add directory study
This commit is contained in:
gohigh
@@ -0,0 +1,313 @@
|
||||
<HTML>
|
||||
<HEAD>
|
||||
<TITLE>The Linux Bootdisk HOWTO: Advanced Bootdisk Creation</TITLE>
|
||||
</HEAD>
|
||||
<BODY bgcolor="#ffffff">
|
||||
<A HREF="Bootdisk-HOWTO-3.html"><IMG SRC="prev.gif" ALT="Previous"></A>
|
||||
<A HREF="Bootdisk-HOWTO-5.html"><IMG SRC="next.gif" ALT="Next"></A>
|
||||
<A HREF="Bootdisk-HOWTO.html#toc4"><IMG SRC="toc.gif" ALT="Contents"></A>
|
||||
<HR>
|
||||
<H2><A NAME="s4">4. Advanced Bootdisk Creation</A></H2>
|
||||
|
||||
|
||||
<H2><A NAME="ss4.1">4.1 Overview</A></H2>
|
||||
|
||||
<P>Previous Sections of this document covered the basics of creating
|
||||
boot/root disks, and are applicable to nearly all kernels up to the
|
||||
present (2.0, the latest stable kernel).</P>
|
||||
<P>Kernels 1.3.48+ involved a substantial rewrite of the ramdisk code,
|
||||
adding significant new capabilities. These kernels could automatically
|
||||
detect compressed filesystems, uncompress them and load them into the
|
||||
ramdisk on boot-up. Root filesystems could be placed on a second disk,
|
||||
and as of kernel 1.3.98 or so, ramdisks are dynamically expandable.</P>
|
||||
<P>Altogether, these new capabilities mean that boot disks can contain
|
||||
substantially more than they used to. With compression, a 1722K disk may
|
||||
now hold up to about 3.5 megs of files. As anyone who has created
|
||||
bootdisks knows, much time is spent pruning down the file set and finding
|
||||
trimmed-down versions of files that will all fit in a small filesystem.
|
||||
With the new capabilities this is no longer such a concern.</P>
|
||||
<P>Unfortunately, creating bootdisks to exploit these new features is
|
||||
slightly more difficult now. To build a compressed filesystem on a
|
||||
floppy, the filesystem has to be built on another device and then
|
||||
compressed and transferred to the floppy. This means a few more steps.</P>
|
||||
<P>The basic strategy is to create a compressed root filesystem, copy the
|
||||
kernel to the floppy disk, tell the kernel where to find the root
|
||||
filesystem, then copy the compressed root filesystem to the floppy.</P>
|
||||
<P>Here's a simple ASCII drawing of what the disk will look like:
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
|<--- zImage --->|<------ Compressed root filesystem -------->|
|
||||
|________________|____________________________________________|
|
||||
Floppy disk space
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
</P>
|
||||
<P>Here are the steps to create the boot floppy:</P>
|
||||
|
||||
|
||||
<H2><A NAME="ss4.2">4.2 Creating a root filesystem</A></H2>
|
||||
|
||||
<P>The root filesystem is created pretty much the same way as outlined in
|
||||
Section 2.3 of this document. The primary difference is that you can no
|
||||
longer create a root filesystem directly on a floppy -- you must create
|
||||
it on a separate device larger than the floppy area it will occupy.</P>
|
||||
|
||||
<H3>Choosing a device</H3>
|
||||
|
||||
<P>In order to build such a root filesystem, you need a spare device that is
|
||||
large enough. There are several choices:
|
||||
<UL>
|
||||
<LI>If you have an unused hard disk partition that is large enough
|
||||
(several megabytes), this is the easiest solution.
|
||||
Alternatively, if you have enough physical RAM you can simply
|
||||
turn off swapping and build the filesystem in your swap
|
||||
partition.
|
||||
|
||||
However, most people don't have a spare partition and can't afford
|
||||
to turn swapping off, so...</LI>
|
||||
<LI>Use a loopback device. A loopback device allows a disk file on
|
||||
an existing filesystem to be treated as a device. In order to
|
||||
use loopback devices you need specially modified mount
|
||||
and unmount programs. You can find these at:
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
ftp://ftp.win.tue.nl:/pub/linux/util/mount-2.5X.tar.gz
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
|
||||
where X is the latest modification letter.
|
||||
|
||||
One you've installed these special mount/umount binaries,
|
||||
create a temporary file on a hard disk with enough capacity (eg,
|
||||
/tmp/fsfile).
|
||||
|
||||
Use this file name in place of DEVICE below.
|
||||
When you issue a mount command you must include the
|
||||
option "-o loop" to tell mount to use a loopback device.
|
||||
For example:
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
mount -o loop -t ext2 /tmp/fsfile /mnt
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
|
||||
|
||||
will mount /tmp/fsfile (via a loopback device) at the mount point
|
||||
/mnt. A 'df' will confirm this.</LI>
|
||||
<LI>A final option is to use the ramdisk (DEVICE = /dev/ram or
|
||||
/dev/ramdisk). In this case, RAM is used to simulate a disk
|
||||
drive. The ramdisk must be large enough to hold a
|
||||
filesystem of the appropriate size. Check your Lilo configuration file
|
||||
(/etc/lilo.conf) for a line like:
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
RAMDISK_SIZE = nnn
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
|
||||
which determines how much RAM will be allocated.</LI>
|
||||
</UL>
|
||||
</P>
|
||||
<P>After you've chosen one of these options, prepare the device with:
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
dd if=/dev/zero of=DEVICE bs=1k 4096
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
</P>
|
||||
<P>This command zeroes out the device. This step is important because the
|
||||
filesystem on the device will be compressed later, so all unused
|
||||
portions should be filled with zeroes to achieve maximum compression.</P>
|
||||
<P>Next, create the filesystem with:
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
mke2fs /dev/hdb5 -m 0 DEVICE
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
</P>
|
||||
<P>(If you're using a loopback device, the disk file you're using should be
|
||||
supplied in place of this DEVICE. In this case, mke2fs will ask if you
|
||||
really want to do this; say yes.)</P>
|
||||
<P>Then mount the device:
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
mount -t ext2 DEVICE /mnt
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
</P>
|
||||
<P>Proceed as before, copying files into /mnt, as specified in Section 2.3.</P>
|
||||
|
||||
<H3>Compressing the filesystem</H3>
|
||||
|
||||
<P>After you're done copying files into the root filesystem, you need to
|
||||
copy it back out and compress it. First, umount it:
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
umount /mnt
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
</P>
|
||||
<P>(Technically you can copy the filesystem without unmounting it first,
|
||||
but that's somewhat dangerous, and bad practice.)</P>
|
||||
<P>Next, copy data off the device to a disk file. Call the disk file
|
||||
<CODE>rootfs</CODE>:
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
dd if=DEVICE of=rootfs
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
</P>
|
||||
<P>Then compress it. Use the '-9' option of gzip for maximal compression:
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
gzip -9 rootfs
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
</P>
|
||||
<P>This may take several minutes. When it finishes, you'll have a file
|
||||
rootfs.gz that is your compressed root filesystem.</P>
|
||||
<P>If you're tight on disk space you can combine dd and gzip:
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
dd if=DEVICE bs=1k | gzip -9 > rootfs.gz
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
</P>
|
||||
|
||||
|
||||
<H2><A NAME="ss4.3">4.3 Calculating the space</A></H2>
|
||||
|
||||
<P>At this point, check the space to make sure both the kernel and the root
|
||||
filesystem will fit on the floppy. An "ls -l" will show how many bytes
|
||||
each occupies; divide by 1024 to determine how many blocks each will
|
||||
need. For partial blocks, be sure to round up to the next block.</P>
|
||||
<P>For example, if the kernel size is 453281 bytes, it will need
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
ceil(453281 / 1024) = 443
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
|
||||
blocks, so it will occupy blocks 0-442 on the floppy disk. The
|
||||
compressed root filesystem will begin at block 443. Remember this
|
||||
block number for the commands to follow; call it ROOTBEGIN.</P>
|
||||
<P>You must tell the kernel where on the floppy to find the root
|
||||
filesystem. Inside the kernel image is a ramdisk word that specifies
|
||||
where the root filesystem is to be found, along with other options. The
|
||||
word is defined in /usr/src/linux/arch/i386/kernel.setup.c and is
|
||||
interpreted as follows:
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
bits 0-10: Offset to start of ramdisk, in 1024 byte blocks
|
||||
(This is ROOTBEGIN, calculated above)
|
||||
bits 11-13: unused
|
||||
bit 14: Flag indicating that ramdisk is to be loaded
|
||||
bit 15: Flag indicating to prompt for floppy
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
</P>
|
||||
<P>(If bit 15 is set, on boot-up you will be prompted to place a new floppy
|
||||
disk in the drive. This is necessary for a two-disk boot set, discussed
|
||||
below in Section 4.5. For now, this will be zero.)</P>
|
||||
<P>If the root filesystem is to begin at block 443, the ramdisk word is
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
1BB (hex) 443 (decimal) (bits 0-10)
|
||||
+ 4000 (hex) Ramdisk load flag (bit 14)
|
||||
----------
|
||||
= 41BB (hex)
|
||||
=16827 (decimal)
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
</P>
|
||||
<P>This ramdisk word will be set in the kernel image using the "rdev -r"
|
||||
command in the next section.</P>
|
||||
|
||||
|
||||
<H2><A NAME="ss4.4">4.4 Copying files to the floppy</A></H2>
|
||||
|
||||
|
||||
<P>At this point you're ready to create the boot floppy. First copy the
|
||||
kernel:
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
dd if=zImage of=/dev/fd0
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
</P>
|
||||
<P>Next, tell the kernel to find its root filesystem on the floppy:
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
rdev /dev/fd0 /dev/fd0
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
</P>
|
||||
<P>Next, you have to set the ramdisk word in the kernel image now residing
|
||||
on the floppy. The ramdisk word is set using the "rdev -r" command.
|
||||
Using the figure calculated in 4.3:
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
rdev -r zImage 16827
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
</P>
|
||||
<P>Finally, place the root filesystem on the floppy after the kernel. The
|
||||
<CODE>dd</CODE> command has a <CODE>seek</CODE> option that allows you to specify how many
|
||||
blocks to skip:
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
dd if=rootfs.gz of=/dev/fd0 bs=1k seek=443
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
</P>
|
||||
<P>(The value 443 is ROOTBEGIN from Section 4.3.)</P>
|
||||
<P>Wait for the floppy drive to finish writing, and you're done.</P>
|
||||
|
||||
|
||||
<H2><A NAME="ss4.5">4.5 Making a two-disk set.</A></H2>
|
||||
|
||||
<P>If you want more space, you can make a two-disk boot set. In this case,
|
||||
the first floppy disk will contain the kernel alone, and the second will
|
||||
contain the compressed root filesystem. With this configuration you can use a
|
||||
compressed filesystem of up to 1440K.</P>
|
||||
<P>A two-disk set is created using a simple variation of the instructions
|
||||
above. First, you must set the ramdisk PROMPT flag to 1 to instruct the
|
||||
kernel to prompt and wait for the second floppy. The root filesystem
|
||||
will begin at byte 0 of the second floppy. </P>
|
||||
<P>From section 4.3, the ramdisk PROMPT flag (bit 15) will be set to 1, and
|
||||
the ramdisk offset will be zero. In our example the new calculation
|
||||
would be:
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
4000 (hex) Ramdisk load flag (bit 14)
|
||||
+ 8000 (hex) Ramdisk prompt flag (bit 15)
|
||||
------------
|
||||
= C000 (hex)
|
||||
=49152 (decimal)
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
</P>
|
||||
<P>which would be used in the 'rdev -r' calculation as before.</P>
|
||||
<P>Follow the instructions of 4.4, above, but after issuing the 'rdev -r'
|
||||
command, put a new floppy in the drive and issue the command:
|
||||
<BLOCKQUOTE><CODE>
|
||||
<PRE>
|
||||
dd if=rootfs.gz of=/dev/fd0
|
||||
</PRE>
|
||||
</CODE></BLOCKQUOTE>
|
||||
</P>
|
||||
<P>The <CODE>seek</CODE> option is not needed since the root filesystem starts at block
|
||||
zero.</P>
|
||||
|
||||
|
||||
<HR>
|
||||
<A HREF="Bootdisk-HOWTO-3.html"><IMG SRC="prev.gif" ALT="Previous"></A>
|
||||
<A HREF="Bootdisk-HOWTO-5.html"><IMG SRC="next.gif" ALT="Next"></A>
|
||||
<A HREF="Bootdisk-HOWTO.html#toc4"><IMG SRC="toc.gif" ALT="Contents"></A>
|
||||
<hr>
|
||||
Space for these documents provided by
|
||||
<A href="http://www.spade.com/">Ben Spade</A>
|
||||
in support of the Linux community.
|
||||
|
||||
</BODY>
|
||||
</HTML>
|
||||
Reference in New Issue
Block a user