[linux-lvm] Snapshots and disk re-use

Jonathan Tripathy jonnyt at abpni.co.uk
Thu Feb 24 16:41:59 UTC 2011

On 24/02/11 15:20, Jonathan Tripathy wrote:
> 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.
Oh, I forgot to mention. My LVM sits on top of linux software RAID1. 
Does that complicate things at all?

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