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Re: [linux-lvm] LVM snapshot - "dd" file size



A compressed dd can still be quite large, as there still might contain non zero values..

if you compress the image, a possible way to make the resulting files smaller is to:

- do a: 'dd if=/dev/zero of=/dummyfile' on the filesystem first (you might want to use 'bs' and 'count' parameters)
- if you topped off the disk, delete the all zero dummyfile.

The zipped result might be smaller. (both with dd and with tar..)

Regards,
Denie

----- Original Message ----- From: <dave frop net>
To: "LVM general discussion and development" <linux-lvm redhat com>
Sent: Wednesday, April 16, 2008 11:04 PM
Subject: Re: [linux-lvm] LVM snapshot - "dd" file size


this has nothing to do with LVM.

dd knows nothing about files, it just copies the entire device (or file or whatever).

tar will work, if you use the right command line switches. Search around on the web for pointers on how to use it.

Another intermediate way would be to compress the image you get with dd.

On Wed, Apr 16, 2008 at 04:59:05PM +0200, Thomas Bellembois wrote:
Hi!

I am using LVM partitions on a Debian Xen system.
I use LVM snapshots to clone and backup my virtual machines.

I have noticed that the ISO file created with the "dd" command it much
bigger that the partition used space (actually 4.7GB for the ISO - 500MB
used space). No problem if I mount the LVM snapshot and "tar" all of the
data.

I have googled the question and read that "dd" also copy "not used space".
Why is the ISO file so big ? Is the "tar" method less safe ?
Is there a better solution ?

Thanks for your answers.

Regards,

Thomas



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