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Old 12-21-2007, 07:03 PM   #1
Registered: May 2006
Location: USA
Distribution: FreeBSD Ubuntu Debian
Posts: 137

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how to determine block size when using dd

How do I figure out what the maximum value of bs can be for a given hard drive when using dd to mirror hard drives?

I know there was some math you could plug into python using the output of fdisk to figure out how high a limit you could put on bs= when running dd to speed up a disk to disk copy. It had something to do with not making it so high that the last block didn't hang off the end of the disk.

I can't find it in any of my googling and I don't want to wait for this thing to finish at 512 block sizes...
Old 12-21-2007, 07:37 PM   #2
Registered: May 2006
Location: USA
Distribution: FreeBSD Ubuntu Debian
Posts: 137

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Rep: Reputation: 15
I just searched for the last 30 minutes and can't find it. The last time I did this for some other backup 1M was safe, and you don't get much faster if you go any higher. I'm just gonna go with 1M and compare the sizes listed in fdisk and hopefully the two drives will match. Then I'll boot all the partitions to make absolutely sure.

Honestly it probably doesn't even really matter. I think the swap partition was last on the disk, which is where any discrepancies would show up anyway...
Old 12-22-2007, 07:16 PM   #3
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Registered: Oct 2004
Distribution: FreeBSD Arch
Posts: 1,871

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Good thread for dd.

Last edited by teckk; 12-22-2007 at 07:18 PM.


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