btrfs data replication

Introduction

The btrfs replication scheme is based on btrfs send/receive. OpenSVC uses btrfs snapshots to ensure incremental send/receive replication.

Source and targets maintain a btrfs snapshot @sent to identify the last incremental synchronization done properly. This state is used to check that the deltas application sequence is not broken. If broken, alerts are emited and a full sync must be scheduled

Command set

syncnodes
Initial synchronization or incremental synchronisation. First one will be a Initial synchronisation, next will be incremental

Status

up
Last synchronization occured less than sync_max_delay minutes ago.
warn
Last synchronization occured more than sync_max_delay minutes ago.

Internals

A replication action applies the following logic:

  • create @tosent readonly snapshots on sender

  • for each subvol-remote pair

    • send/receive the @tosent snap
    • rotate @tosent to @sent on remote
    • recursive clean-up of the destination final location on remote
    • install the @sent subvol as rw snapshots on remotes
  • rotate @tosent to @sent on sender

Implementation details

There are some shortcomings in the current btrfs send/receive implementation that opensvc works around :

  • no recursive snapshot/delete/send : for now you have to declare one sync resource per subvol, even if they are organised as a tree.
  • btrfs receive is easily confused when looking for a subvol parent id : for now opensvc has to mount the ID5 root vol on <OSVCVAR>/btrfs/<label> and uses a flat/root-parented snapshot hosting. Once the btrfs feature matures we might move to a .osvcsnap/ dedicated subvol mounted in <OSVCVAR>/btrfs/<label>.osvcnap/ instead.

Examples

Extract of a typical btrfs sync env file

[DEFAULT]
nodes = deb1.opensvc.com deb2.opensvc.com
app = OSVCLAB
service_type = DEV

[fs#1]
type = btrfs
dev = /dev/vdb
mnt = /btrfssvc
mnt_opt = subvol=btrfssvc

[sync#1]
type = btrfs
src = data:btrfssvc
dst = data:btrfssvc
target = nodes

[sync#2]
type = btrfs
src = data:btrfssvc/child
dst = data:btrfssvc/child
target = nodes

Full synchronization

root@deb1.opensvc.com # svcmgr -s btrfssvc syncnodes
* BTRFSSVC.SYNC#1 - INFO - btrfs subvolume snapshot -r /var/lib/opensvc/btrfs/data/btrfssvc /var/lib/opensvc/btrfs/data/btrfssvc@tosend
* BTRFSSVC.SYNC#1 - INFO - output:
Create a readonly snapshot of '/var/lib/opensvc/btrfs/data/btrfssvc' in '/var/lib/opensvc/btrfs/data/btrfssvc@tosend'

* BTRFSSVC.SYNC#2 - INFO - btrfs subvolume snapshot -r /var/lib/opensvc/btrfs/data/btrfssvc/child /var/lib/opensvc/btrfs/data/btrfssvc_child@tosend
* BTRFSSVC.SYNC#2 - INFO - output:
Create a readonly snapshot of '/var/lib/opensvc/btrfs/data/btrfssvc/child' in '/var/lib/opensvc/btrfs/data/btrfssvc_child@tosend'

* BTRFSSVC.SYNC#1 - INFO - btrfs send /var/lib/opensvc/btrfs/data/btrfssvc@tosend | /usr/bin/ssh -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 deb2.opensvc.com btrfs receive /var/lib/opensvc/btrfs/data
* BTRFSSVC.SYNC#1 - INFO - /usr/bin/ssh -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 deb2.opensvc.com mv /var/lib/opensvc/btrfs/data/btrfssvc@tosend /var/lib/opensvc/btrfs/data/btrfssvc@sent
* BTRFSSVC.SYNC#1 - INFO - /usr/bin/ssh -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 deb2.opensvc.com btrfs subvolume delete /var/lib/opensvc/btrfs/data/btrfssvc/child && btrfs subvolume delete /var/lib/opensvc/btrfs/data/btrfssvc
* BTRFSSVC.SYNC#1 - INFO - output:
Delete subvolume '/var/lib/opensvc/btrfs/data/btrfssvc/child'
Delete subvolume '/var/lib/opensvc/btrfs/data/btrfssvc'

* BTRFSSVC.SYNC#1 - INFO - /usr/bin/ssh -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 deb2.opensvc.com btrfs subvolume snapshot /var/lib/opensvc/btrfs/data/btrfssvc@sent /var/lib/opensvc/btrfs/data/btrfssvc
* BTRFSSVC.SYNC#1 - INFO - output:
Create a snapshot of '/var/lib/opensvc/btrfs/data/btrfssvc@sent' in '/var/lib/opensvc/btrfs/data/btrfssvc'

* BTRFSSVC.SYNC#1 - INFO - mv /var/lib/opensvc/btrfs/data/btrfssvc@tosend /var/lib/opensvc/btrfs/data/btrfssvc@sent
* BTRFSSVC.SYNC#1 - INFO - update state file with snap uuid 203
* BTRFSSVC.SYNC#1 - INFO - /usr/bin/scp -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 /var/lib/opensvc/btrfssvc_sync#1_btrfs_state deb2.opensvc.com:/var/lib/opensvc/btrfssvc_sync\#1_btrfs_state
* BTRFSSVC.SYNC#2 - INFO - btrfs send /var/lib/opensvc/btrfs/data/btrfssvc_child@tosend | /usr/bin/ssh -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 deb2.opensvc.com btrfs receive /var/lib/opensvc/btrfs/data
* BTRFSSVC.SYNC#2 - INFO - /usr/bin/ssh -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 deb2.opensvc.com mv /var/lib/opensvc/btrfs/data/btrfssvc_child@tosend /var/lib/opensvc/btrfs/data/btrfssvc_child@sent
* BTRFSSVC.SYNC#2 - INFO - /usr/bin/ssh -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 deb2.opensvc.com btrfs subvolume snapshot /var/lib/opensvc/btrfs/data/btrfssvc_child@sent /var/lib/opensvc/btrfs/data/btrfssvc/child
* BTRFSSVC.SYNC#2 - INFO - output:
Create a snapshot of '/var/lib/opensvc/btrfs/data/btrfssvc_child@sent' in '/var/lib/opensvc/btrfs/data/btrfssvc/child'

* BTRFSSVC.SYNC#2 - INFO - mv /var/lib/opensvc/btrfs/data/btrfssvc_child@tosend /var/lib/opensvc/btrfs/data/btrfssvc_child@sent
* BTRFSSVC.SYNC#2 - INFO - update state file with snap uuid 204
* BTRFSSVC.SYNC#2 - INFO - /usr/bin/scp -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 /var/lib/opensvc/btrfssvc_sync#2_btrfs_state deb2.opensvc.com:/var/lib/opensvc/btrfssvc_sync\#2_btrfs_state
* BTRFSSVC - INFO - exec 'svcmgr -s btrfssvc --waitlock 3600 postsync' on node deb2.opensvc.com

Incremental synchronization

root@deb1.opensvc.com # svcmgr -s btrfssvc syncnodes
* BTRFSSVC.SYNC#1 - INFO - btrfs subvolume snapshot -r /var/lib/opensvc/btrfs/data/btrfssvc /var/lib/opensvc/btrfs/data/btrfssvc@tosend
* BTRFSSVC.SYNC#1 - INFO - output:
Create a readonly snapshot of '/var/lib/opensvc/btrfs/data/btrfssvc' in '/var/lib/opensvc/btrfs/data/btrfssvc@tosend'

* BTRFSSVC.SYNC#2 - INFO - btrfs subvolume snapshot -r /var/lib/opensvc/btrfs/data/btrfssvc/child /var/lib/opensvc/btrfs/data/btrfssvc_child@tosend
* BTRFSSVC.SYNC#2 - INFO - output:
Create a readonly snapshot of '/var/lib/opensvc/btrfs/data/btrfssvc/child' in '/var/lib/opensvc/btrfs/data/btrfssvc_child@tosend'

* BTRFSSVC.SYNC#1 - INFO - btrfs send -i /var/lib/opensvc/btrfs/data/btrfssvc@sent -p /var/lib/opensvc/btrfs/data/btrfssvc@sent /var/lib/opensvc/btrfs/data/btrfssvc@tosend | /usr/bin/ssh -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 deb2.opensvc.com btrfs receive /var/lib/opensvc/btrfs/data
* BTRFSSVC.SYNC#1 - INFO - /usr/bin/ssh -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 deb2.opensvc.com btrfs subvolume delete /var/lib/opensvc/btrfs/data/btrfssvc@sent
* BTRFSSVC.SYNC#1 - INFO - output:
Delete subvolume '/var/lib/opensvc/btrfs/data/btrfssvc@sent'

* BTRFSSVC.SYNC#1 - INFO - /usr/bin/ssh -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 deb2.opensvc.com mv /var/lib/opensvc/btrfs/data/btrfssvc@tosend /var/lib/opensvc/btrfs/data/btrfssvc@sent
* BTRFSSVC.SYNC#1 - INFO - /usr/bin/ssh -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 deb2.opensvc.com btrfs subvolume delete /var/lib/opensvc/btrfs/data/btrfssvc/child && btrfs subvolume delete /var/lib/opensvc/btrfs/data/btrfssvc
* BTRFSSVC.SYNC#1 - INFO - output:
Delete subvolume '/var/lib/opensvc/btrfs/data/btrfssvc/child'
Delete subvolume '/var/lib/opensvc/btrfs/data/btrfssvc'

* BTRFSSVC.SYNC#1 - INFO - /usr/bin/ssh -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 deb2.opensvc.com btrfs subvolume snapshot /var/lib/opensvc/btrfs/data/btrfssvc@sent /var/lib/opensvc/btrfs/data/btrfssvc
* BTRFSSVC.SYNC#1 - INFO - output:
Create a snapshot of '/var/lib/opensvc/btrfs/data/btrfssvc@sent' in '/var/lib/opensvc/btrfs/data/btrfssvc'

* BTRFSSVC.SYNC#1 - INFO - btrfs subvolume delete /var/lib/opensvc/btrfs/data/btrfssvc@sent
* BTRFSSVC.SYNC#1 - INFO - output:
Delete subvolume '/var/lib/opensvc/btrfs/data/btrfssvc@sent'

* BTRFSSVC.SYNC#1 - INFO - mv /var/lib/opensvc/btrfs/data/btrfssvc@tosend /var/lib/opensvc/btrfs/data/btrfssvc@sent
* BTRFSSVC.SYNC#1 - INFO - update state file with snap uuid 206
* BTRFSSVC.SYNC#1 - INFO - /usr/bin/scp -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 /var/lib/opensvc/btrfssvc_sync#1_btrfs_state deb2.opensvc.com:/var/lib/opensvc/btrfssvc_sync\#1_btrfs_state
* BTRFSSVC.SYNC#2 - INFO - btrfs send -i /var/lib/opensvc/btrfs/data/btrfssvc_child@sent -p /var/lib/opensvc/btrfs/data/btrfssvc_child@sent /var/lib/opensvc/btrfs/data/btrfssvc_child@tosend | /usr/bin/ssh -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 deb2.opensvc.com btrfs receive /var/lib/opensvc/btrfs/data
* BTRFSSVC.SYNC#2 - INFO - /usr/bin/ssh -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 deb2.opensvc.com btrfs subvolume delete /var/lib/opensvc/btrfs/data/btrfssvc_child@sent
* BTRFSSVC.SYNC#2 - INFO - output:
Delete subvolume '/var/lib/opensvc/btrfs/data/btrfssvc_child@sent'

* BTRFSSVC.SYNC#2 - INFO - /usr/bin/ssh -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 deb2.opensvc.com mv /var/lib/opensvc/btrfs/data/btrfssvc_child@tosend /var/lib/opensvc/btrfs/data/btrfssvc_child@sent
* BTRFSSVC.SYNC#2 - INFO - /usr/bin/ssh -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 deb2.opensvc.com btrfs subvolume snapshot /var/lib/opensvc/btrfs/data/btrfssvc_child@sent /var/lib/opensvc/btrfs/data/btrfssvc/child
* BTRFSSVC.SYNC#2 - INFO - output:
Create a snapshot of '/var/lib/opensvc/btrfs/data/btrfssvc_child@sent' in '/var/lib/opensvc/btrfs/data/btrfssvc/child'

* BTRFSSVC.SYNC#2 - INFO - btrfs subvolume delete /var/lib/opensvc/btrfs/data/btrfssvc_child@sent
* BTRFSSVC.SYNC#2 - INFO - output:
Delete subvolume '/var/lib/opensvc/btrfs/data/btrfssvc_child@sent'

* BTRFSSVC.SYNC#2 - INFO - mv /var/lib/opensvc/btrfs/data/btrfssvc_child@tosend /var/lib/opensvc/btrfs/data/btrfssvc_child@sent
* BTRFSSVC.SYNC#2 - INFO - update state file with snap uuid 207
* BTRFSSVC.SYNC#2 - INFO - /usr/bin/scp -o StrictHostKeyChecking=no -o ForwardX11=no -o BatchMode=yes -o ConnectTimeout=10 /var/lib/opensvc/btrfssvc_sync#2_btrfs_state deb2.opensvc.com:/var/lib/opensvc/btrfssvc_sync\#2_btrfs_state
* BTRFSSVC - INFO - exec '/etc/opensvc/btrfssvc --waitlock 3600 postsync' on node deb2.opensvc.com