[linux-lvm] CoW under the hood

Zdenek Kabelac zkabelac at redhat.com
Mon Dec 3 11:39:15 UTC 2018

Dne 03. 12. 18 v 7:17 Far Had napsal(a):
> Hi,
> I'm trying to understand how LVM thick snapshots work under the hood. I can 
> see that when new write requests are comming to the original volume that has a 
> snapshot, the current contents of storage blocks copy to the snapshot space 
> then the new data overwrites on the volume. No problem so far.
> the question is when I delete a file on a volume that has snapshot, the 
> content is gone on the volume so the pointer which points from volume object 
> to that certain blocks on the volume should now point to empty blocks. On the 
> other hand I can see that the contents of deleted parts from the original 
> volume did not copy to the snapshot space, meaning that the snapshot pointers 
> that were pointing to that blocks on the original volume, still point that 
> direction.
> In other words the pointers from both snapshot and original volume are 
> pointing to the same blocks on the storage media but they don't show the same 
> contents when you mount them on the filesystem.
> Please enlighten me about this situation


I assume going through some info from this page might put some light into it:


In particular http://people.redhat.com/agk/talks/FOSDEM_2005/
from slide 15  there is some info about snapshots you might find useful.



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