"Jeffrey J. Kosowsky" <[email protected]> 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" <[email protected]> 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
Out of the Box Solutions, Inc.
Creative IT Solutions Made Simple!
http://www.OutOfTheBoxSolutions.com
[email protected]
22108 Harper Ave.
St. Clair Shores, MI 48080
Office: (800)750-4OBS (4627)
Cell: (586)945-8796
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