[Discuss] A really interesting chain of functionality

[blu at nedharvey.com (Edward Ned Harvey)]

> From: discuss-bounces+blu=nedharvey.com at blu.org [mailto:discuss-
> bounces+blu=nedharvey.com at blu.org] On Behalf Of Cole Tuininga
> 
> On 9/25/11 10:49 AM, Edward Ned Harvey wrote:
> > Personally, I do the same thing in ZFS using zvol's for improved
> performance
> > and reliability and ability to snapshot.  (LVM snapshot is a terrible
> > thing.)
> 
> Could you elaborate?  As somebody who is only passingly familiar with
> this level of things, I'm curious to learn what makes LVM's snapshot so
> terrible.

It's been a while since I did this, so please forgive me wherever I
misspeak.  The man page has all the correct details.  But as I remember
it...

In LVM, if you want to do snapshots, you must first reserve all the space
that could possibly ever be used for the snapshot differentials.  When you
create a snap, it just creates another block device that looks the same as
the primary block device at the time...  And the more active your primary
volume, the more quickly the space in your snapshot device gets consumed.
When you run out of space in your snapshot device, your snapshot simply
disappears.  You can't resize it on the fly, and it doesn't dynamically
allocate.  And it doesn't self-mount.

So yes, there are some situations where it might be useful (create a
snapshot in order to run backup of the snapshot instead of backing up the
live filesystem device)...  But given the fact that your snapshot could
suddenly disappear, it seems like an unlikely thing you'd want to do.  And
given the lack of automatic mounting or anything...  You're not likely to
use it for general purpose automatically available historical snapshots for
users to self-restore from backup without IT assistance, either.

There is no incremental send.  There is no send at all.

IMHO, the most useful features of snapshots are (a) users ability to
self-restore files without assistance, (b) ability to instantly efficiently
send incremental backups to whatever backup destination, (c) cloning
filesystems (such as VM guests) instantly...  What else...  Well the point
is, LVM snapshots don't do anything useful, and they waste an *awful* lot of
disk space to do it.