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Tux

Example 3: mount multiple instances of one filesystem

The device /dev/sda2 can be formatted as a filesystem and populated with lots of data:

  # mkfs   /dev/sda2
  # mount  /dev/sda2 /mnt/myfs

  # cp .... lotsofdata .... /mnt/myfs

  # umount /dev/sda2
This filesystem can now be mounted several times via the cowloop driver to obtain multiple writable instances of the same filesystem:
  # modprobe cowloop
  # cowdev -a /dev/sda2 /fsdir/inst0.cow
  /dev/cow/0     # the command returns this name now
  # cowdev -a /dev/sda2 /fsdir/inst1.cow
  /dev/cow/1     # the command returns this name now
  # cowdev -a /dev/sda2 /fsdir/inst2.cow
  /dev/cow/2     # the command returns this name now
  # cowdev -l
  cowdevice     read-only file     copy-on-write file              
  ---------------------------------------------------
  /dev/cow/0    /dev/sda2          /fsdir/inst0.cow                
  /dev/cow/1    /dev/sda2          /fsdir/inst1.cow                
  /dev/cow/2    /dev/sda2          /fsdir/inst2.cow                

  # mount /dev/cow/0 /mnt/fs0
  # mount /dev/cow/1 /mnt/fs1
  # mount /dev/cow/2 /mnt/fs2
  # df
  Filesystem   1K-blocks      Used Available Use% Mounted on
  ....
  /dev/cow/0     2016044   1057832    855800  56% /mnt/fs0
  /dev/cow/1     2016044   1057832    855800  56% /mnt/fs1
  /dev/cow/2     2016044   1057832    855800  56% /mnt/fs2
Every instance can be modified independent of the other instances, while the underlying filesystem on device /dev/sda2 will not be modified at all.

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