[linux-lvm] exposing snapshot block device
Mikulas Patocka
mpatocka at redhat.com
Mon Sep 7 13:09:08 UTC 2020
On Fri, 4 Sep 2020, Tomas Dalebjörk wrote:
> hi
> I tried to perform as suggested
> # lvconvert —splitsnapshot vg/lv-snap
> works fine
> # lvconvert -s vg/lv vg/lv-snap
> works fine too
>
> but...
> if I try to converting cow data directly from the meta device, than it doesn’t work
> eg
> # lvconvert -s vg/lv /dev/mycowdev
> the tool doesn’t like the path
> I tried to place a link in /dev/vg/mycowdev -> /dev/mycowdev
> and retried the operations
> # lvconveet -s vg/lv /dev/vg/mycowdev
> but this doesn’t work either
>
> conclusion even though the cow device is an exact copy of the cow
> device that I have saved on /dev/mycowdev before the split, it wouldn’t
> work to use to convert back as a lvm snapshot
>
> not sure if I understand the tool correctly, or if there are other
> things needed to perform, such as creating virtual information about the
> lvm VGDA data on the first of this virtual volume named /dev/mycowdev
AFAIK LVM doesn't support taking existing cow device and attaching it to
an existing volume. When you create a snapshot, you start with am empty
cow.
Mikulas
> let me know what more steps are needed
>
> beat regards Tomas
>
> Sent from my iPhone
>
> On 7 Nov 2019, at 18:29, Tomas Dalebjörk <tomas.dalebjork at gmail.com> wrote:
>
> Great, thanks!
>
> Den tors 7 nov. 2019 kl 17:54 skrev Mikulas Patocka <mpatocka at redhat.com>:
>
>
> On Tue, 5 Nov 2019, Tomas Dalebjörk wrote:
>
> > Thanks,
> >
> > That really helped me to understand how the snapshot works.
> > Last question:
> > - lets say that block 100 which is 1MB in size is in the cow device, and a write happen that wants to something or all data on that region of block
> 100.
> > Than I assume; based on what have been previously said here, that the block in the cow device will be overwritten with the new changes.
>
> Yes, the block in the cow device will be overwritten.
>
> Mikulas
>
> > Regards Tomas
>
>
>
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