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Tux

Functional overview

For every cowdevice the combination of a read-only file and a cowfile should be assigned. This assignment can be made dynamically (while the cowloop module is loaded) with the command cowdev; this command can also be used to deactivate a cowdevice again.
For the first cowdevice an assigment can be made already during loading of the cowloop module by specifying one read-only file and one cowfile as parameters for the modprobe command.

Every combination of a read-only file and corresponding cowfile is made available as a new cowdevice (e.g. two independent cowdevices /dev/cow/0 and /dev/cow/1 in the figure below).

driver schematics

The cowloop driver accesses the read-only file as a sequence of 1 Kb blocks.

Concerning the example of cowdevice /dev/cow/0 in the figure above:
Initially every block read from this device is retrieved from the read-only file (e.g. block 3). When a block is written to the cowdevice, the cowloop driver writes the block to the copy-on-write file. On reading, the cowloop driver first examines the cowfile to see whether a modified version of the requested block is present. Hence modified blocks are retrieved from the cowfile (e.g. block 1), otherwise the original block is retrieved from the read-only file (e.g. block 2).

 

Suppose that the underlying read-only file contains a filesystem, it can be mounted read-write with the command

  mount /dev/cow/0 /mnt/fs0

All modifications on data-blocks and metadata-blocks of the filesystem are written to the copy-on-write file. After the filesystem has been unmounted and the cowdevice deactivated, the read-only file still contains the original (untouched) filesystem. However when it is used in combination with the cowfile via the cowloop driver, the modified filesystem appears again.

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