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Status information

As soon as a cowdevice is activated, status information can be read from the file /proc/cow/n (n is the number of the cowdevice /dev/cow/n):

  $ cat /proc/cow/5
     cowloop version:      2.14

     number of opens:         1
       pid of thread:      2533

      read-only file: /dev/sda2
            rdoreads:      3129

  copy-on-write file: /tmp/myfs1.cow
       state cowfile:     dirty
       bitmap-blocks:        32 (of 1024 bytes)
    cowblocks in use:       762 (of 1024 bytes)
            cowreads:      1007
           cowwrites:      1770
Most relevant values are:
  • cowloop version
    Version of the current cowloop driver.
  • number of opens
    Number of times that the cowdevice is currently open. The cowdevice can only be deactivated when this value is 0.
  • read-only file
    Name of the file specified as read-only file for this cowdevice.
  • rdoreads
    Number of read-requests issued on the read-only file since it has been activated.
  • copy-on-write file
    Name of the file specified as cowfile for this cowdevice.
  • state cowfile
    Current state of the cowfile. The state 'dirty' means that the bitmap in memory has been modified but not yet flushed to the cowfile. The bitmap is automatically flushed when the cowdevice is deactivated.
    With the command cowsync a flush of the bitmap can be forced without deactivating the cowdevices. After that the state 'clean' is shown until the driver modifies the in-memory bitmap for that cowdevice again.
  • bitmap blocks
    Number of blocks (1 Kbytes) used for the bitmap.
  • cowblocks in use
    Number of modified blocks (1 Kbytes) in use in the cowfile. This value can be used to determine the filling-degree of the sparse copy-on-write file.
  • cowreads
    Number of read-requests issued on the copy-on-write file since it has been activated.

  • cowwrites
    Number of write-requests issued on the copy-on-write file since it has been activated.

Note: from version 3.3. on the use of the /proc file system has been discontinued, and the debugfs has taken its place. The debugfs is usually mounted under /sys/kernel/debug. This change was required by general kernel driver policy.




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