Hi. Is this possible: ?
I have a block device on the local host /dev/something with data on it. I have a block device on the remote host remote:/dev/something with the exact same size but no relevant data on it. I have room on the local host in /data or an extra block device (could be loop). What I want to do: 1. Stop programs and umount /dev/something 2. mdadm --create /dev/md0 --level=mirror --force --raid-devices=1 \ --metadata-file /data/raid_something /dev/something → Now I have /dev/md0 that is an exact image of /dev/something, with changes on it synced instantaneously. 3. mount /dev/md0 and restart programs. → Now I have everything running again completely normally after a very short service interruption. But behind the scenes files operations go through /dev/md0 before reaching /dev/something. If I want to go back, I de-configure /dev/md0 and can start using /dev/something directly again. 4. mdadm --add /dev/md0 remote:/dev/something && mdadm --grow /dev/md0 --raid-devices=2 → The kernel starts syncing the contents of /dev/md0=/dev/something onto remote:/dev/something. 5. Wait a long time. → When the sync is done, /dev/something and remote:/dev/something are exact copies of each other, with /dev/md0 a view of both, and every write to /dev/md0 is almost instantaneously synced onto them. 6. umount /dev/md0 && mdadm --stop /dev/md0 && stop services 7. ssh remote → mount /dev/something and start programs. → Now the programs are running on remote instead of local. The points of all this: - The service interruption is only between 1 and 3 and 6 and 7, very short. The blocking step is 5 while the RAID synchronizes, but the system is working normally during that time, just with slightly reduced performance due to the syncing. - The system keeps track of the sync, I do not have to start again or figure out where it stopped like I would have to if I just copied /dev/something over SSH. To say it differently, what I would like: - A kind-of-RAID1 (it does not have to actually be mdadm), where all devices are mirror of each other. - The metadata and bitmaps of the RAID resides separately. - The contents of the underlying devices are exactly identical and the RAID device shows it exactly. - When creating the RAID, one of the underlying is considered already valid and its data is preserved. - Underlying devices can be on the network; they can be write-only. Does anything like this exist? Regards, -- Nicolas George
