Re: snapshot merging?

[Tomasz Chmielewski <mangoo@wpkg.org>]

Zheng Yan schrieb:
> Tomasz Chmielewski wrote:
>> At the beginning of August, Mikulas Patocka posted to linux-kernel
>> mailing list about adding snapshot merging to LVM[1].
>>
>> Basicaly, snapshot merging means that it is possible to turn a snapshot
>> back into its origin.
>>
>> Using LVM, however, means that you need to have free place outside of
>> the filesystem (i.e., in physical volume) to make snapshots, which is
>> not always possible on workstations and some servers.
>>
>> Is something similar available, or planned, for btrfs? I didn't find
>> anything similar on "btrfs design" page in the wiki.
>>
> Writeable snapshot has been available in btrfs for a long time.

Writeable snapshots and snapshot merging are two different things.


>> [1] http://kerneltrap.org/Linux/LVM_Snapshot_Merging
>>
> Writeable snapshot means you can mount/modify snapshots as you like.
> In other words, btrfs does snapshot merging automatically.

With LVM, you can also modify snapshots as you like, but it doesn't mean 
LVM supports snapshot merging right now (that is, you have to apply the 
patches described on the page above to have "snapshot merging" feature).


Maybe let me show you an example:

0) you have your "original" root filesystem
1) you make a snapshot of your rootfs
2) you upgrade your distribution (version 1.0 -> version 2.0-alpha)
3) you realise that some of the things don't work as expected - you mark 
that your snapshot should be turned into an original
4) you reboot (as your rootfs is mounted, you have to) and from now on 
your snapshot turns into the original from point 0.


Of course, one could boot off a live CD and copy the snapshot into the 
original, but that involves additional operating system to be booted.

Or, you could copy data from snapshot to the original while your system 
is running, but that could be risky for running programs (you replace 
libc, lots of libraries, binaries etc.).


-- 
Tomasz Chmielewski
http://wpkg.org