Replied inline:
On 2014/04/24 12:30 AM, Robert White wrote:
So the backup/restore system described using snapshots is incomplete
because the final restore is a copy operation. As such, the act of
restoring from the backup will require restarting the entire backup
cycle because the copy operation will scramble the metadata
consanguinity.
The real choice is to restore by sending the snapshot back via send
and receive so that all the UIDs and metadata continue to match up.
But there's no way to "promote" the final snapshot to a non-snapshot
subvolume identical to the one made by the original btrfs subvolume
create operation.
btrfs doesn't differentiate snapshots and subvolumes. They're the same
first-class citizen. A snapshot is a subvolume that just happens to have
some data (automagically/naturally) deduplicated with another subvolume.
Consider a file system with __System as the default mount (e.g. btrfs
subvolume create /__System). You make a snapshot (btrfs sub snap -r
/__System /__System_BACKUP). Then you send the backup to another file
system with send receive. Nothing new here.
The thing is, if you want to restore from that backup, you'd
send/receive /__System_BACKUP to the new/restore drive. But that
snapshot is _forced_ to be read only. So then your only choice is to
make a writable snapshot called /__System. At this point you have a
tiny problem, the three drives aren't really the same.
The __System and __System_BACKUP on the final drive are subvolumes of
/, while on the original system / and /__System were full subvolumes.
There's no such thing as a "full" subvolume. Again, they're all
first-class citizens. The "real" root of a btrfs is always treated as a
subvolume, as are the subvolumes inside it too. Just because other
subvolumes are contained therein it doesn't mean they're diminished
somehow. You cannot have multiple subvolumes *without* having them be a
"sub" volume of the real "root" subvolume.
It's dumb, it's a tiny difference, but it's annoying. There needs to
be a way to promote /__System to a non-snapshot status.
If you look at the output of "btrfs subvolume list -s /" on the
various drives it's not possible to end up with the exact same system
as the original.
From a user application perspective, the system *is* identical to the
original. That's the important part.
If you want the disk to be identical bit for bit then you want a
different backup system entirely, one that backs up the hard disk, not
the files/content.
On the other hand if you just want to have all your snapshots restored
as well, that's not too difficult. Its pointless from most perspectives
- but not difficult.
There needs to be either an option to btrfs subvolume create that
takes a snapshot as an argument to base the new device on, or an
option to receive that will make a read-write non-snapshot subvolume.
This feature already exists. This is a very important aspect of how
snapshots work with send / receive and why it makes things very
efficient. They work just as well for a restore as they do for a backup.
The flag you are looking for is "-p" for "parent", which you should
already be using for the backups in the first place:
From backup host:
$ btrfs send -p /backup/path/yesterday /backup/path/last_backup |
<netcat or whatever you choose>
From restored host:
$ <netcat or whatever you choose> | btrfs receive /tmp/btrfs_root/
Then you make the non-read-only snapshot of the restored subvolume.
[snip]
--
__________
Brendan Hide
http://swiftspirit.co.za/
http://www.webafrica.co.za/?AFF1E97
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