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Tux

Introduction

The cowloop driver allows block-devices or files to be used in a read-write fashion without modifying the block-device or file itself. Modified data-blocks are not written back to the original read-only device, instead they are written to a separate file called the copy-on-write file or cowfile.
The cowloop driver offers (default) 16 cowdevices. To every cowdevice a read-only device (file) can be assigned with a corresponding cowfile to store the modifications for that read-only device.

The cowloop driver offers various possibilities, such as:

  • A filesystem stored on a non-writable medium (on a write-protected floppy, in a file on a cdrom/dvd) can still be mounted read-write via a cowdevice.

  • One filesystem can be used as a basis for various independent sessions that can be active in parallel.
    For each session a separate cowdevice is prepared. Every cowdevice could use the same read-only device (or file) holding the filesystem, while every cowdevice uses a dedicated cowfile holding the specific modifications for that session. Each cowdevice is mounted separately via the cowloop driver and unmounted again after use.

  • A database stored in a file or on a block-device can be used read-write without modifying the original contents.

  • ......
The cowloop driver can be used for any kind of filesystem-type (even non-filesystem data can be accessed in a read-write fashion). Obviously the filesystem-module that lies on top of the cowloop driver should allow write-accesses to that particular type of filesystem.

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