[linux-lvm] Snapshots and disk re-use

Jonathan Tripathy jonnyt at abpni.co.uk
Thu Feb 24 15:20:35 UTC 2011

On 24/02/11 15:13, Stuart D. Gathman wrote:
> On Thu, 24 Feb 2011, Jonathan Tripathy wrote:
>>> Yes. To be more pedantic, the COW has copies of the original contents
>>> of blocks written to the origin since the snapshot.  That is why you
>>> need to clear it to achieve your stated purpose.  The origin blocks
>>> are written normally to the *-real volume (you can see these in
>>> the /dev/mapper directory).
>> But didn't you say that there is only one copy of the files stored physically
>> on disk?
> Yes.  When you make the snapshot, there is only one copy, and the COW table
> is empty.  AS YOU WRITE to the origin, each chunk written is saved to
> *-cow first before being written to *-real.
Got ya. So data that is being written to the origin, while the snapshot 
exists, is the data that may leak, as it's saved to the COW first, then 
copied over to real.

Hopefully an expert will let me know weather its safe to zero the COW 
after I’ve finished with the snapshot.

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