[dm-devel] [RFC] dm snapshot based asynchronous replication

[christophe.varoqui at free.fr]

dm developers,

I'm in need of an asynchronous replication method for active/passive clusters using direct-attached disks. Please advise if you know tools (other than drdb an rsync) to do the job. (How goes dm-replicator btw ?).

Pending an existing project, I toyed with a replication scheme I'd like to sketch here for comments and fool-proofing.

Source and destination are logical volumes : lv_src => lv_dst
Upon cold start lv_src and lv_dst are unsynchronized.

==== pseudo-code start
snap lv_src (lv_src_snap0)
full copy lv_src_snap0 on lv_dst
while true:
  wait n seconds
  snap lv_src (lv_src_snap1)
  for each changed chunk in lv_src_snap0 cow table:
    copy chunk from lv_src_snap1 to lv_dst
  drop lv_src_snap0, and rename lv_src_snap1 to lv_src_snap0
==== pseudo-code end

Snapshoting the lv_dst would make sense as a rollback safety net ... even if the rollback is a bit hard pending the snapshot merging  patchset inclusion.

If this sketch looks sane to you, I'm interested in how to list the changed chunks for a snap. A pointer to existing code would do great.

Regards,
cvaroqui