On 05/21/2013 08:55 AM, Charles Polisher wrote:
On Sun, May 12, 2013 at 05:01:39PM -0400, John Stoffel wrote:
Edward> Focus on reliability. ;-)
No, you want to focus on restorability of your data in face of
disaster. It does no good at all to have a reliable backup system if
you can't get the data out.
I don't recall seeing upthread mention of the purported BER of
LTO tapes: 1 error in 10^18 bits. I wish my experience confirmed
this. Recent attempts to restore files (HP tape library, LTO-4,
Backup Exec) resulted in a large number of tapes declared un-
catalogable. I never recovered the data. I do not believe this
reflects the underlying reliability of all tape. I'm a tape
believer. But not in the tape systems in my shop. Possible
contributing factors: lossy optical fiber links, equipment
maintenance issues, unlimited tape lifetimes. YMMV.
The LTO BER is based on a number of fairly ideal environmental
assumptions (temperature, humidity, maximum acceleration, etc.). It also
assumes no problems with the drives themselves. Of our three spats of
bad tapes, we've encountered the following:
1. We had a set of bad tapes that were all present in a single offsite
container shortly before the errors occurred. Our assumption is that the
offsite courier dropped the container.
2. We also did not understand how to implement drive auto-cleaning in
our new STK libraries. Back when we had a Spectra Logic library, our
backup software (TSM) would pick up on when our drives needed to be
cleaned and would clean them automatically. When we got our new STK
libraries, we kept that configuration. It turns out, however, that STK
libraries do not report drive cleaning status back to software and
handle cleaning internally. We had more and more drives reporting SCSI
errors back to TSM, and when we dug through the STK error/warn logs we
discovered that they were all indicating they needed cleaning. Once we
setup internal library cleaning, these problems went away.
3. We had one batch of bad tapes. Out of 400 tapes in the batch, 12 of
them so far have had problems (this is about 10x worse than our tape
population as a whole). We've bought lots of tape from this vendor and
have had no other problems, so we're assuming there's some manufacturing
defect with them.
Skylar
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