[linux-lvm] CoW under the hood

[linux-lvm] CoW under the hood

[Far Had]

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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

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Re: [linux-lvm] CoW under the hood

[Zdenek Kabelac]

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

Hi

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

http://people.redhat.com/agk/talks/


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

Regards

Zdenek