1T file in 188G file system !

mah454 · 03-16-2011, 05:49 AM

Hello ...
I use this command for make image file :

Code:

#dd if=/dev/zero of=disk.img seek=2G count=1

and use this command for check file size :

Code:

#ls -l
-rw-r--r-- 1 root   root   1099511627776 2011-03-16 14:14 disk.img
#ls -lh
-rw-r--r-- 1 root   root   1.0T 2011-03-16 14:15 disk.img

I make this file in /home partition :

Code:

#df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda5             188G  124G   55G  70% /home

Please compare file and filesystem size !
What is this mean ?

michaelk · 03-16-2011, 07:21 AM

seek skips n blocks (using the specified output block size) from beginning of output file before copying. Since you did not specify the block size vie the bs option the default is 512. So:
file size = 2G * BS
1099511627776=2*1024*1024*1024*512

The correct syntax to create a 2G file is:
dd if=/dev/zero of=disk.img bs=2G count=1

I will let the experts chime in on what is actually happening but my logical guess is the actual disk space is only 512 i.e default block size. Assuming the command completed successfully and obviously if a 1TB file was written you would see errors or worse yet file system corruption.

Guttorm · 03-16-2011, 08:38 AM

Explanation here:

http://en.wikipedia.org/wiki/Sparse_file

michaelk · 03-16-2011, 09:04 AM

Of course... Thanks...

mah454 · 03-17-2011, 12:08 AM

Ok ...
Thank You