[linux-lvm] exposing snapshot block device

[Zdenek Kabelac]

Dne 23. 10. 19 v 13:24 Tomas Dalebjörk napsal(a):
> I have tested FusionIO together with old thick snapshots.
> I created the thick snapshot on a separate old traditional SATA drive, just to 
> check if that could be used as a snapshot target for high performance disks; 
> like a Fusion IO card.
> For those who doesn't know about FusionIO; they can deal with 150-250,000 IOPS.
> 
> And to be honest, I couldn't bottle neck the SATA disk I used as a thick 
> snapshot target.
> The reason for why is simple:
> - thick snapshots uses sequential write techniques
> 
> If I would have been using thin snapshots, than the writes would most likely 
> be more randomized on disk, which would have required more spindles to coop 
> with this.
> 
> Anyhow;
> I am still eager to hear how to use an external device to import snapshots.
> And when I say "import"; I am not talking about copyback, more to use to read 
> data from.

Format of 'on-disk' snapshot metadata for old snapshot is trivial - being some
header + pairs of dataoffset-TO-FROM -  I think googling will reveal couple
python tools playing with it.

You can add pre-created COW image to LV  with  lvconvert --snapshot
and to avoid 'zeroing' metadata use option -Zn
(BTW in the same way you can detach snapshot from LV with --splitsnapshot so 
you can look how the metadata looks like...)

Although it's pretty unusual why would anyone create first the COW image with 
all the special layout and then merge it to LV - instead of directly 
merging...   There is only the 'little' advantage of minimizing 'offline' time 
of such device   (and it's the reason why --split exists).

Regards

Zdenek