Re: Understanding the two-year release cycle as a desktop user (and a Debian newcomer)
On Thu, 27 Feb 2020 08:54:02 +1100
Keith Bainbridge <keithrbau@gmail.com> wrote:
>
>
> I'm having trouble understanding how LVM snapshots works on / if it
> is outside the LVM.
>
I don't think anyone has picked this up. Yes, you do need additional
space.
I've only done it a few times. It basically involves leaving enough
unallocated space to accommodate whatever you want to do with the
snapshot.
What I used it for was on-line system backup. You can generally get away
with just copying a Linux system while it is running, if not much is
going on at the time, particularly if you temporarily stop SQL servers
and the like. Various volatile files won't be consistent, being copied
at different times, but if they are general housekeeping files which are
constantly being re-written, then the contents won't matter much after
a restore and reboot. Yes, some inconsistencies can be critical, and
restoring it in full over your working system is a bit risky. Using it
to restore individual files is generally safe, for most people a full
restore from backup would be a last resort (OK, second last, just before
reinstalling). But I've booted an online copy on another machine on
several occasions without (as far as I know) any issues.
If you want an absolutely consistent backup (e.g. copying to a bigger
hard drive), you either shut down and take it with an external tool, or
you can use LVM. When you enable the snapshot, the region of interest is
frozen, further disc writes are basically done to a journal, not to the
frozen region, and the journal is reconciled with its parent region when
you close the snapshot. In the meantime, you can take a backup where no
disc files will change while you're doing it.
You need enough space for the snapshot to store everything that may
written until you terminate the snapshot, but with modern discs you can
generally allocate as much space as the largest region you want to
back up, which is the safe option. Performance will be a bit reduced
during the snapshot period, but that depends on what the system is
being used for, how much writing is going on, and in any case a system
backup itself will slow things down a lot. Windows Server partitions
with Volume Shadow enabled do something like this continuously, managing
daily snapshots and deleting old ones up to the limit of allocated
space.
Read the current docs for exact details of how to do this.
--
Joe
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