5. File Systems, Disks, and Drives
5.1. How To Get Linux to Work with a Disk.
If your disk is an IDE or EIDE drive,
you should read the file /usr/src/linux/drivers/block/README.ide (part of the Linux kernel source code). This README contains many helpful hints about IDE drives. Many modern IDE controllers do
translation between `physical' cylinders/heads/sectors,
and `logical' ones.
SCSI disks are accessed by linear block numbers. The BIOS invents some `logical'
cylinder/head/sector fiction to support DOS.
Older IBM PC-compatible BIOS's will usually not be able to access partitions
which extend beyond 1024 logical cylinders,
and will make booting a Linux kernel from
such partitions using LILO problematic at
best.
You can still use such partitions for Linux or other operating systems that access
the controller directly.
It's recommend that you create at least one Linux partition entirely under the
1024 logical cylinder limit, and boot from
that. The other partitions will then be okay.
Also there seems to be a bit of trouble with the newer Ultra-DMA drives. I haven't
gotten the straight scoop on them—but
they are becoming a very common problem at
the SVLUG installfests. When you can get 8
to 12 Gig drives for $200 to $300 it's no
wonder.
[Jim Dennis]
5.2. How To Undelete Files.
In general, this is very hard to do on unices
because of their multitasking nature. Undelete
functionality for the ext2fs file system is being worked on, but don't hold your breath.
There are a number of packages available which instead provide new commands for
deleting and copying which move deleted files
into a `wastebasket' directory. The files
can be recovered until cleaned out automatically
by background processing.
The Midnight Commander file manager provides an undelete facility that uses Ext2
file system library functions and an undelete
directory for each file system. Commercial
distribution packages of MC may or may not
have this feature enabled, so be sure to look
in the source code distribution for instructions
on how to enable the undelete feature.
Alternatively, you can search the raw disk device which holds the file system
in question. This is hard work, and you will
need to be logged in as root to do this. But
it can be done. Run grep on the raw device; e.g.:
grep -b 'bookmarks' /dev/hda |
If the data has not been overwritten, you should be able to recover it with a
text editor.
[Dave Cinege, Daniel Novotny]
5.3. How To Make Backups.
You can back up a directory hierarchy
or complete file system to any media using
GNU tar or cpio, the standard *nix tools for this purpose. tar seems to be the more commonly used program currently, and includes command line
options to make compressed, incremental, and
multi-volume backups. Complete information
is contained in the documentation, which is
in GNU Texinfo format.
The free program, Amanda, receives a lot of mentions on Usenet. Its home page is http://www.amanda.org/.
Several commercial backup utilities also exist. They are often included in commercial
distributions.
5.4. How To Resize a Partition (Non-Destructively).
Use the FIPS.EXE program, included with most Linux distributions,under MS-DOS.
GNU parted, a partition editor, is stable enough for non-guru, mere-mortal use with relative
confidence. Source code for the latest version
is at: ftp://ftp.gnu.org/pub/gnu/parted/. There's also a boot disk image for resizing root partitions and for running
parted on non-Linux machines. The disk image may be easier for beginners. Building
from source could require some extra configuration.
Parted also has tutorial-style, plain-text documentation for Linux and FAT (MS-DOS)
file systems.
Also, some commercial distributions come with their own partitioning software,
like Partition Magic.
5.6. How To Create a File System on a Floppy.
To format a 3.5-inch, high density
floppy:
$ fdformat /dev/fd0H1440
$ mkfs -t ext2 -m 0 /dev/fd0H1440 1440 |
For a 5.25 inch floppy, use fd0h1200 and 1200 as appropriate. For the B: drive use fd1 instead of fd0.
The -m 0 option tells mkfs.ext2 not to reserve any space on the disk for the superuser—usually the last
10% is reserved for root.
The first command performs a low-level format. The second creates an empty file
system. You can mount the floppy like a hard
disk partition and simply cp and mv files, etc.
Device naming conventions generally are the same as for other unices. They can
be found in Matt Welsh's Installation and Getting Started guide. Refer to (``Where Is the Documentation?'') A more detailed and technical description is Linux Allocated Devices by H. Peter Anvin, hpa@zytor.com, which is included in LaTeX and ASCII form in the kernel source distribution
(probably in /usr/src/kernel/Documentation/), as devices.tex and devices.txt.
5.9. Linux Prints Nasty Messages about Inodes, Blocks, and the Like.
You may have a corrupted file system,
probably caused by not shutting Linux down
properly before turning off the power or resetting.
You need to use a recent shutdown program
to do this—for example, the one included
in the util-linux package, available on sunsite
and tsx-11.
If you're lucky, the program fsck (or e2fsck or xfsck as appropriate if you don't have the automatic fsck front-end) will be able to repair your file system. If you're unlucky, the file
system is trashed, and you'll have to re-initialize
it with mkfs (or mke2fs, mkxfs, etc.), and restore from a backup.
NB: don't try to check a file system that's mounted read/write—this includes
the root partition, if you don't see
VFS: mounted root ... read-only |
at boot time.
5.10. The Swap Area Isn't Working.
When you boot (or enable swapping
manually) you should see
Adding Swap: NNNNk swap-space |
If you don't see any messages at all you are probably missing
(the command to enable swapping) in your /etc/rc.local or /etc/rc.d/* (the system
startup scripts), or have forgotten to make
the right entry in /etc/fstab:
for example.
If you see:
Unable to find swap-space signature |
you have forgotten to run mkswap. See the manual page for details; it works much like mkfs.
Running, free in addition to showing free memory, should display:
total used free
Swap: 10188 2960 7228 |
If typing ``cat /proc/swaps'' reveals only file or partition names, but no swap space information, then
the swap file or partition needs re-initialization.
Use fdisk (as root) to determine which partition on a hard drive has been designated as
the swap partition. The partition still needs
to be initialized with mkswap before enabling it with swapon.
[Andy Jefferson, Steve Withers]
5.11. How To Add Temporary Swap Space.
In addition to a swap partition,
Linux can also use a swap file. Some programs,
like g++, can use huge amounts of virtual memory, requiring the temporary creation of
extra space. To install an extra 64 MB of
swap space, for example, use the following
shell commands:
# dd if=/dev/zero of=/swap bs=1024 count=65535
# mkswap /swap
# swapon /swap |
The count= argument to dd determines how big the swap file will be. In this example the swap file's name
is /swap, but the name and location are, generally, arbitrary, depending only on the
file system's available space and your having
write permissions in the directory.
When you don't need the swap space any more, remove it with the following statements:
# swapoff /swap
# rm /swap |
Take a look also at the Installation HOWTO and Installation & Getting Started for detailed instructions.
If that still doesn't provide enough swap space, refer to (``How To Have More Than 128Mb of Swap.'')
5.12. How To Remove LILO So the System Boots DOS Again?
The lilo program (not the complete LILO package), uses the command line option -u to uninstall the LILO boot loader. You have to supply the device name of the device you installed
LILO on, for example:
This rewrites the original, pre-LILO master boot record back to the first hard drive, from the boot record saved
in /boot/boot.0300. If you installed LILO to a partition as a secondary boot loader, for example, /dev/hda1, lilo re-installs the original boot sector from the save file /boot/boot.0301. Refer to the lilo manual page for details. Thanks to Villy Kruse for reminding me to update this
answer.
If you have an earlier version of LILO, you will have to use the DOS (MS-DOS 5.0 or later, or OS/2) FDISK /MBR (which is not documented). This will restore a standard MS-DOS Master Boot Record. If you have DR-DOS 6.0, go into FDISK.EXE in the normal way and then select the Re-write Master Boot Record option.
If you create a boot floppy during the Windows installation process, make sure
that it contains the programs FDISK.EXE, FORMAT.COM, and SYS.COM, and use that to re-install MS-DOS on the hard disk.
If you don't have MS-DOS or DR-DOS, you need to have the boot sector that LILO
saved when you first installed it. You did
keep that file, didn't you? It's probably
called boot.0301 or some such. Type:
dd if=boot.0301 of=/dev/hda bs=445 count=1 |
(or /dev/sda if you're using a SCSI disk). This may also wipe out your partition table, so
beware! If you're desperate, you could use
dd if=/dev/zero of=/dev/hda bs=512 count=1 |
This will erase your partition table and boot sector completely: you can then
reformat the disk using your favorite software.
But this will render the contents of your
disk inaccessible—you'll lose it all
unless you're an expert.
Note that the DOS MBR boots whichever (single!) partition is flagged as ``active.'' You may need
to use fdisk to set and clear the active flags on partitions appropriately.
5.13. Why Does fdformat Require Superuser Privileges?
The system call to format a floppy
can only be done as root, regardless of the
permissions of /dev/fd0*. If you want any user to be able to format a floppy, try getting the fdformat2 program. This works around the problems by being setuid to root.
5.16. What Is /proc/kcore?
None of the files in /proc are really there—they're all, ``pretend,'' files made up by the kernel,
to give you information about the system and
don't take up any hard disk space.
/proc/kcore is like an ``alias'' for the memory in your computer. Its size is the same as
the amount of RAM you have, and if you read
it as a file, the kernel does memory reads.
5.17. The AHA1542C Doesn't Work with Linux.
The option to allow disks with more
than 1024 cylinders, which the AHA1542C card
can recognize, is only required as a workaround
for a PC-compatible BIOS misfeature and should
be turned `off' under Linux. For older Linux
kernels you need to turn off most of the `advanced
BIOS' options—all but the one about
scanning the bus for bootable devices.
