snapshot send with parent question

snapshot send with parent question

[Michael Welsh Duggan]

Assume the following scenario:
There exists a read-only snapshot called a.
A read-write snapshot called b is created from a, and is then modified.
A read-only snapshot of b is created, called c.
A btrfs send is done for c, with a marked as its parent.

Will the send data only contain the differences between a and c?  My
experiments seem to indicate no, but I have no confidence that I am not
doing something else correctly.

Also, when a btrfs receive gets a stream containing the differences
between a (parent) and c, does it only look at the relative pathname
differences between a and c in order to determine the matching parent on
the receiving side?

-- 
Michael Welsh Duggan
(md5i@md5i.com)

Re: snapshot send with parent question

[Alex Lyakas]

Michael,
btrfs-send doesn't really know or care how did you manage to get from
a to c. It is able to compare any two RO subvolumes (not necessarily
related by snapshot operations), and produce a stream of commands that
transfer a-content into c-content.

Send assumes that at a receive side, you have a snapshot identical to
"a". Then receive side locates the a-snapshot (by a "received_UUID"
field) and creates a RW snapshot out of it. This snapshot would be
identical to "c", after applying the stream of commands. Then receive
side applies the stream of commands (in strict order), and at the end
sets the RW snapshot to be RO. At this point, this snapshot should be
identical to c.

The stream of commands most probably will not be identical to
operations that you did in order to get from "a" into "c". But it will
transfer "a"-content into "c"-content (leave alone possible bugs),
which is what's important.

Of course, if a and c are related via snapshot operations, then
btrfs-send will be much more efficient, in terms that it will be able
to skip entire btrfs subtrees (look at "btrfs_compare_trees"), thus
avoiding many additional comparisons that some other tool like rsync
would have done.

Thanks,
Alex.



On Sun, Apr 20, 2014 at 1:00 AM, Michael Welsh Duggan <mwd@md5i.com> wrote:
> Assume the following scenario:
> There exists a read-only snapshot called a.
> A read-write snapshot called b is created from a, and is then modified.
> A read-only snapshot of b is created, called c.
> A btrfs send is done for c, with a marked as its parent.
>
> Will the send data only contain the differences between a and c?  My
> experiments seem to indicate no, but I have no confidence that I am not
> doing something else correctly.
>
> Also, when a btrfs receive gets a stream containing the differences
> between a (parent) and c, does it only look at the relative pathname
> differences between a and c in order to determine the matching parent on
> the receiving side?
>
> --
> Michael Welsh Duggan
> (md5i@md5i.com)
>
> --
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