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Re: Escaping Data Rot
Steven Hirsch wrote:
D Finnigan wrote:
Michael J. Mahon wrote:
A corollary is that the best backup medium for a hard disk is another
hard disk. It is much cheaper and infinitely more convenient to do a
disk-to-disk copy than to play with changing removeable optical disks.
Agreed. I've always had the notion that the safest place to store digital
data is on a hard disk system. It's sealed, secure, and high-capacity. Of
course, tape can have a nice long life-span as well.
No spinning storage device comes within an order-of-magnitude of
commercial tape media (e.g. LTO-2,3,4) in terms of durability and
lifespan. There's a good reason that literally all enterprise-level
backup schemes use tape libraries instead of a rack of drives.
Of course, my off-site backup drives are not spinning... ;-)
And the lifespan of tapes is greater than the lifespan of the
technology used to record them! No debate on that point. But
the only real "archival" storage technique is frequent recopying
onto technologically current media--whether disk or tape--so the
actual life of the media is not a determining factor.
Hard disks come with their read/write technology, so the only
technology "generation" issue is their attachment--ATA for decades,
only now transitioning to SATA--clearly destined to be pervasive for
decades to come.
Prices for used LTO-2 drives and tapes have hit the "sweet spot" in the
past year. I can backup my entire server (300GB RAID-5 array) on a
single tape in less than an hour. The tape goes in my briefcase and
travels to the office with me so there's always an off-site copy.
How about 1.5TB? That fits on one <$90 drive and backs up in under
two hours. And I *always* have three copies of my data, so I can
tolerate the rare failure of a single drive. (A multiple-drive
failure, if they are stored apart, is astronomically unlikely.)
Educate me on the costs: How much for a drive? How much for each tape?
What is the "sweet spot" tape capacity for incompressible data?
I presume that you read back after each backup to verify that both drive
and tape are actually working? (A software division of my company also
used tape backup, and found that for six months, they had been making
unreadable backups!) This is a case where just making multiple backups
doesn't solve the problem!
Reading back is generally not necessary for hard disk copies, since the
necessary updates to the file system data structures during a copy
ensure that the drive is both reading and writing correctly.
I can't help but think that the practices of large IT departments are
strongly shaped by tradition (to put it politely). It seems to me that
most high-availability database systems would be best backed up using
a "floating triple-mirror" scheme, in which the system is never off-line
and the backups are all onto (other) hard disks on off-site mirrored
systems. A journaled database and file system mesh beautifully with
this approach.
The enterprise server backup problem is also functionally quite a bit
different from the file archive backup problem. Backups are generally
used for two things: to recover files lost to "operator error", and
to restore a system database to a known-good point after a severe
software or hardware failure, or for hardware upgrade.
The inherent "browsability" of a hard disk backup makes it very well
suited to the recovery of particular files or groups of files, and to
periodic "syncing" of file groups, which is a good match to the way
individual users maintain their files.
I'm sure tapes can be used in this way, but this is not the typical
"backup" scenario. Certainly data cannot easily be updated in place.
-michael
NadaNet and AppleCrate II: parallel computing for Apple II computers!
Home page: http://home.comcast.net/~mjmahon
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