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Loading and unloading cowloop

The command

  modprobe cowloop [maxcows=...] [rdofile=... cowfile=... [option=r]]
takes care that the cowloop driver is loaded into the kernel.

The parameter maxcows= can be used to specify how many cowdevices should be supported in parallel. The default value is 16, accessible via the cowdevices /dev/cow/0 till /dev/cow/15.
The current maximum value is 254. Version 3.3. will increase the maximum to 1022. (more info on how to modify this maximum is in the file HOWTO.install).

It is possible to activate the first cowdevice /dev/cow/0 already while loading the cowloop driver:

  • rdofile=
    The pathname of the read-only file or filesystem. The read-only file(system) may be of type regular or block-device.
  • cowfile=
    The pathname of the copy-on-write file used as storage-space for modified blocks of the read-only file(system). The copy-on-write file is always a regular file.

    If the given copy-on-write file does not yet exist, a new file with this name will be created by the cowloop driver.

    If an existing regular file is specified as cowfile, its contents will be used again for the current read-only file (the cowfile is supposed to contain modified blocks from an earlier session for the same read-only file).

  • option=
    Notice that an existing cowfile must be in a consistent state. This means that the previous session should have been finished properly by deactivating the cowdevice via the command cowdev or by unloading the cowloop driver (command rmmod).
    When the cowfile has not been properly closed, the cowloop driver refuses to assign it again unless the parameter option=r is given with the command modprobe. This option forces that the cowfile is repaired automatically if it appears to be inconsistent. In that case the command modprobe might take some time!

    Notice that this option should only be used for special purposes. Inconsistent cowfiles should preferably be repaired with the command cowrepair before activating the cowdevice.

When failures occur during driver-loading, more information about the reason of the failure can be found in the /var/log/messages syslog file.


The command

    rmmod cowloop
takes care that the cowloop driver is unloaded again (all active cowdevices will be deactivated automatically).




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