[CentOS] Rsync and differential Backups

Fri Nov 13 20:59:11 UTC 2015
J Martin Rushton <martinrushton56 at btinternet.com>

Hash: SHA1

On 13/11/15 17:55, Gordon Messmer wrote:
> On 11/13/2015 01:46 AM, J Martin Rushton wrote:
>> If you really_need_  the guarantee of a snapshot, consider either
>> LVM or RAID1. Break out a volume from the RAID set, back it up,
>> then rebuild.
> FFS, don't do the latter.  LVM is the standard filesystem backing
> for Red Hat and CentOS systems, and fully supports consistent
> snapshots without doing half-ass shit like breaking a RAID volume.
> Breaking a RAID volume doesn't make filesystems consistent, so when
> you try to mount it, you might have a corrupt filesystem, or
> corrupt data. Breaking the RAID will duplicate UUIDs of filesystems
> and the name of volume groups.  There are a whole bunch of
> configurations where it just won't work.  At best, it's unreliable.
> Never do this.  Don't advise other people to do it.  Use LVM
> snapshots (or ZFS if that's an option for you). 
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Maybe I should have been clearer: use (LVM) OR (RAID1 and break).
Don't use LVM and break, that would be silly.

I hope I'm wrong, but you wouldn't be thinking of mounting the broken
out copy on a the same system would you?  You must never do that, not
even during disaster recovery.  Use dd or similar on the disk, not the
mounted partitions - isn't that obvious?  I wasn't trying to give step
by step instructions.

Way before LVM existed we used this technique to back up VAXes (and
later Alphas) under VMS using "volume shadowing" (ie RAID1). It worked
quite happily for several years with disks shared across the cluster.
IIRC it was actually recommended by DEC, indeed a selling point, but I
don't have any manuals to hand to confirm that nowadays! One thing I
did omit was you MUST sync first (there was an equivalent VMS command,
don't ask me now), and also ensure that as the disks are added back a
full catchup copy occurs.  You may consider it half a mule's
droppings, but it is, after all, what happens if you loose a spindle
and hot replace.
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