W dniu 2012-01-22 05:21, Joel C. Ewing pisze:
I've said it before, but will say it again: modern tape media has such a
large capacity that a single dfhsm cartridge can contain an incredibly
large number of datasets. It is almost inevitable that loss of a single
dfhsm ML2 or Backup cartridge will impact something you care about or
can't afford to lose (or are legally required to retain), and it is also
inevitable that single cartridges will occasionally be physically
damaged. Over the long haul, you really can't afford NOT to duplex all
dfhsm carts (and for that matter, non-dfhsm carts that contain data you
can't afford to lose).
90% Agreed, some remarks:
1. Cart capacity is not so relevant. The problem exists with any cart
capacity, the difference is in a likehood, but the likehood is quite
similar (same order of magnitude).
2. It worth to separate two cases:
a) Backup copy loss. It's more or less like loss of spare in a RAID
group, you did not lose your data, you lost redundancy. Additional event
must happen to make backup copy needed for recovery. Of course there
could be some legal requirement to have such copy, in such case he
requirement is broken.
b) Archive copy, ML2 (without backup). In such case lost of cartridge
simply means lost of data.
I always vote fo duplexing data on tapes, for the same reasons why we
use RAID protection on DASD. I mean real media, for example virtual
tapes are usually on RAID-protected disks.
--
Radoslaw Skorupka
Lodz, Poland
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