[Discuss] LVM Re: A really interesting chain of functionality

[richb at pioneer.ci.net (Rich Braun)]

Edward Ned Harvey wrote:
> 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.

Actually, you don't.  A snapshot can be extended just like any other logical
volume.  What I do to manage storage resources is this:  over-allocate when an
application is first deployed.  Once I get a sense of how much is really
required, then I can crank future deployments down to use more-realistic
amounts.  Always use Nagios (or equivalent) to monitor remaining storage. 
Make sure the operations team knows how to respond to alerts, which in this
case is a simple matter of typing "lvextend <foo>" -- a straightforward
training exercise because the same command applies to normal ext4 filesystems
which run low on storage (the late shift will be told, if they can't find
files to clean out, just run lvextend to add a few GB, followed by resize2fs.)

So an example might be a 30GB volume for source-code control, for which I
might over-allocate 15GB to a snapshot.  Once I saw that usage never goes past
2GB in a typical month, I might generate future snapshots with 4GB and set an
alert to go off once it goes past 2GB.

-rich