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

Mark Woodward markw at mohawksoft.com
Thu Oct 4 15:02:28 UTC 2012

On 10/04/2012 10:44 AM, Bryn M. Reeves wrote:
> Hash: SHA1
> 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 LVM2 code that ships with most distros still uses the old interfaces 
and still (Ubuntu 12.04) seemst to create a COW file with SnAp

>> 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.

Where are these files to be found?
> Regards,
> Bryn.
> Version: GnuPG v1.4.12 (GNU/Linux)
> Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/
> =wsdZ

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