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<TITLE>Developer.com - Online Reference Library - 0672311739:RED HAT LINUX 2ND EDITION:Filesystems, Disks, and Other Devices</TITLE>
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<!-- TITLE=RED HAT LINUX 2ND EDITION //-->
<!-- AUTHOR=DAVID PITTS ET AL //-->
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<!-- PUBLICATION DATE=1998 //-->
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<A NAME="PAGENUM-204"><P>Page 204</P></A>
<P>The last two filesystems are special; the first
(/proc) is a special filesystem provided by the
kernel as a way of providing information about the system to user programs. The information
in the /proc filesystem is used in order to make utilities such as
ps, top, xload, free, netstat, and so on work. Some of the "files" in
/proc are really enormous (for example,
/proc/kcore) but don't worry—all the information in the
/proc filesystem is generated on-the-fly by the
Linux kernel as you read it; no disk space is wasted. You can tell that they are not real files because,
for example, root can't give them away with chown.
</P>
<P>The final "filesystem" isn't, in fact, a filesystem at all; it is an entry that indicates a disk
partition used as swap space. Swap partitions are used to implement virtual memory. Files can
also be used for swap space. The names of the swap files go in the first column where the
device name usually goes.
</P>
<P>The two numeric columns on the right relate to the operation of the
dump and fsck commands, respectively. The dump command compares the number in column five (the
dump interval) with the number of days since that filesystem was last backed up so that it can inform the
system administrator that the filesystem needs to be backed up. Other backup software—for
example, Amanda—can also use this field for the same purpose. (Amanda can be found at the URL
<a href="http://www.cs.umd.edu/projects/amanda/amanda.html">http://www.cs.umd.edu/projects/amanda/amanda.html</A>
.) Filesystems without a dump interval field are assumed to have a dump interval of zero, denoting "never dump." For more
information, see the manual page for dump.
</P>
<P>The sixth column is the fsck pass and indicates which filesystems can be checked in parallel
at boot time. The root filesystem is always checked first, but after that, separate drives can
be checked simultaneously, Linux being a multitasking operating system. There is no point,
however, in checking two filesystems on the same hard drive at the same time, because this
would result in lots of extra disk head movement and wasted time. All the filesystems that have
the same pass number are checked in parallel, from 1 upward. Filesystems with a 0 or missing
pass number (such as the floppy and CD-ROM drives) are not checked at
all.
</P>
<H3><A NAME="ch11_ 9">
Creating New Filesystems
</A></H3>
<P>When you install Red Hat Linux, the installation process makes some new filesystems and
sets the system up to use them. When you later come to set up new filesystems under Linux,
you will be coming to it for the first time.
</P>
<P>Many operating systems don't distinguish between the preparation of the device's surface
to receive data (formatting) and the building of new filesystems. Linux does distinguish
between the two, principally because only floppy disks need formatting in any case, and also
because Linux offers as many as half a dozen different filesystems that can be created (on any
block device). Separately providing the facility of formatting floppy disks in each of these
programs would be poor design and would require you to learn a different way of doing it for each
kind of new filesystem. The process of formatting floppy disks is dealt with separately (see the
section "Floppy Disks," later in this chapter).
</P>
<A NAME="PAGENUM-205"><P>Page 205</P></A>
<P>Filesystems are initially built by a program that opens the block device and writes some
structural data to it so that, when the kernel tries to mount the filesystem, the device contains
the image of a pristine filesystem. This means that both the kernel and the program used to
make the filesystem must agree on the correct filesystem structure.
</P>
<P>Linux provides a generic command, mkfs, that enables you to make a filesystem on a
block device. In fact, because UNIX manages almost all resources with the same set of
operations, mkfs can be used to generate a filesystem inside an ordinary file! Because this is unusual,
mkfs asks for confirmation before proceeding. When this is done, you can even mount the
resulting filesystem using the loop device (see the section "Mounting Filesystems on Files," later in
this chapter).
</P>
<P>Because of the tremendous variety of filesystems available, almost all the work of building
the new filesystem is delegated to a separate program for each; however, the generic
mkfs program provides a single interface for invoking them all. It's not uncommon to pass options to the
top-level mkfs (for example, -V to make it show what commands it executes or
-c to make it check the device for bad blocks). The generic
mkfs program also enables you to pass options to
the filesystem-specific mkfs. There are many of these filesystem-dependent options, but most
of them have sensible defaults, and you normally would not want to change them. The only
options you might want to pass to mke2fs, which builds
ext2 filesystems, are -m and -i. The -m option specifies how much of the filesystem is reserved for root's use (for example, for
working space when the system disk would otherwise have filled completely). The
-i option is more rarely exercised and is used for setting the balance between inodes and disk blocks; it is
related to the expected average file size. As stated previously, the defaults are reasonable for most
purposes, so these options are used only in special circumstances:
</P>
<!-- CODE //-->
<PRE>
# mkfs -t ext2 /dev/fd1
mke2fs 1.10, 24-Apr-97 for EXT2 FS 0.5b, 95/08/09
Linux ext2 filesystem format
Filesystem label=
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
# mount -t ext2 /dev/fd1 /mnt/floppy
# ls -la /mnt/floppy
total 14
drwxr-xr-x 3 root root 1024 Aug 1 19:49 .
drwxr-xr-x 7 root root 1024 Jul 3 21:47 ..
drwxr-xr-x 2 root root 12288 Aug 1 19:49 lost+found
# umount /mnt/floppy
</PRE>
<!-- END CODE //-->
<A NAME="PAGENUM-206"><P>Page 206</P></A>
<P>Here, you see the creation and mounting of an
ext2 filesystem on a floppy. The structure
of the filesystem as specified by the program's defaults are shown. There is no volume
label, and there are 4096 bytes (4KB) per inode (360
¥ 4 = 1440). The block size is 1KB and 5
percent of the disk is reserved for root. These are the defaults (which are explained in the
mke2fs manual page). After you have created a filesystem, you can use
dumpe2fs to display information about an ext2 filesystem, but remember to pipe the result through a pager such as
less because this output can be very long.
</P>
<P>After creating the filesystem on this floppy, you can include it in the filesystem table by
changing the existing line referring to a vfat filesystem on
/dev/fd1 to the following:
</P>
<!-- CODE SNIP //-->
<PRE>
/dev/fd1 /mnt/floppy ext2 user,sync,errors=continue 0 0
</PRE>
<!-- END CODE SNIP //-->
<P>The first three columns are the device, mount point, and filesystem type, as shown
previously. The options column is more complex than previous ones. The
user option indicates that users are allowed to mount this filesystem. The
sync option indicates that programs writing to
this filesystem wait while each write finishes, and only then continue. This might seem
obvious, but it is not the normal state of affairs. The kernel normally manages filesystem writes in
such a way as to provide high performance (data still gets written to the device of course, but it
doesn't necessarily happen immediately). This is perfect for fixed devices such as hard disks, but
for low-capacity removable devices such as floppy disks it's less beneficial. Normally, you write
a few files to a floppy and then unmount it and take it away. The unmount operation must
wait until all data has been written to the device before it can finish (and the disk can then be
removed). Having to wait like this is off-putting, and there is always the risk that someone
might copy a file to the floppy, wait for the disk light to go out, and remove it. With
asynchronous writes, some buffered data might not have yet been written to disk. Hence, synchronous
writes are safer for removable media.
</P>
<P>
The ext2 filesystem has a configurable strategy for errors. If an
ext2 filesystem encounters an error (for example, a bad disk block) there are three possible responses to the error:
</P>
<BLOCKQUOTE>
Remount the device read-only—For filesystems that contain mostly nonessential
data (for example, /tmp, /var/tmp, or news spools), remounting the filesystem read-only
so that it can be fixed with fsck is often the best choice.
<BR><BR>
Panic—Continuing regardless in the face of potentially corrupted system
configuration files is unwise, so a kernel panic (that is, a controlled crash—or
emergency landing, if you prefer) can sometimes be appropriate.
<BR><BR>
Ignore it—Causing a system shutdown if a floppy disk has a bad sector is a
little excessive, so the continue option tells the kernel to "carry on regardless" in
this situation. If this actually does happen, the best thing to do is to use the
-c option of e2fsck, for example, with fsck -t ext2 -c
/dev/fd1. This runs e2fsck, giving it the
-c option, which invokes the command badblocks to test the device for bad
disk blocks. After this is done, e2fsck does its best to recover from the
situation.
</BLOCKQUOTE>
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