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Re: [BackupPC-users] Block-level rsync-like hashing dd?
2011-04-12 18:47:52
"Jeffrey J. Kosowsky" <backuppc AT kosowsky DOT org>
wrote on 04/12/2011 05:10:15 PM:
> Timothy J Massey wrote at about 15:40:05 -0400 on Tuesday, April 12,
2011:
> > "Jeffrey J. Kosowsky" <backuppc AT kosowsky DOT org>
wrote on 04/10/201101:57:01
> > PM:
> >
> > > The only problem with dd is that you would generally
need to either
> > > make a "snapshot" (e.g., using lvm2) or
shutdown BackupPC and unmount
> > > the drives to assure a perfect partition copy.
> >
> > You always have to unmount (for it to work correctly, anyway),
whether
> > you're dd'ing the partition raw or using LVM. The
difference with LVM is
> > that you only have to have it umounted for a brief moment,
and without it
> > for the entire time of the DD.
>
> Well with LVM, if you have a dedicated BackupPC partition, it should
> be sufficient to make sure no backup (or backuppc nightly process)
is
> running and that you are not at one of the round-number o'clock wakeup
> times, then just doing a 'sync' followed by a lvm-snapshot should
be
> sufficient. I agree though that unmounting would be simpler though
and
> less likely to make mistakes.
Last I looked, if you dd a LVM snapshot of an EXT2/3
partition that is mounted R/W, it will be marked dirty on mount. If
this is not the case, then yes, quiescing BackupPC is enough.
However, we're talking about *backup* here. I'm
not a big fan of "should be OK": I'd rather unmount the
partition! :)
> > > I'm not sure though how a 'dd' of an lvm2 snapshot
works and what you
> > > would need to do on the new drive to get it to mount.
> >
> > The same thing you'd have to do in any other case: mount
it! :)
>
> Well with 'dd' I typically do *not* mount it. I do a dd on the
> unmounted partition (to ensure it is stable). I guess my question
is
> whether I can do the same on an unmounted lvm partition using
> /dev/lvm-pv/lvm-vg format. I imagine I can...
A snapshot partition is indistinguishable from the
same partition before the snapshot. So go nuts.
> That being said, I have found that at least on my slow machines,
> lvm-snapshots add a significant slowdown tax...
It's not a "slow machine" thing, it's a
spindle thing. If you put the log volume on a dedicated spindle (and
*not* a crummy USB drive!), the performance is high, even on slow machines
(such as a VIA EPIA system).
Most people think, "Oh look: I'll keep
some space free at the end of my VG for snapshots!" Epic fail,
basically no matter *how* fast the hardware is... unless you've got
a full SAN below it--in which case, why are you screwing around with LVM?!?
That's what your NetApp Filer/EMC Clariion/whatever is for! :)
Timothy J. Massey
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