice 4 and mark it as unused:
"p 4 0 0 0"
Example: to set slice 1 to a Fx slice, starting at sector 1 for 2503871 sectors (note: these numbers will be rounded upwards and downwards to correspond to head and cylinder boundaries):
"p 1 165 1 2503871"
Example: to make slice 1 the active slice:
The entire utility should be made more user-friendly.
Most users new to Fx do not understand the difference between ``slice'' and ``partition'' causing difficulty to adjust.
You cannot use this command to completely dedicate a disk to Fx . The bsdlabel(8) command must be used for this.
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Created 1996-2022 by Maxim Chirkov
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