Dne 03. 11. 22 v 8:35 Zhiyong Ye napsal(a):
Hi all,
I am new to dm-devel. When using dm-thin via lvm, I found it difficult to
share dm-thin on multiple hosts.
The background is that I want to implement live migration of VMs in the lvm
+ iSCSI environment, in which lvmlockd is used to coordinate access to
shared storage. There are multiple hosts in the cluster using the same iSCSI
connection, and the VMs running in this environment use thinlv volumes. But
if want to live migrate the vm, it will be difficult since thinlv which from
the same thin pool can only be exclusive active on one host.
I'm curious why the dm-thin doesn't support multi-host shared access, just
like the dm-linear does. If I try to write a new dm-thin target or make some
modifications based on the existing dm-thin target to support this feature,
is it technically feasible?
dm-thin is tightly connected with thin-pool - it's not standalone unit
that can live 'without' thin-pool.
Thin-pool currently does NOT support 'shared/multihost' activation and must
be activated exclusively on a single host together with all its thin volumes.
So at this moment there is no way you can activate thin-pool on multiple hosts
without causing a serious metadata corruption to the thin-pool (as there will
be 2 parallel unlocked stream updates).
There has been also implemented some basic detection to prevent 'multihost'
activation and avoid damage to the metadata as you are not the first one who
tries to activate thin-pool on more then one machine at a time.
There have been some ideas how to allow 'mutlihost' thin-pool support - but
they are relatively very difficult to implement with low amount of actual
users/customers - so they are somewhat queued for later time
Meanwhile potential idea how to implement such thing might be to look at
blk-archive project (https://github.com/jthornber/blk-archive)
Regards
Zdenek
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