Linux : Virtual filesystem – Loopback device

June 25, 2008

Sometimes you may need additional local partition or local mountable drives, but you dont have free blocks in the parition table. In that case virtual filesystem helps. You can create a virtual filesystem and mount them as a loopback device. Here are the steps to do it.

Create a empty file with the mount of disk space you need. Here I have created a 1G file.

[root@unixfoo23 ~]# dd if=/dev/zero of=/root/myfs1 bs=1024 count=1048576
1048576+0 records in
1048576+0 records out

[root@unixfoo23 ~]# ls -l /root/myfs1
-rw-r–r– 1 root root 1073741824 Jun 25 08:32 /root/myfs1

[root@unixfoo23 ~]# du -sh /root/myfs1
1.1G /root/myfs1
[root@unixfoo23 ~]#
Create a filesystem on the virtual device (/root/myfs1). I have selected ext3 , whereas you can create the filesystem of your choice.

[root@unixfoo23 ~]# mkfs.ext3 /root/myfs1
mke2fs 1.35 (28-Feb-2004)
/root/myfs1 is not a block special device.
Proceed anyway? (y,n) y
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
131072 inodes, 262144 blocks
13107 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=268435456
8 block groups
32768 blocks per group, 32768 fragments per group
16384 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376

Writing inode tables: done
Creating journal (8192 blocks): done
Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 28 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.

[root@unixfoo23 ~]# file /root/myfs1
/root/myfs1: Linux rev 1.0 ext3 filesystem data (large files)
[root@unixfoo23 ~]#
Mount the filesystem as a loopback device.[root@unixfoo23 ~]# mount -o loop /root/myfs1 /mnt
[root@unixfoo23 ~]# df /mnt
Filesystem 1K-blocks Used Available Use% Mounted on
/root/myfs1 1032088 34092 945568 4% /mnt
[root@unixfoo23 ~]#
If you need this permanantly , you can add this to /etc/fstab.