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<!DOCTYPE HTML PUBLIC "-//Norman Walsh//DTD DocBook HTML 1.0//EN">
<HTML
><HEAD
><TITLE
>Filesystems</TITLE
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>The Linux System Administrators' Guide</TH
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><TR
><TD
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ALIGN="left"
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><A
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><HR
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WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN1029"
>Filesystems</A
></H1
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN1031"
>What are filesystems?</A
></H2
><P
>A <I
CLASS="GLOSSTERM"
>filesystem</I
> is the methods and
data structures that an operating system uses to keep track
of files on a disk or partition; that is, the way the files
are organized on the disk. The word is also used to refer to a
partition or disk that is used to store the files or the type of
the filesystem. Thus, one might say ``I have two filesystems''
meaning one has two partitions on which one stores files, or
that one is using the ``extended filesystem'', meaning the type
of the filesystem.</P
><P
>The difference between a disk or partition and the filesystem
it contains is important. A few programs (including,
reasonably enough, programs that create filesystems) operate
directly on the raw sectors of a disk or partition; if there
is an existing file system there it will be destroyed or
seriously corrupted. Most programs operate on a filesystem,
and therefore won't work on a partition that doesn't contain
one (or that contains one of the wrong type).</P
><P
>Before a partition or disk can be used as a filesystem, it
needs to be initialized, and the bookkeeping data structures need
to be written to the disk. This process is called
<I
CLASS="GLOSSTERM"
>making a filesystem</I
>.</P
><P
>Most UNIX filesystem types have a similar general
structure, although the exact details vary quite a bit.
The central concepts are <I
CLASS="GLOSSTERM"
>superblock</I
>,
<I
CLASS="GLOSSTERM"
>inode</I
>, <I
CLASS="GLOSSTERM"
>data block</I
>,
<I
CLASS="GLOSSTERM"
>directory block</I
>, and <I
CLASS="GLOSSTERM"
>indirection
block</I
>. The superblock contains information
about the filesystem as a whole, such as its size (the exact
information here depends on the filesystem). An inode contains
all information about a file, except its name. The name is
stored in the directory, together with the number of the inode.
A directory entry consists of a filename and the number of
the inode which represents the file. The inode contains the
numbers of several data blocks, which are used to store the
data in the file. There is space only for a few data block
numbers in the inode, however, and if more are needed, more
space for pointers to the data blocks is allocated dynamically.
These dynamically allocated blocks are indirect blocks; the name
indicates that in order to find the data block, one has to find
its number in the indirect block first.</P
><P
>UNIX filesystems usually allow one to create a
<I
CLASS="GLOSSTERM"
>hole</I
> in a file (this is done with
<TT
CLASS="FUNCTION"
>lseek</TT
>; check the manual page), which means
that the filesystem just pretends that at a particular place in
the file there is just zero bytes, but no actual disk sectors are
reserved for that place in the file (this means that the file
will use a bit less disk space). This happens especially often
for small binaries, Linux shared libraries, some databases, and
a few other special cases. (Holes are implemented by storing a
special value as the address of the data block in the indirect
block or inode. This special address means that no data block
is allocated for that part of the file, ergo, there is a hole
in the file.)</P
><P
>Holes are moderately useful. On the author's system,
a simple measurement showed a potential for about 4 MB of
savings through holes of about 200 MB total used disk space.
That system, however, contains relatively few programs and no
database files.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN1048"
>Filesystems galore</A
></H2
><P
>Linux supports several types of filesystems. As of this
writing the most important ones are:
<DIV
CLASS="GLOSSLIST"
><DL
><DT
><B
>minix</B
></DT
><DD
><P
> The oldest, presumed to be the most reliable, but quite
limited in features (some time stamps are missing, at
most 30 character filenames) and restricted in
capabilities (at most 64 MB per filesystem).
</P
></DD
><DT
><B
>xia</B
></DT
><DD
><P
> A modified version of the minix filesystem that lifts
the limits on the filenames and filesystem sizes,
but does not otherwise introduce new features. It is
not very popular, but is reported to work very well.
</P
></DD
><DT
><B
>ext2</B
></DT
><DD
><P
> The most featureful of the native Linux filesystems,
currently also the most popular one. It is designed to
be easily upwards compatible, so that new versions
of the filesystem code do not require re-making the
existing filesystems.
</P
></DD
><DT
><B
>ext</B
></DT
><DD
><P
> An older version of ext2 that wasn't upwards
compatible. It is hardly ever used in new installations
any more, and most people have converted to ext2.
</P
></DD
></DL
></DIV
>
</P
><P
>In addition, support for several foreign filesystem exists,
to make it easier to exchange files with other operating
systems. These foreign filesystems work just like native
ones, except that they may be lacking in some usual UNIX
features, or have curious limitations, or other oddities.
<DIV
CLASS="GLOSSLIST"
><DL
><DT
><B
>msdos</B
></DT
><DD
><P
> Compatibility with MS-DOS (and OS/2 and Windows NT)
FAT filesystems.
</P
></DD
><DT
><B
>usmdos</B
></DT
><DD
><P
> Extends the msdos filesystem driver under
Linux to get long filenames, owners,
permissions, links, and device files. This allows a normal
msdos filesystem to be used as if it were a
Linux one, thus removing the need for a separate
partition for Linux.
</P
></DD
><DT
><B
>iso9660</B
></DT
><DD
><P
> The standard CD-ROM filesystem; the popular Rock Ridge
extension to the CD-ROM standard that allows longer file
names is supported automatically.
</P
></DD
><DT
><B
>nfs</B
></DT
><DD
><P
> A networked filesystem that allows sharing a filesystem
between many computers to allow easy access to the
files from all of them.
</P
></DD
><DT
><B
>hpfs</B
></DT
><DD
><P
> The OS/2 filesystem.
</P
></DD
><DT
><B
>sysv</B
></DT
><DD
><P
> SystemV/386, Coherent, and Xenix filesystems.
</P
></DD
></DL
></DIV
>
</P
><P
>The choice of filesystem to use depends on the situation. If
compatibility or other reasons make one of the non-native
filesystems necessary, then that one must be used. If one can
choose freely, then it is probably wisest to use ext2, since
it has all the features but does not suffer from lack of
performance.</P
><P
>There is also the proc filesystem, usually accessible as
the <TT
CLASS="FILENAME"
>/proc</TT
> directory, which is not really a
filesystem at all, even though it looks like one. The
proc filesystem makes it easy to access certain kernel
data structures, such as the process list (hence the name).
It makes these
data structures look like a filesystem, and that filesystem
can be manipulated with all the usual file tools. For example,
to get a listing of all processes one might use the
command
<PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>ls -l /proc</B
></TT
>
<TT
CLASS="COMPUTEROUTPUT"
>total 0
dr-xr-xr-x 4 root root 0 Jan 31 20:37 1
dr-xr-xr-x 4 liw users 0 Jan 31 20:37 63
dr-xr-xr-x 4 liw users 0 Jan 31 20:37 94
dr-xr-xr-x 4 liw users 0 Jan 31 20:37 95
dr-xr-xr-x 4 root users 0 Jan 31 20:37 98
dr-xr-xr-x 4 liw users 0 Jan 31 20:37 99
-r--r--r-- 1 root root 0 Jan 31 20:37 devices
-r--r--r-- 1 root root 0 Jan 31 20:37 dma
-r--r--r-- 1 root root 0 Jan 31 20:37 filesystems
-r--r--r-- 1 root root 0 Jan 31 20:37 interrupts
-r-------- 1 root root 8654848 Jan 31 20:37 kcore
-r--r--r-- 1 root root 0 Jan 31 11:50 kmsg
-r--r--r-- 1 root root 0 Jan 31 20:37 ksyms
-r--r--r-- 1 root root 0 Jan 31 11:51 loadavg
-r--r--r-- 1 root root 0 Jan 31 20:37 meminfo
-r--r--r-- 1 root root 0 Jan 31 20:37 modules
dr-xr-xr-x 2 root root 0 Jan 31 20:37 net
dr-xr-xr-x 4 root root 0 Jan 31 20:37 self
-r--r--r-- 1 root root 0 Jan 31 20:37 stat
-r--r--r-- 1 root root 0 Jan 31 20:37 uptime
-r--r--r-- 1 root root 0 Jan 31 20:37 version</TT
>
<TT
CLASS="PROMPT"
>$</TT
></PRE
>
(There will be a few extra files that don't correspond to
processes, though. The above example has been shortened.)</P
><P
>Note that even though it is called a filesystem, no part of
the proc filesystem touches any disk. It exists only in the
kernel's imagination. Whenever anyone tries to look at any
part of the proc filesystem, the kernel makes it look as if
the part existed somewhere, even though it doesn't. So, even
though there is a multi-megabyte <TT
CLASS="FILENAME"
>/proc/kcore</TT
> file,
it doesn't take any disk space.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN1104"
>Which filesystem should be used?</A
></H2
><P
>There is usually little point in using many different
filesystems. Currently, ext2fs is the most popular one, and
it is probably the wisest choice. Depending on the overhead
for bookkeeping structures, speed, (perceived) reliability,
compatibility, and various other reasons, it may be advisable
to use another file system. This needs to be decided on a
case-by-case basis.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN1107"
>Creating a filesystem</A
></H2
><P
>Filesystems are created, i.e., initialized, with the <B
CLASS="COMMAND"
>mkfs</B
>
command. There is actually a separate program for each filesystem
type. <B
CLASS="COMMAND"
>mkfs</B
> is just a front end that runs the appropriate
program depending on the desired filesystem type. The type is
selected with the <SPAN
CLASS="OPTION"
>-t fstype</SPAN
> option.</P
><P
>The programs called by <B
CLASS="COMMAND"
>mkfs</B
> have slightly
different command line interfaces. The common and most important
options are summarized below; see the manual pages for more.
<DIV
CLASS="GLOSSLIST"
><DL
><DT
><B
><SPAN
CLASS="OPTION"
>-t fstype</SPAN
></B
></DT
><DD
><P
> Select the type of the filesystem.
</P
></DD
><DT
><B
><SPAN
CLASS="OPTION"
>-c</SPAN
></B
></DT
><DD
><P
> Search for bad blocks and initialize the bad
block list accordingly.
</P
></DD
><DT
><B
>-l filename</B
></DT
><DD
><P
> Read the initial bad block list from the name file.
</P
></DD
></DL
></DIV
>
</P
><P
>To create an ext2 filesystem on a floppy, one would give the
following commands:
<PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>fdformat -n /dev/fd0H1440</B
></TT
>
<TT
CLASS="COMPUTEROUTPUT"
>Double-sided, 80 tracks, 18 sec/track. Total capacity 1440 kB.
Formatting ... done</TT
>
<TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>badblocks /dev/fd0H1440 1440 $>$ bad-blocks</B
></TT
>
<TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>mkfs -t ext2 -l bad-blocks /dev/fd0H1440</B
></TT
>
<TT
CLASS="COMPUTEROUTPUT"
>mke2fs 0.5a, 5-Apr-94 for EXT2 FS 0.5, 94/03/10
360 inodes, 1440 blocks
72 blocks (5.00%) reserved for the super user
First data block=1
Block size=1024 (log=0)
Fragment size=1024 (log=0)
1 block group
8192 blocks per group, 8192 fragments per group
360 inodes per group
Writing inode tables: done
Writing superblocks and filesystem accounting information: done</TT
>
<TT
CLASS="PROMPT"
>$</TT
></PRE
>
First, the floppy was formatted (the <SPAN
CLASS="OPTION"
>-n</SPAN
> option
prevents validation, i.e., bad block checking). Then bad blocks
were searched with <B
CLASS="COMMAND"
>badblocks</B
>, with the output
redirected to a file, <TT
CLASS="FILENAME"
>bad-blocks</TT
>. Finally,
the filesystem was created, with the bad block list initialized
by whatever <B
CLASS="COMMAND"
>badblocks</B
> found.</P
><P
>The <SPAN
CLASS="OPTION"
>-c</SPAN
> option could have been used with
<B
CLASS="COMMAND"
>mkfs</B
> instead of <B
CLASS="COMMAND"
>badblocks</B
>
and a separate file. The example below does that.
<PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>mkfs -t ext2 -c /dev/fd0H1440</B
></TT
>
<TT
CLASS="COMPUTEROUTPUT"
>mke2fs 0.5a, 5-Apr-94 for EXT2 FS 0.5, 94/03/10
360 inodes, 1440 blocks
72 blocks (5.00%) reserved for the super user
First data block=1
Block size=1024 (log=0)
Fragment size=1024 (log=0)
1 block group
8192 blocks per group, 8192 fragments per group
360 inodes per group
Checking for bad blocks (read-only test): done
Writing inode tables: done
Writing superblocks and filesystem accounting information: done</TT
>
<TT
CLASS="PROMPT"
>$</TT
></PRE
>
The <SPAN
CLASS="OPTION"
>-c</SPAN
> option is more convenient than a separate use of
<B
CLASS="COMMAND"
>badblocks</B
>, but <B
CLASS="COMMAND"
>badblocks</B
> is necessary for checking
after the filesystem has been created.</P
><P
>The process to prepare filesystems on hard disks or
partitions is the same as for floppies, except that the formatting
isn't needed.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="MOUNT-AND-UMOUNT"
>Mounting and unmounting</A
></H2
><P
>Before one can use a filesystem, it has to be <I
CLASS="GLOSSTERM"
>mounted</I
>.
The operating system then does various bookkeeping things to
make sure that everything works. Since all files in UNIX are
in a single directory tree, the mount operation will make it
look like the contents of the new filesystem are the contents of
an existing subdirectory in some already mounted filesystem.</P
><P
>For example, <A
HREF="x1029.html#HD-MOUNT-ROOT"
>Figure 4-3</A
> shows three
separate filesystems, each with their own root directory.
When the last two filesystems are mounted below <TT
CLASS="FILENAME"
>/home</TT
>
and <TT
CLASS="FILENAME"
>/usr</TT
>, respectively, on the first filesystem, we
can get a single directory tree, as in
<A
HREF="x1029.html#HD-MOUNT-ALL"
>Figure 4-4</A
>.</P
><DIV
CLASS="FIGURE"
><P
><B
><A
NAME="HD-MOUNT-ROOT"
>Figure 4-3. Three separate filesystems.</A
></B
></P
><P
><IMG
SRC="hd-mount-separate.gif"></P
></DIV
><DIV
CLASS="FIGURE"
><P
><B
><A
NAME="HD-MOUNT-ALL"
>Figure 4-4. <TT
CLASS="FILENAME"
>/home</TT
> and <TT
CLASS="FILENAME"
>/usr</TT
> have been mounted.</A
></B
></P
><P
><IMG
SRC="hd-mount-mounted.gif"></P
></DIV
><P
>The mounts could be done as in the following example:
<PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>mount /dev/hda2 /home</B
></TT
>
<TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>mount /dev/hda3 /usr</B
></TT
>
<TT
CLASS="PROMPT"
>$</TT
></PRE
>
The <B
CLASS="COMMAND"
>mount</B
> command takes two arguments.
The first one is the device file corresponding to the disk
or partition containing the filesystem. The second one is
the directory below which it will be mounted. After these
commands the contents of the two filesystems look just
like the contents of the <TT
CLASS="FILENAME"
>/home</TT
> and
<TT
CLASS="FILENAME"
>/usr</TT
> directories, respectively. One would
then say that ``<TT
CLASS="FILENAME"
>/dev/hda2</TT
> <I
CLASS="GLOSSTERM"
>is
mounted on</I
> <TT
CLASS="FILENAME"
>/home</TT
>'', and
similarly for <TT
CLASS="FILENAME"
>/usr</TT
>. To look at either
filesystem, one would look at the contents of the directory
on which it has been mounted, just as if it were any other
directory. Note the difference between the device file,
<TT
CLASS="FILENAME"
>/dev/hda2</TT
>, and the mounted-on directory,
<TT
CLASS="FILENAME"
>/home</TT
>. The device file gives access to the
raw contents of the disk, the mounted-on directory gives access
to the files on the disk. The mounted-on directory is called
the <I
CLASS="GLOSSTERM"
>mount point</I
>.</P
><P
>Linux supports many filesystem types. <B
CLASS="COMMAND"
>mount</B
> tries to
guess the type of the filesystem. You can also use the
<SPAN
CLASS="OPTION"
>-t fstype</SPAN
> option to specify the type directly;
this is sometimes necessary, since the heuristics <B
CLASS="COMMAND"
>mount</B
>
uses do not always work. For example, to mount an MS-DOS
floppy, you could use the following command:
<PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>mount -t msdos /dev/fd0 /floppy</B
></TT
>
<TT
CLASS="PROMPT"
>$</TT
></PRE
>
</P
><P
>The mounted-on directory need not be empty, although it
must exist. Any files in it, however, will be inaccessible by
name while the filesystem is mounted. (Any files that have
already been opened will still be accessible. Files that
have hard links from other directories can be accessed using
those names.) There is no harm done with this, and it can even
be useful. For instance, some people like to have <TT
CLASS="FILENAME"
>/tmp</TT
>
and <TT
CLASS="FILENAME"
>/var/tmp</TT
> synonymous, and make <TT
CLASS="FILENAME"
>/tmp</TT
> be a symbolic
link to <TT
CLASS="FILENAME"
>/var/tmp</TT
>. When the system is booted, before
the <TT
CLASS="FILENAME"
>/var</TT
> filesystem is mounted, a <TT
CLASS="FILENAME"
>/var/tmp</TT
> directory
residing on the root filesystem is used instead. When <TT
CLASS="FILENAME"
>/var</TT
>
is mounted, it will make the <TT
CLASS="FILENAME"
>/var/tmp</TT
> directory on the root
filesystem inaccessible. If <TT
CLASS="FILENAME"
>/var/tmp</TT
> didn't exist on the
root filesystem, it would be impossible to use temporary files
before mounting <TT
CLASS="FILENAME"
>/var</TT
>.</P
><P
>If you don't intend to write anything to the filesystem, use
the <SPAN
CLASS="OPTION"
>-r</SPAN
> switch for <B
CLASS="COMMAND"
>mount</B
> to do a <I
CLASS="GLOSSTERM"
>readonly
mount</I
>. This will make the kernel stop any attempts at
writing to the filesystem, and will also stop the kernel from
updating file access times in the inodes. Read-only mounts
are necessary for unwritable media, e.g., CD-ROM's.</P
><P
>The alert reader has already noticed a slight
logistical problem. How is the first filesystem (called the <I
CLASS="GLOSSTERM"
>root
filesystem</I
>, because it contains the root directory) mounted,
since it obviously can't be mounted on another filesystem?
Well, the answer is that it is done by magic.
<A
NAME="AEN1217"
HREF="#FTN.AEN1217"
>[1]</A
>
The root filesystem is magically mounted at boot time,
and one can rely on it to always be mounted. If the
root filesystem can't be mounted, the system does not boot.
The name of the filesystem that is magically mounted as root
is either compiled into the kernel, or set using LILO or
<B
CLASS="COMMAND"
>rdev</B
>.</P
><P
>The root filesystem is usually first mounted readonly.
The startup scripts will then run <B
CLASS="COMMAND"
>fsck</B
>
to verify its validity, and if there are no problems, they
will <I
CLASS="GLOSSTERM"
>re-mount</I
> it so that writes will
also be allowed. <B
CLASS="COMMAND"
>fsck</B
> must not be run on a
mounted filesystem, since any changes to the filesystem while
<B
CLASS="COMMAND"
>fsck</B
> is running <I
CLASS="EMPHASIS"
>will</I
>
cause trouble. Since the root filesystem is mounted readonly
while it is being checked, <B
CLASS="COMMAND"
>fsck</B
> can fix any
problems without worry, since the remount operation will flush
any metadata that the filesystem keeps in memory.</P
><P
>On many systems there are other filesystems that should
also be mounted automatically at boot time. These are specified
in the <TT
CLASS="FILENAME"
>/etc/fstab</TT
> file; see the fstab man
page for details on the format. The details of exactly when the
extra filesystems are mounted depend on many factors, and can be
configured by each administrator if need be; see
<A
HREF="c1582.html"
>Chapter 6</A
>.</P
><P
>When a filesystem no longer needs to be mounted, it can be
unmounted with <B
CLASS="COMMAND"
>umount</B
>.
<A
NAME="AEN1232"
HREF="#FTN.AEN1232"
>[2]</A
>
<B
CLASS="COMMAND"
>umount</B
> takes one argument:
either the device file or the mount point.
For example, to unmount the directories of
the previous example, one could use the commands
<PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>umount /dev/hda2</B
></TT
>
<TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>umount /usr</B
></TT
>
<TT
CLASS="PROMPT"
>$</TT
></PRE
>
</P
><P
>See the man page for further instructions on how to
use the command. It is imperative that you always unmount a
mounted floppy. <I
CLASS="EMPHASIS"
>Don't just pop the floppy out of
the drive!</I
> Because of disk caching, the data is
not necessarily written to the floppy until you unmount it,
so removing the floppy from the drive too early might cause the
contents to become garbled. If you only read from the floppy,
this is not very likely, but if you write, even accidentally,
the result may be catastrophic.</P
><P
>Mounting and unmounting requires super user privileges, i.e.,
only root can do it. The reason for this is that if any
user can mount a floppy on any directory, then it is rather easy
to create a floppy with, say, a Trojan horse disguised as
<TT
CLASS="FILENAME"
>/bin/sh</TT
>, or any other often used program. However, it is
often necessary to allow users to use floppies, and there are
several ways to do this:
<P
></P
><UL
><LI
><P
>Give the users the root password. This is
obviously bad security, but is the easiest solution. It works
well if there is no need for security anyway, which is the case
on many non-networked, personal systems.</P
></LI
><LI
><P
>Use a program such as <B
CLASS="COMMAND"
>sudo</B
> to allow users to
use mount. This is still bad security, but doesn't
directly give super user privileges to
everyone.
<A
NAME="AEN1252"
HREF="#FTN.AEN1252"
>[3]</A
>
</P
></LI
><LI
><P
>Make the users use <B
CLASS="COMMAND"
>mtools</B
>, a package for manipulating
MS-DOS filesystems, without mounting them. This works
well if MS-DOS floppies are all that is needed,
but is rather awkward otherwise.
</P
></LI
><LI
><P
>List the floppy devices and their allowable mount points
together with the suitable options in <TT
CLASS="FILENAME"
>/etc/fstab</TT
>.
</P
></LI
></UL
>
The last alternative can be implemented by adding a line like
the following to the \fn{/etc/fstab} file:
<PRE
CLASS="SCREEN"
>/dev/fd0 /floppy msdos user,noauto 0 0</PRE
>
The columns are: device file to mount, directory to mount
on, filesystem type, options, backup frequency (used by
<B
CLASS="COMMAND"
>dump</B
>), and <B
CLASS="COMMAND"
>fsck</B
> pass number
(to specify the order in which filesystems should be checked
upon boot; 0 means no check).</P
><P
>The <SPAN
CLASS="OPTION"
>noauto</SPAN
> option stops this mount to be done
automatically when the system is started (i.e., it stops
<B
CLASS="COMMAND"
>mount -a</B
> from mounting it). The <SPAN
CLASS="OPTION"
>user</SPAN
> option
allows any user to mount the filesystem, and, because of security
reasons, disallows execution of programs (normal or setuid)
and interpretation of device files from the mounted filesystem.
After this, any user can mount a floppy with an msdos
filesystem with the following command:
<PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>mount /floppy</B
></TT
>
<TT
CLASS="PROMPT"
>$</TT
></PRE
>
The floppy can (and needs to, of course) be unmounted with
the corresponding \cmd{umount} command.</P
><P
>If you want to provide access to several types of floppies,
you need to give several mount points. The settings can be
different for each mount point. For example, to give access
to both MS-DOS and ext2 floppies, you could have the following
to lines in <TT
CLASS="FILENAME"
>/etc/fstab</TT
>:
<PRE
CLASS="SCREEN"
>/dev/fd0 /dosfloppy msdos user,noauto 0 0
/dev/fd0 /ext2floppy ext2 user,noauto 0 0</PRE
>
For MS-DOS filesystems (not just floppies), you probably want
to restrict access to it by using the <SPAN
CLASS="OPTION"
>uid</SPAN
>,
<SPAN
CLASS="OPTION"
>gid</SPAN
>, and <SPAN
CLASS="OPTION"
>umask</SPAN
> filesystem
options, described in detail on the <B
CLASS="COMMAND"
>mount</B
>
manual page. If you aren't careful, mounting an MS-DOS filesystem
gives everyone at least read access to the files in it, which
is not a good idea.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN1278"
>Checking filesystem integrity with <B
CLASS="COMMAND"
>fsck</B
></A
></H2
><P
>Filesystems are complex creatures, and as such, they
tend to be somewhat error-prone. A filesystem's correctness and
validity can be checked using the <B
CLASS="COMMAND"
>fsck</B
> command.
It can be instructed to repair any minor problems it finds, and to
alert the user if there any unrepairable problems. Fortunately,
the code to implement filesystems is debugged quite effectively,
so there are seldom any problems at all, and they are usually
caused by power failures, failing hardware, or operator errors;
for example, by not shutting down the system properly.</P
><P
>Most systems are setup to run <B
CLASS="COMMAND"
>fsck</B
>
automatically at boot time, so that any errors are detected
(and hopefully corrected) before the system is used. Use of
a corrupted filesystem tends to make things worse: if the
data structures are messed up, using the filesystem will
probably mess them up even more, resulting in more data loss.
However, <B
CLASS="COMMAND"
>fsck</B
> can take a while to run on big
filesystems, and since errors almost never occur if the system
has been shut down properly, a couple of tricks are used to
avoid doing the checks in such cases. The first is that if
the file <TT
CLASS="FILENAME"
>/etc/fastboot</TT
> exists, no checks
are made. The second is that the ext2 filesystem has a special
marker in its superblock that tells whether the filesystem
was unmounted properly after the previous mount. This allows
<B
CLASS="COMMAND"
>e2fsck</B
> (the version of <B
CLASS="COMMAND"
>fsck</B
>
for the ext2 filesystem) to avoid checking the filesystem if
the flag indicates that the unmount was done (the assumption
being that a proper unmount indicates no problems). Whether the
<TT
CLASS="FILENAME"
>/etc/fastboot</TT
> trick works on your system
depends on your startup scripts, but the ext2 trick works
every time you use <B
CLASS="COMMAND"
>e2fsck</B
>. It has to be
explicitly bypassed with an option to <B
CLASS="COMMAND"
>e2fsck</B
>
to be avoided. (See the <B
CLASS="COMMAND"
>e2fsck</B
> man page for
details on how.)</P
><P
>The automatic checking only works for the
filesystems that are mounted automatically at boot time.
Use <B
CLASS="COMMAND"
>fsck</B
> manually to check other filesystems,
e.g., floppies.</P
><P
>If <B
CLASS="COMMAND"
>fsck</B
> finds unrepairable problems,
you need either in-depth knowlege of how filesystems work in
general, and the type of the corrupt filesystem in particular,
or good backups. The latter is easy (although sometimes tedious)
to arrange, the former can sometimes be arranged via a friend,
the Linux newsgroups and mailing lists, or some other source of
support, if you don't have the know-how yourself. I'd like to
tell you more about it, but my lack of education and experience
in this regard hinders me. The <B
CLASS="COMMAND"
>debugfs</B
>
program by Theodore T'so should be useful.</P
><P
><B
CLASS="COMMAND"
>fsck</B
> must only be run on unmounted
filesystems, never on mounted filesystems (with the exception of
the read-only root during startup). This is because it accesses
the raw disk, and can therefore modify the filesystem without the
operating system realizing it. There <I
CLASS="EMPHASIS"
>will</I
>
be trouble, if the operating system is confused.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN1301"
>Checking for disk errors with <B
CLASS="COMMAND"
>badblocks</B
></A
></H2
><P
>It can be a good idea to periodically check for bad blocks.
This is done with the <B
CLASS="COMMAND"
>badblocks</B
> command. It outputs
a list of the numbers of all bad blocks it can find. This list
can be fed to <B
CLASS="COMMAND"
>fsck</B
> to be recorded
in the filesystem data structures so that the operating system
won't try to use the bad blocks for storing data.
The following example will show how this could be done.
<PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>badblocks /dev/fd0H1440 1440 > bad-blocks</B
></TT
>
<TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>fsck -t ext2 -l bad-blocks /dev/fd0H1440</B
></TT
>
<TT
CLASS="COMPUTEROUTPUT"
>Parallelizing fsck version 0.5a (5-Apr-94)
e2fsck 0.5a, 5-Apr-94 for EXT2 FS 0.5, 94/03/10
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Check reference counts.
Pass 5: Checking group summary information.
/dev/fd0H1440: ***** FILE SYSTEM WAS MODIFIED *****
/dev/fd0H1440: 11/360 files, 63/1440 blocks</TT
>
<TT
CLASS="PROMPT"
>$</TT
></PRE
>
If badblocks reports a block that was already used,
<B
CLASS="COMMAND"
>e2fsck</B
> will try to move the block to another
place. If the block was really bad, not just marginal, the
contents of the file may be corrupted.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN1315"
>Fighting fragmentation</A
></H2
><P
>When a file is written to disk, it can't always be written
in consecutive blocks. A file that is not stored in
consecutive blocks is <I
CLASS="GLOSSTERM"
>fragmented</I
>. It takes longer
to read a fragmented file, since the disk's read-write head
will have to move more. It is desireable to avoid fragmentation,
although it is less of a problem in a system with a good buffer
cache with read-ahead.</P
><P
>The ext2 filesystem attempts to keep fragmentation at a
minimum, by keeping all blocks in a file close together, even if
they can't be stored in consecutive sectors. Ext2 effectively
always allocates the free block that is nearest to other blocks
in a file. For ext2, it is therefore seldom necessary to worry
about fragmentation. There is a program for defragmenting an
ext2 filesystem, see XXX (ext2-defrag) in the bibliography.</P
><P
>There are many MS-DOS defragmentation programs that
move blocks around in the filesystem to remove fragmentation.
For other filesystems, defragmentation must be done by backing
up the filesystem, re-creating it, and restoring the files
from backups. Backing up a filesystem before defragmening is
a good idea for all filesystems, since many things can go wrong
during the defragmentation.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN1321"
>Other tools for all filesystems</A
></H2
><P
>Some other tools are also useful for managing filesystems.
<B
CLASS="COMMAND"
>df</B
> shows the free disk space on one or more
filesystems; <B
CLASS="COMMAND"
>du</B
> shows how much disk space a
directory and all its files contain. These can be used to hunt
down disk space wasters.</P
><P
><B
CLASS="COMMAND"
>sync</B
> forces all unwritten blocks
in the buffer cache (see <A
HREF="x1551.html"
>the section called <I
>The buffer cache</I
> in Chapter 5</A
>) to
be written to disk. It is seldom necessary to do this by
hand; the daemon process <B
CLASS="COMMAND"
>update</B
> does
this automatically. It can be useful in catastrophies,
for example if <B
CLASS="COMMAND"
>update</B
> or its helper
process <B
CLASS="COMMAND"
>bdflush</B
> dies, or if you must
turn off power <I
CLASS="EMPHASIS"
>now</I
> and can't wait for
<B
CLASS="COMMAND"
>update</B
> to run.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN1334"
>Other tools for the ext2 filesystem</A
></H2
><P
>In addition to the filesystem creator (<B
CLASS="COMMAND"
>mke2fs</B
>) and
checker (<B
CLASS="COMMAND"
>e2fsck</B
>) accessible directly or via the
filesystem type independent front ends, the ext2
filesystem has some additional tools that can be useful.</P
><P
><B
CLASS="COMMAND"
>tune2fs</B
> adjusts filesystem parameters. Some of the
more interesting parameters are:
<P
></P
><UL
><LI
><P
> A maximal mount count. <B
CLASS="COMMAND"
>e2fsck</B
> enforces a check when
filesystem has been mounted too many times, even if
the clean flag is set. For a system that is used for
developing or testing the system, it might be a good
idea to reduce this limit.
</P
></LI
><LI
><P
> A maximal time between checks. <B
CLASS="COMMAND"
>e2fsck</B
> can also enforce
a maximal time between two checks, even if the clean
flag is set, and the filesystem hasn't been mounted very
often. This can be disabled, however.
</P
></LI
><LI
><P
> Number of blocks reserved for root. Ext2
reserves some blocks for root so that if the
filesystem fills up, it is still possible to do system
administration without having to delete anything. The
reserved amount is by default 5 percent, which on most disks
isn't enough to be wasteful. However, for floppies there
is no point in reserving any blocks.
</P
></LI
></UL
>
See the <B
CLASS="COMMAND"
>tune2fs</B
> manual page for more
information.</P
><P
><B
CLASS="COMMAND"
>dumpe2fs</B
> shows information about an ext2 filesystem, mostly
from the superblock. <A
HREF="x1029.html#DUMPE2FS-OUTPUT"
>Figure 4-5</A
> shows
a sample output. Some of the information in the output is
technical and requires understanding of how the filesystem
works (see appendix XXX ext2fspaper), but much of
it is readily understandable even for layadmins.</P
><DIV
CLASS="FIGURE"
><P
><B
><A
NAME="DUMPE2FS-OUTPUT"
>Figure 4-5. Sample output from <B
CLASS="COMMAND"
>dumpe2fs</B
></A
></B
></P
><P
CLASS="LITERALLAYOUT"
>dumpe2fs 0.5b, 11-Mar-95 for EXT2 FS 0.5a, 94/10/23<br>
Filesystem magic number: 0xEF53<br>
Filesystem state: clean<br>
Errors behavior: Continue<br>
Inode count: 360<br>
Block count: 1440<br>
Reserved block count: 72<br>
Free blocks: 1133<br>
Free inodes: 326<br>
First block: 1<br>
Block size: 1024<br>
Fragment size: 1024<br>
Blocks per group: 8192<br>
Fragments per group: 8192<br>
Inodes per group: 360<br>
Last mount time: Tue Aug 8 01:52:52 1995<br>
Last write time: Tue Aug 8 01:53:28 1995<br>
Mount count: 3<br>
Maximum mount count: 20<br>
Last checked: Tue Aug 8 01:06:31 1995<br>
Check interval: 0<br>
Reserved blocks uid: 0 (user root)<br>
Reserved blocks gid: 0 (group root)<br>
<br>
Group 0:<br>
Block bitmap at 3, Inode bitmap at 4, Inode table at 5<br>
1133 free blocks, 326 free inodes, 2 directories<br>
Free blocks: 307-1439<br>
Free inodes: 35-360</P
></DIV
><P
><B
CLASS="COMMAND"
>debugfs</B
> is a filesystem debugger.
It allows direct access to the filesystem data structures
stored on disk and can thus be used to repair a disk that is so
broken that <B
CLASS="COMMAND"
>fsck</B
> can't fix it automatically.
It has also been known to be used to recover deleted files.
However, <B
CLASS="COMMAND"
>debugfs</B
> very much requires that
you understand what you're doing; a failure to understand can
destroy all your data.</P
><P
><B
CLASS="COMMAND"
>dump</B
> and <B
CLASS="COMMAND"
>restore</B
> can be used to back up an
ext2 filesystem. They are ext2 specific versions of the
traditional UNIX backup tools. See <A
HREF="c2187.html"
>Chapter 10</A
>
for more information on backups.</P
></DIV
></DIV
><H3
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN1217"
HREF="x1029.html#AEN1217"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>For more
information, see the kernel source or the Kernel Hackers'
Guide.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN1232"
HREF="x1029.html#AEN1232"
>[2]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>It should of course be
<B
CLASS="COMMAND"
>unmount</B
>, but the n mysteriously disappeared in
the 70's, and hasn't been seen since. Please return it to Bell
Labs, NJ, if you find it.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN1252"
HREF="x1029.html#AEN1252"
>[3]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>It requires several seconds of hard
thinking on the users' behalf.</P
></TD
></TR
></TABLE
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