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Example 1: filesystem on write-protected floppy
Obviously, you may read "USB stick" instead of "floppy" here....
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Mount a filesystem on a write-protected floppy read-only to see the list of files: # mount -r /dev/fd0 /mnt/floppy # ls -l /mnt/floppy -rw-r--r-- 1 root root 44 Jun 30 13:14 adjtime -rw-r--r-- 1 root root 1343 Jun 30 13:14 aliases -rw-r----- 1 root root 12288 Jun 30 13:14 aliases.db -rw------- 1 root root 1 Jun 30 13:14 at.deny -rw-r--r-- 1 root root 872 Jun 30 13:14 atsar.conf -rw-r--r-- 1 root root 212 Jun 30 13:14 auto.master # umount /dev/fd0The same filesystem can be mounted read-write via the cowloop driver (already loaded via the command modprobe),
specifying the name of a new cowfile:
# cowdev -a /dev/fd0 /tmp/ses1.cow /dev/cow/0 # mount /dev/cow/0 /mnt/floppy # ls -l /mnt/floppy -rw-r--r-- 1 root root 44 Jun 30 13:14 adjtime -rw-r--r-- 1 root root 1343 Jun 30 13:14 aliases -rw-r----- 1 root root 12288 Jun 30 13:14 aliases.db -rw------- 1 root root 1 Jun 30 13:14 at.deny -rw-r--r-- 1 root root 872 Jun 30 13:14 atsar.conf -rw-r--r-- 1 root root 212 Jun 30 13:14 auto.masterAfter entering the command mount the situation is
as shown in this figure:
# vi /mnt/floppy/adjtime # ls -l /mnt/floppy/adjtime -rw-r--r-- 1 root root 47 Jun 30 13:24 /mnt/floppy/adjtime # rm /mnt/floppy/aliases # umount /dev/cow/0 # cowdev -d /dev/cow/0The contents of the floppy itself is unmodified. The modified blocks of the filesystem (superblock, inodes, bitmaps, data-blocks) are written to the cowfile. Apparently the cowfile is almost half a Megabyte (according to command ls), but it only consumes
18 Kbytes of physical disk space (according to command du):
# ls -l /tmp/ses1.cow -rw------- 1 root root 475584 Jun 30 13:25 /tmp/ses1.cow # du -k /tmp/ses1.cow 18 /tmp/ses1.cowLater on the filesystem can be mounted read-write again via the cowloop driver, using the same cowfile: # cowdev -a /dev/fd0 /tmp/ses1.cow /dev/cow/3 # mount /dev/cow/3 /mnt/floppy # ls -l /mnt/floppy -rw-r--r-- 1 root root 47 Jun 30 13:24 adjtime -rw-r----- 1 root root 12288 Jun 30 13:14 aliases.db -rw------- 1 root root 1 Jun 30 13:14 at.deny -rw-r--r-- 1 root root 872 Jun 30 13:14 atsar.conf -rw-r--r-- 1 root root 212 Jun 30 13:14 auto.masterWhen the cowfile /tmp/ses1.cow is removed before
activating it again via the command "cowdev -a",
the cowloop driver creates a new empty cowfile.
In that case the original floppy-contents appears again after the mount.
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