[PLUG] Disk backups (was: DLT tape drives)

[Russell Senior]

>>>>> "Keith" == Keith Lofstrom <keithl at kl-ic.com> writes:

Keith> I may be sounding like a broken record here, but consider
Keith> switching to disk-to-disk backup.  Faster, cheaper, more
Keith> reliable, more capacity, less system load, easier swapping.  I
Keith> can do full level-0 equivalents on all the machines on my
Keith> network (80GB worth) in 40 minutes, without needing to bring
Keith> anything down to single user.

You don't need to bring anything down to single user to do tape
backups either, at least not the way I do them (tar).  Whatever you
lose doing "live" backups to tape, you also lose doing "live" backups
to disk.

There are still some advantages to tape, one being size.  The external
drives I just put together occupy about the same space as 32 4mm tape
cartridges.  At DDS3 densities, an 80gig disk in an enclosure loses by
a factor of nearly 5.  Tapes are easier to carry off-site.  Not sure
about your "less system load" or "easier swapping" claims either.
Color me sceptical.  _Less_ swapping, certainly.  But otherwise, I am
drifting inexorably towards disk backup too, mostly because practical
tape solutions are too expensive.  I am guessing that an IDE drive is
probably able to survive heat (e.g. from a fire) better than a tape
cartridge also.

BTW, Dirvish is using the same strategy that I invented independently
(and that probably existed long before I thought of it) two years ago:
basically, exploiting hardlinks to save disk space on unchanged
portions of snapshots.  Even before rsync got the explicit support for
doing this, you could fake it by first doing a tree copy using
hardlinks:

  cp -al <src> <dest>

where <src> was the last snapshot, and then rsyncing a new backup to
<dest>.  Since rsync 2.5.6, you can just use the --link-dest option to
do effectively the same thing in one shot.

The tricky thing about these is keeping people from changing anything
in the mounted snapshots.  Because they are hardlinks, a write to the
shared inode changes _all_ the snapshots.  Allowing the file system to
be accessible for reading/recovery and writable for making new
snapshots at the same time is a challenge.

Keith> I will have some Exabyte drives for sale (cheap) pretty soon
Keith> now.

Let me know, I may be interested.


-- 
Russell Senior         ``shtal latta wos ba padre u prett tu nashtonfi
seniorr at aracnet.com      mrlosh''  -- Bashgali Kafir for ``If you have
                         had diarrhoea many days you will surely die.''