Dne 19. 11. 25 v 16:46 Brian J. Murrell napsal(a):
On Wed, 2025-11-19 at 09:07 -0500, Matthew Patton wrote:
That is one hell of a wild and unsupported assumption.
Yeah, I too was going to challenge that statement that I was a "lonely"
outlier in the community of LVM users that would want to be able to
move a thinly-provisioned LV from one PV to another, just as non-
thinly-provisioned LVs can be.
Practically nobody , even seasoned sysadmins would know of this
deficiency.
Indeed, I was skeptical that this was commonly understood as well.
Well by anyone who had not tried it as I did only to find it is not
supported.
It does suck to have to choose between feature sets that are both very
useful.
There is likely a major misunderstanding how thin-provisioning works with lvm2
(and ATM I'm not sure how we can improve our 'man lvmthin' to make this more
clear)
It's NOT lvm2 doing 'thin-provisioning' at lvm2 level.
lvm2 just drives a kernel target device - which you can think of like would
attach some 'magic box' that take space X a gives you back space Y with
some properties (just like when lvm2 controls 'raidX' target).
lvm2 does not see 'how the magic of provisioning' is made - it just consumes
it - thus lvm2 itself has no knowledge about the layout of thin-chunks - which
are often very small compared with lvm2 extent size granularity.
So lvm2 cannot move i.e. 64KiB thin chunk blocks - when the minimal pvmove
granularity if 4MiB.
And while I'm not saying the work cannot be done somehow - it's quite a major
feature request for thin-pool kernel target if that should be an online operation.
To add better example - it's like if you would want to 'extract' the file from
filesytem and let the rest of 'device' compact.
I guess everyone sticks to 'cp' command - which can be seen as equivalent of
'dd' command in our block device level...
Regards
Zdenek
