Tape Backup Prices

[Lennart Sorensen]

On Tue, Nov 30, 2010 at 11:36:37AM -0500, Digimer wrote:
> 800gb native or 2:1 compressed?

Native.

> My biggest problem with tape has always been their tendency to silently  
> fail. Sure, doing monthly mock restores help to catch this, but clients  
> rarely remember to do so.

LTO has the read head behind the write head and does an immediate
read verify as part of the write cycle.  So no silent failures on LTO.
In fact if will write out extra error recovery code and copies of the
data if it detects small read errors on the tape.

DDS/DAT tapes on the other hand do fail a lot, and silently.  Most people
don't do read after write checking because it doubles the backup time
(LTO was VERY clever in their drive design), and being helical scan there
is lots of wear on the tape and drive head.  LTO is linear and has almost
no wear on the tape and head.  Tapes last much longer on LTO and rarely
fail, and because you can even deal with bad spots on a tape transparently
at write time, the tape really does stay useable much longer.

One thing you can't do with LTO is bulk erase.  There are permanent
servo tracks on the tape that the bulk eraser gets rid off turning the
tape useless.  Very effective to permanently erase your tape, but you
aren't able to ever reuse it again.  I have a box sitting on my shelf
that says "Do not degauss this package!" on it.

> These days, I'd recommend an external drive carrier with eSATA for  
> backup. You should easily get 40+MB/sec. Use rsync and backups happen  
> very quickly. If you really want speed, then install an SSD into the  
> eSATA carrier. Even modest SSDs boast ~200MB/sec write speeds.

eSATA is certainly nice.  USB is too slow to compare with decent tape
drives.  rsync is nice too, but it does mean that as a trade off in
making a backup update much much faster, you don't actually check that
the existing data on the drive really is what it should be, unless you
tell rsync to verify everything by reading, although that certainly
slows it down a lot.  That's the tradeoff.

-- 
Len Sorensen
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