Creating a Virtual File System and Mounting it with a Loopback Device.

Creating a Virtual File System and Mounting it with a Loopback Device.

 STEP 1 (Construct a 10MB file)

          $ dd if=/dev/zero of=/tmp/disk-image count=20480

         By default dd uses block of 512 so the size will be 20480*512

    STEP 2 (Make an ext2 or ext3 file system) -- ext2 shown here.

          $ mke2fs -q

         or if you want ext3

          $ mkfs -t ext3 -q /tmp/disk-image

         yes, you can even use reiser, but you'll need to create a bigger
         disk image. Something like "dd if=/dev/zero of=/tmp/disk-image count=50480".

          $ mkfs -t reiserfs -q /tmp/disk-image

         Hit yes for confirmation.  It only asks this because it's a file


    STEP 3 (Create a directory "virtual-fs" and mount. This has to be done as root)

          $ mkdir /virtual-fs
          $ mount -o loop=/dev/loop0 /tmp/disk-image /virtual-fs

        SPECIAL NOTE: if you mount a second device you will have to increase the
                      loop count: loop=/dev/loop1, loop=/dev/loop2, ... loop=/dev/loopn

         Now it operates just like a disk. This virtual filesystem can be mounted
         when the system boots by adding the following to the "/etc/fstab" file. Then,
         to mount, just type "mount /virtual-fs".

                /tmp/disk-image /virtual-fs ext2               rw,loop=/dev/loop0 0 0

    STEP 4 (When done, umount it)

          $ umount /virtual-fs


    SPECIAL NOTE: If you are using Fedora core 2, in the /etc/fstab you can take
             advantage of acl properties for this mount. Note the acl next to the
             rw entry. This is shown here with ext3.

                /tmp/disk-image     /virtual-fs ext3    rw,acl,loop=/dev/loop1 0 0

             Also, if you are using Fedora core 2 and above, you can mount the file
             on a cryptoloop.

               $ dd if=/dev/urandom of=disk-aes count=20480


               $ modprobe loop
               $ modprobe cryptoloop
               $ modprobe aes

               $ losetup -e aes /dev/loop0 disk-aes
               $ mkfs -t ext2 /dev/loop0
               $ mount -o loop,encryption=aes disk-aes 


             If you do not have Fedora core 2, then, you can build the kernel from source
             with some of the following options (not complete, yet)
              reference:
    http://cvs.sourceforge.net/viewcvs.py/cpearls/cpearls/src/posted_on_sf/acl/ehd.pdf?rev=1.1&view=log

                     Cryptographic API Support (CONFIG_CRYPTO)
                     generic loop cryptographic (CONFIG_CRYPTOLOOP)
                     Cryptographic ciphers (CONFIG_CIPHERS)
                     Enable one or more  ciphers  (CONFIG CIPHER .*) such as AES.


    HELPFUL INFORMATION: It is possible to bind mount partitions, or associate the
                    mounted partition to a directory name.

                 # mount --bind  /virtual-fs      /home/mchirico/vfs

            Also, if you want to see what filesystems are currently mounted, "cat" the
            file "/etc/mtab"

                 $ cat /etc/mtab

 

 Example with reiserfs file system 

 Assume you have a reisers files system created from a disk file, which means you have done something like the following:

         # dd if=/dev/zero of=disk-rfs count=102400
         # losetup  /dev/loop4 ./disk-rfs
         # mkfs -t reiserfs /dev/loop4
         # mkdir /fs2
         # mount -o loop,acl ./disk-rfs /fs2

    Now, you can run reiserfstune. But, first you will need to umount fs2

         # umount /fs2
         # reiserfstune ./disk-rfs

    Or you can run the debug command

         # debugreiserfs -J ./disk-rfs

    Now, suppose you run through a lot of the debug options on
    http://www.namesys.com/ and you destroy this file.

    You can recreate the file and delete the loop device.

         # dd if=/dev/zero of=disk-rfs count=102400
         # losetup -d /dev/loop4
         # mount -o loop,acl ./disk-rfs /fs2

    Now, try working with some of the ACL options - you can only do this
    with the latest kernel and tools -- Fedora Core 2 will work.

    Assume you have 3 users, donkey, chirico and bozo2. You can give
    everyone rights to this file system as follows:

         # setfacl -R -m d:u:donkey:rwx,d:u:chirico:rwx,d:u:bozo2:rwx /fs2