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Linux forum: How does partimage distinguish between free vs. empty blocks

by: wfamily_7 September 25, 2011 7:07 PM PDT

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How does partimage distinguish between free vs. empty blocks

by wfamily_7 - 9/25/11 7:07 PM

I'm trying to figure out exactly how partimage distinguishes
between a free block (containing old/garbage data) and an empty (zero)
block when it comes to disk imaging.

In other words, since "dd" copies all the blocks, what technique does
partimage use that enables it to only clone the used blocks?
Everywhere I've looked online says partimage has this ability, but HOW does it do it?

Any ideas?


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"Read the source Luke"

by R. Proffitt Moderator - 9/25/11 7:15 PM

In Reply to: How does partimage distinguish between free vs. empty blocks by wfamily_7

http://www.partimage.org/Download

I see source code.

More at http://www.openxource.com/crossings/2005/02/21/read_the_source_luke/brief
Bob


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Sorta Found Something

by wfamily_7 - 9/25/11 10:43 PM

In Reply to: "Read the source Luke" by R. Proffitt Moderator

I already did, but it's not exactly easy to decipher what it's doing.

Anyway, after some research on the source code I have a slight clue as to how it's doing this.

Partimage basically looks for the bits that are set ("1") and treats those as used bits. Everything else is treated as unused/"0"/empty. Look for "isBitSet" function in the source code.

I think that's how it works...still need more research to figure it out exactly. Anyway, hoped this helped some people.


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So far that's how most tests for "empty" work.

by R. Proffitt Moderator - 9/26/11 1:14 PM

In Reply to: Sorta Found Something by wfamily_7

And it's a good reason to go to the trouble of ZERO FILLING A DRIVE if you have the time when starting fresh.
Bob


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