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List:       evms-devel
Subject:    Re: [Evms-devel] read-only PVs, storeable snapshot-to-snapshot diffs
From:       Matt Zimmerman <mdz () debian ! org>
Date:       2002-05-13 14:20:33
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On Mon, May 13, 2002 at 05:21:05PM +1200, Mark van Walraven wrote:

> Eduard, have you seen rdiff?  You can use rdiff to do the following:
> generate a signature file for a snapshot; generate a delta file from a
> later snapshot and the signature; and reconstruct the earlier snapshot
> contents from the original and the delta.
> 
> Of course, rdiff must scan the entire earlier snapshot to calculate the
> signature, and also scan later snapshots to generate deltas.  It's a bit
> sad to scan for differences when the volume manager knows the changes
> since the snapshots were made (and uses that knowledge to provide the
> snapshot image).  I'd love to see a way of extracting those differences
> directly, from userspace.

But since rdiff operates at this higher level, it also has advantages.  For
example, if a large file is changed only in one place, but the entire file
is rewritten (e.g., by rsync), the volume manager would not realize that the
chunks are the same, only that they have been written to.  Whereas, rdiff
does a direct comparison of the files and notices which bits were changed.

I agree that it could be useful to extract a map of which chunks have been
modified; this could perhaps be used to optimize things like rdiff so that
they can skip comparisons of large sections of the filesystem.

-- 
 - mdz

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