Hi all
I am trying to use dm snapshots to provide copy-on-write (CoW) support
using a static (read-only) origin which is to be shared by several CoW
overlays.
So, I prepare a (sparse) loop-back image and device with the origin in
/dev/loop0, and create an overlay onto a 'real' logical volume, using:
echo "0 $((152704*1024*1024/512)) snapshot /dev/loop0
/dev/mapper/VolGroup_store_3 p 128" | dmsetup create store3_pref
This seems to work OK, and indeed if I recreate the same dm device after
reboot or power cycle, I seem to recover the overlay changes.
So, where is the overlay mapping stored? I believe that this is known as
the 'exception store', and I can see some stuff about it in the source,
but where does a persistent overlay map end up?
Also, how is the overlay (snapshot) device used? Is there a one-to-one
mapping of (logical device) block offsets onto the overlay (snapshot)
device? If so, where does the overlay map fit in?
Lastly, how do I flush the overlay mapping? Suppose I have had quite
enough of the (modified) snapshot and I want to revert to the original
origin in a snapshot using the same overlay (snapshot) device. I notice
empirically that if I 'dmsetup remove' then 'dmsetup create ... n ...',
then 'dmsetup remove ...' once more, and finally 'dmsetup create ... p
...', then I have a clean snapshot of the origin again. I'm guessing
there's a better way ;)
Is this a sensible way to implement CoW? I realise that my use case is
an inversion of the original intention of snapshots, where the origin
was intended to be modified and the snapshot remained static (for
back-up purposes). In my CoW usage, the origin remains static and I
mutate several snapshots of the origin.
Thanks for any pointers
Roland
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