[linux-lvm] exposing snapshot block device

[Tomas Dalebjörk]

Wow!

Impressing.
This will make history!

If this is possible, than we are able to implement a solution, which can do:
- progressive block level incremental forever (always incremental on block
level : this already exist)
- instant recovery to point in time (using the mentioned methods you just
described)

For example, lets say that a client wants to restore a file system, or a
logical volume to how it looked a like yesterday.
Eventhough there are no snapshot, nor any data.
Than the client (with some coding); can start from an empty volume, and
re-attach a cow device, and convert that using lvconvert --merge, so that
the copying can be done in background using the backup server.

If you forget about "how we will re-create the cow device"; and just
focusing on the LVM ideas of re-attaching a cow device.
Do you think that I have understood it correctly?


Den tors 24 okt. 2019 kl 18:01 skrev Zdenek Kabelac <zkabelac at redhat.com>:

> 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
>
>
>
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