[Sheepdog] [PATCH 0/5] modify the usage of snapshot and cloning

Chris Webb chris at arachsys.com
Sun Jan 10 22:45:58 CET 2010

MORITA Kazutaka <morita.kazutaka at lab.ntt.co.jp> writes:

> What I thought is that the apply command moves the position of HEAD.
> For example,
>   $ shepherd info -t vdi -f tree john
>   john---[tag_1]-+-[tag_2]---HEAD
>                  `-[tag_3]
>   $ shepherd apply john:tag_1 delta_file
>   (New image is created and it becomes HEAD. Previous HEAD becomes a snapshot.)
>   $ shepherd info -t vdi -f tree john
>   john---[tag_1]-+-[tag_2]---[tag_4]
>                  +-[tag_3]
>                  `-HEAD
> This behavior is the same as when writing data to the snapshot.

That's a nice behaviour too: effectively 'import a snapshot'. I like it!

> Using a standardized format may be better such as VCDIFF.

>From the metadata, all you know are which blocks are identical and which
blocks have been written to and are potentially different. You're going to
have to read the changed blocks to send them anyway, but wouldn't doing
VCDIFF require reading the whole image including unchanged data blocks as
well to do the pattern matching, so losing all the disk IO advantages?

Best wishes,


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