[linux-lvm] LVM pages at TLDP

[Mark Ballard]

Dear alewis

May I make a suggestion that may make a small improvement to your helpful
pages on LVM at The Linux Documentation Project?

I found your pages while searching for help on snapshots and mirrors in the
hope that I might find a foolproof way of moving the encrypted lvm disk on
my desktop to a safe place - from where it can still be booted! - while I
upgrade the O/S. (There are good reasons for this, from my perspective*).

Your documentation page, "Taking a backup using snapshots" (
http://www.tldp.org/HOWTO/LVM-HOWTO/snapshots_backup.html), helpfully
describes how to create the snapshot.

Then, however, you say ' I assume you will have a more sophisticated backup
strategy than this!', which I would say is an unwelcome assumption for
documentaion on how to backup. I would rather you assumed I where not at
all sophisticated and had very little clue about what I was going to do
next. That was indeed  why I was reading your documenation page in the
first place.

So I do want to do a sophisticated backup. But I don't know how. I have a
vague understanding that there might be some use in creating a tar ball of
a snapshot, rather than, say, simply making a tar ball of a partition. But
I would hope to find some explanation of the options that are available at
this point and which are advisible for different purposes - with the
reasons spelt out. That is why I have come here from the man pages. I
would, for example, suspect that my best option at this point might be to
dd the snapshot to my secondary drive. But if I did that, would it be a
snapshot of a volume group, or just one lv within a vg? Would then I need
to do a snapshot for each lv in my vg? Why not do a snapshot of a pv? And
most importantly, if I used dd to copy my logical volume, would I be
foolish to do it while my my o/s was running off it)? Or would I be
expected to run off a live disk or something in order to perform the
operation? I am anyway kind of surprised that lvm doesn't have a tool to
render a snapshot into a fully-fledged copy on another disk. Wouldn't
lvconvert do this? Perhaps something like rsync is the most sensible, even
after creating a snapshot. But then why would you bother creating a
snapshot for a backup then, and not simply rsync each lv? And while we are
on the subject, I had thought mirror might do this job, but the man pages
weren't much help to me, and I've found scant info on it elsewhere.

So thank you for getting me this far. I reckon I'll sleep on it.

mballard.

* The reasons are twofold. While it seemed that a snapshot or mirror might
be the way to do it, the description of snapshots in the lvm man pages is
atrocious. I was fortunately led to your page on snapshot backups by the
Ubuntu wiki, which has a refreshingly comprehensible description of
snapshots: https://wiki.ubuntu.com/Lvm

(it's author, flimm, should be encouraged, begged, paid or forced to write
the manpages imho. But flimm doesn't have a public email address - his
Ubuntu wiki points those searching for a contact address to
https://launchpad.net/~flimm, which says his address is not public. The lvm
manpages meanwhile have no address on them at all. They are presumably
unmanaged? And I guess users are irrelevant nowadays, so contact is not
necessary?).

Pre-briefed by flimms Wiki, your brief discussion on setting up a snapshot
was a helpful and informative digression, and it was easy to follow your
instructions at this point. (The thing that is not readily apparent, and
which is explained none too well in most of the standard documentation is
that snapshots do not get allocated their own dedicated physical space such
as an lv or a partition. This is unintuitive behaviour when you are used to
specifying some physical location for something to be dumped).

Secondly, (to be the long-winded article, and in case user experience
stories are of any use to you - if not, I hope you switched off before you
got here): I had resized partitions, physical and logical volumes some
while ago, despite reading some text somewhere that advised lvm couldn't
really cope with such an operation and would have to be setup afresh or it
would run like an old dog. It has seemed to be running like an old dog ever
since. But it is an old dog anyway, and I also needed to upgrade my o/s.
But I cannot interrupt my work. So I needed to find a way to recreate my
current installation on a secondary disk - and to be able to continue
working from it in the interim and further, in the event of a mishap -
while I wiped my main disk and installed a fresh lvm and o/s.

And no the RedHat documenation isn't much help on this either. That looks
like it was written for enterprise-class petrol heads. Probably not much
use for anyone on bicycle power.
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