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<A NAME="1685"> </A><H2><A NAME="SECTION00586000000000000000">Checking filesystem integrity with <tt>fsck</tt></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 <tt>fsck</tt><A NAME="1687"> </A> 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>
Most systems are setup to run <tt>fsck</tt><A NAME="1689"> </A> 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,
<tt>fsck</tt><A NAME="1691"> </A> 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>/etc/fastboot</tt><A NAME="1693"> </A>
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 <tt>e2fsck</tt><A NAME="1695"> </A> (the version of <tt>fsck</tt><A NAME="1697"> </A> 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>/etc/fastboot</tt><A NAME="1699"> </A> trick works on your system depends on
your startup scripts, but the ext2 trick works every time
you use <tt>e2fsck</tt><A NAME="1701"> </A>--it has to be explicitly bypassed with
an option to <tt>e2fsck</tt><A NAME="1703"> </A> to be avoided. (See the <em>e2fsck</em><A NAME="1705"> </A>
man page for details on how.)
<P>
The automatic checking only works for the filesystems that are
mounted automatically at boot time. Use <tt>fsck</tt><A NAME="1707"> </A> manually
to check other filesystems, e.g., floppies.
<P>
If <tt>fsck</tt><A NAME="1709"> </A> 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 <tt>debugfs</tt><A NAME="1711"> </A> program by Theodore T'so
should be useful.
<P>
<tt>fsck</tt><A NAME="1713"> </A> 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 <em>will</em> be trouble,
if the operating system is confused.
<P>
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<P><ADDRESS>
<I>Lars Wirzenius <BR>
Sat Nov 15 02:32:11 EET 1997</I>
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