Re: [gentoo-dev] Init systems portage category

[Richard Freeman <rich0@gentoo.org>]

Jesús Guerrero wrote:
> 
> In my opinion, if we really want to speak about a way to implement that
> kind of snapshoting, we should start thinking about providing a better
> integration with lvm, from the root. lvm can take care of the snapshots on
> a non-expensive way, and it would be relatively easy to implement. However
> a lot of stuff would need to be re-documented, starting from the handbook,
> and the init system.

LVM snapshotting is extremely wasteful - it has no knowledge of the 
underlying structure of a filesystem.  For example, if I moved all the 
files around on a fairly full ext3 filesystem, an LVM snapshot would 
consume the full size of the filesystem.  However, a filesystem-level 
solution wouldn't need to store a single byte of data since nothing 
actually changed.

Also - a snapshot restoration obliterates ALL data on the partition that 
has changed since the snapshot was taken - so unless the essential files 
are on a separate partition it won't work out well.

LVM snapshots really seem to be a solution to atomic backups - if you 
unmount, snapshot, and remount a filesystem then you can run a 
self-consistent backup off of the snapshot with minimal downtime.  The 
wasted space isn't a big deal since the snapshot would be deleted before 
it grew too far.

Finally - I'm not to eager to try out lvm2 again anytime soon - I lost a 
ton of data when something glitched and wiped out data across multiple 
lvm partitions.  I know that the error must have been in the lvm layer 
(not md or the filesystem), because when I fscked and repaired one 
filesystem, another filesystem instantly became hosed (on a separate lvm 
partition).  Somehow the partitions had gotten scrambled together and 
the fsck was crossing partition boundaries.  Plus, dmesg was dumping all 
kinds of compliants at the md layer about the lvm device trying to 
access out-of-range clusters.  Googling I found a few other reports of 
similar behavior - it seems extremely rare, but very nasty.

Fortunately the most important stuff on my PC was backed up (good 
planning), but it was still a pain - I lost tons of DVR recordings since 
I don't back that up (not worth the cost vs the value of the data).  Now 
I just run ext3 on top of md and I haven't had any problems.

You're right that btrfs will definitely help.  However, being able to 
create a personal stage1 tarball at will would certainly also be useful, 
and it wouldn't actually consume much disk space.