Re: [linux-lvm] access or interface to list of blocks that have, changed via C.O.W.?

[Mark Woodward <markw@mohawksoft.com>]

On 10/04/2012 10:44 AM, Bryn M. Reeves wrote:
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> On 10/04/2012 11:17 AM, Mark Woodward wrote:
>> I was going to try to answer in line, but decided that it would be
>> too much work. There are utilities to extract the exception table
>> out of the LVM2 snapshot, and if you can code in almost any
>> language, you can write your own. It is dead simple. You can google
>> for ddsnap and zumastore to
> ddsnap was the Zumastor snapshot tool which uses a different in-kernel
> snapshot target and metadata format. It does not share code with
> current (or historic afaik) LVM2/device-mapper and has been dead for a
> number of years (last commits around 2008).
>
>> get the code. It old and not supported currently, but still works.
>> I
> Really? I'd be surprised if it even builds against modern kernels or
> device-mapper.
>
>> The format of the array is simple: old_address (The offset in the
>> volume) followed by the new_address (the offset in the COW device).
>> An array of all the "old_address" values is the changed block list.
>> You don't even need to worry about the data if you can really get a
>> file list by blocks.
> If you really want to poke into the CoW store format I'd start by
> reading dm-snap-persistent.c which is the traditional device-mapper
> snapshot format.
>
> Snapshots using the thinp target use the metadata format described in
> dm-thin-metadata.c.

Sorry, I got pulled away a bit.

Let me understand. The CoW format changed, but the header signature 
stayed the same? I'm having a bit of an issue understanding this. The 
"old" LVM format CoW device had the signature "SnAp" and that had a 
fairly well understood layout.  Has this or has this not changed? I had 
some code that worked in the 2.6 series of kernels using the LVM tools. 
However, when I run it on a later kernel version (3.2.0) it seems not to 
work. I am more than willing to say its a bug on my end, but can you 
clarify whether or not it is expected that this format would change? I 
am planning a server upgrade and I rely on the snapshot format for my 
differential backup tools.

As for the thin provisioned snapshot I have been long waiting for this. 
Is it "production ready?" if so, in what kernel version moving forward?
>
> Regards,
> Bryn.
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