[linux-lvm] exposing snapshot block device

[Dalebjörk, Tomas]

Hi Mikulas,

Thanks for the replies

I am confused now with the last message?

LVM doesn't support taking existing cow device and attaching it to an 
existing volume?

Isn't that what "lvconvert --splitsnapshot" & "lvconvert -s" is ment to 
be doing?

lets say that I create the snapshot on a different device using these steps:

root at src# lvcreate -s -L 10GB -n lvsnap vg/lv /dev/sdh
root at src# lvconvert ---splitsnapshot vg/lvsnap
root at src# echo "I now move /dev/sdb to another server"
root at tgt# lvconvert -s newvg/newlv vg/lvsnap


Regards Tomas

Den 2020-09-07 kl. 15:09, skrev Mikulas Patocka:
>
> 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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