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Date: Thu, 24 Feb 2011 16:41:59 +0000
From: Jonathan Tripathy <jonnyt@abpni.co.uk>
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Subject: Re: [linux-lvm] Snapshots and disk re-use
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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=20
>>> physically
>>> on disk?
>> Yes.  When you make the snapshot, there is only one copy, and the COW=20
>> 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=20
> snapshot exists, is the data that may leak, as it's saved to the COW=20
> first, then copied over to real.
>
> Hopefully an expert will let me know weather its safe to zero the COW=20
> after I=EF=BF=BDve finished with the snapshot.
>
>
Oh, I forgot to mention. My LVM sits on top of linux software RAID1.=20
Does that complicate things at all?