file as raw device

Sometimes there is a need to have a file act as raw device , here is a simple trick that
you can take in order to achieve that goal (all commands should run as root) :

1. create an empty file using dd command ,
with the required size ( that can be change later on )

~# dd if=/dev/zero of=1G.img bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 3.28652 s, 327 MB/s

2. create a file system on that file using mkfs -F

~# mkfs.ext4 -F 1G.img
mke2fs 1.42 (29-Nov-2011)
Discarding device blocks: done
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
65536 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
8192 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376Allocating group tables: done
Writing inode tables: done
Creating journal (8192 blocks): done
Writing superblocks and filesystem accounting information: done

3. mount the file as it was a device

~# mkdir mymount
~# mount 1G.img mymount/
~# mount |grep mymount
/tmp/1G.img on /tmp/mymount type ext4 (rw)

And that is it , no more to do . now lets say you want to extend this partition/file , you can do it
with 2 simple commands , but first you need to umount the file .

1. check the fs and clean it before resize

~# e2fsck -f 1G.img
e2fsck 1.42 (29-Nov-2011)
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
1G.img: 11/65536 files (0.0% non-contiguous), 12635/262144 blocks

2. resize the file

~# resize2fs 1G.img 2G
resize2fs 1.42 (29-Nov-2011)
Resizing the filesystem on 1G.img to 524288 (4k) blocks.
The filesystem on 1G.img is now 524288 blocks long.
~# ls -lh 1G.img -rw-r--r-- 1 root root 2.0G Jun 19 22:31 1G.img
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