Creating a filesystem inside a regular file in Linux

CREATING THE FILESYSTEM INSIDE A REGULAR FILE

Follow the next steps to create a regular file that contains a filesystem (also know as RAW DISC, IMG DISC or IMAGE DISC). 
This kind of filesystem inside a file is used in Virtualization like XEN or when you copy a harddisk from /dev/hdxx or /dev/sdxx to a regular file.

1) Create an empty file (filled with zeros) with 100Mbytes size: 

# dd if=/dev/zero of=fakedisk bs=1M count=100
Change the "count" value to the desired space.

2) Create the partitions with fdisk:

# fdisk fakedisk
Command (m for help): n     (create a new partition)
Command (m for help): p     (create a primary partition)
Command (m for help): 1     (create the first partition of the disk)
Command (m for help): enter (use default values)
Command (m for help): enter (use default values)
Command (m for help): t -> 83 (use 83 for linux filesystems like ext2,ext3,ext4)
Command (m for help): w     (write changes to disk)

3) Using fdisk, note the "Start" number of the partition, in this case is 2048.

# fdisk -l fakedisk

Disk fakedisk: 104 MB, 104857600 bytes
191 heads, 50 sectors/track, 21 cylinders, total 204800 sectors
Units = setores of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xb5cd89fb

Dispositivo Boot   Start     End      Blocks   Id  System
fakedisk1          2048      204799   101376   83  Linux

4) Mount the partition, using the noted number and multiply by 512.

# losetup /dev/loop0 fakedisk -o $((2048 * 512))

5) Format the partition with the filesystem

# mkfs.ext3 /dev/loop0

6) Verify if the partition is recognized as ext3:

# file -s /dev/loop0

/dev/loop0: sticky Linux rev 1.0 ext3 filesystem data, UUID=c52cc60c-9419-4906-95a7-c742937b7769

7) Mount the partition

# mkdir /mnt/partition1

# mount /dev/loop0 /mnt/partition1

Case loop0 is already used, you can use loop1, loop2, ...

Now you can use the /mnt/partition directory like any other mounted point.

RESIZING YOUR PARTITION INSIDE REGULAR FILE

Now if you want to increase your partition/filesystem inside a regular file, follow the steps:

1) Unmount the partition

# umount /mnt/partition1

2) Increase the regular file that contains the filesystem with zeroes at the end. 
In this example we'll increase the file that holds the filesystem to more 100MB.

# dd if=/dev/zero bs=1M count=100 >> fakedisk

3) Reload the loop device mapping to recognize the new free space: 

# losetup -d /dev/loop0

# losetup /dev/loop0 fakedisk -o $((2048 * 512))

4) Check the Filesystem Integrity

# fsck -f /dev/loop0

fsck de util-linux 2.20.1

e2fsck 1.41.12 (17-May-2010)

Pass 1: Checking inodes, blocks, and sizes

Pass 2: Checking directory structure

Pass 3: Checking directory connectivity

Pass 4: Checking reference counts

Pass 5: Checking group summary information

/dev/loop0: 11/25376 files (0.0% non-contiguous), 8874/101376 blocks

5) Resize the filesystem

# resize2fs /dev/loop0

resize2fs 1.41.12 (17-May-2010)

Resizing the filesystem on /dev/loop0 to 203776 (1k) blocks.

The filesystem on /dev/loop0 is now 203776 blocks long.

7) Mount the loop0 device:

# mount /dev/loop0 /mnt/partition1

8) Verify the partition with the new size:

# df -h
Sist. Arq.                Tam  Usad Dispon. Uso% Montado em
rootfs                     19G   13G    4.9G  73% /
/dev/loop0                193M  5.6M    178M   4% /mnt/partition1

Done.

Now you have the partition with more space.