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Old 02-25-2005, 05:32 PM   #1
slinky2004
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Registered: Oct 2004
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using dd to backup?


i'm trying to backup my windows NTFS partition using a knoppix disk, a secondary hard drive and the dd command. i've been using this command: "dd if=/dev/hda | gzip > /mnt/hdb1/hda1bak.img.gz", which i've seen on the wiki here, http://wiki.linuxquestions.org/wiki/Dd. i have a few questions/problems that i'm hoping you guys could help me with:

1. i've seen several examples and they seem to change the bs to 512, 1024, etc. without saying why. do i need to change the bs depending on the drive i'm grabbing? for example if my NTFS drive was formatted with 1024 cluster sizes, would i have to set the bs to 1024?

2. also, some descriptions have said the bs was the number of bytes read at a time and other say the number of blocks read at at time. and what is the bs measured in? bytes, kilobytes, megabytes?

3. why does dd take so long? i did a zero-fill on my 160 gig hard drive with this command "dd if=/dev/zero of=/dev/hda bs=1M" and it took eleven hours to finish. also, when i try to grab an image of my NTFS partition(only 40 gigs).

4. i tried to increase the bs size so that it might go faster. i incresed it from the default to 3M, so the command i ran was "dd if=/dev/hda1 bs=3M | gzip > /mnt/hdb1/hda1bak.img.gz". this took a very long time to finish running and when it finished it gave me an error about the filesize being too large.

will somebody help me plz?

Last edited by slinky2004; 02-25-2005 at 06:14 PM.
 
Old 02-26-2005, 02:52 PM   #2
dwight1
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Registered: Feb 2005
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dd's default block size is 512 bytes per block. Try this to verify it:
dd if=/dev/zero of=foo count=1

`ls -l foo` should show it's 512 bytes long

The reason why things are taking so long is probably because you're not using an optimal block size.

Increasing the block size doesn't necessarily make dd go faster. What you want to do is to have the block size correspond to one cylinder of the hard disk. This will minimize the number of seeks that each head on the hard disk has to perform.

You should be able to get the disk parameters (C/H/S) from the manufacturer. You can also try to get the disk parameters from hdparm, but these might be misleading.

To verify it, you can try the tiobench or other benchmarks.

For example:
/sbin/hdparm -g /dev/hda
gives:
/dev/hda:
geometry = 2584/240/63, sectors = 39070080, start = 0

If one belived that I really had 240 heads on this device, I'd do the following, for 1 cylinder at a time:

In order to do transfers 1 cylinder at a time, I would do the following
dd if=/dev/hda of=/dev/null bs=15120

15120 = 240 * 63

HTH
 
  


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