Up and again,
a little more testing showed that "compression enabled" is slower and
fits *less* data than having it disabled.
I dumbly didn�t save the screen results, but I can repeat the tests
and post the exact messages, if needed. For now, what I can say is
compression dropped the transfers from about 11,8 MB/s to 10MB/s and
reduced the capacity from about 95GB to 85GB. I repeated 2 times each
test, with little variation, and maintained block sizes of 64kb,
filtering with dd:
To store:
# dd if=/home/bkp/backup_050425.tar of=/dev/esa0 bs=64k
To restore:
# tar -b 128 -xvOf /dev/esa0 | dd of=/dev/null
Note: I don�t know if the tape rewinding+ejection time counted in the
total time. I used /dev/null to be sure HD performance would not
interfere. In fact, writing to the tape was about 1% slower than reading.
Would this be expectable?
Tulio
Tulio Guimar�es da Silva wrote:
Hi Gary,
ouch! That�s quite disappointing... :( We had already noticed this
kind of behaviour with DDS-* tapes, but we got some progress varying
the block size... and yup, I�m really using gzipped data. :S
For AIT-3, however, i thought this hardware compression was something
about using lower tape�s phisical-rolling speeds or alikes, but I
could never really find anything concrete about the methods... the
only one thing I found was they could use "variable block sizes", but
that�s all. Again, not many details. Anyway, I�m giving up the idea of
compression for now.
If something, I�m noticeing that (at least with -b 10) it becomes (a
lot) slower with time, but I guess this would be more of a question to
the -performance list.
Add: while writing this message, I remembered to check the 700V�s
"Product Specification Manual", and they mention something about
dual-partitions, but it seems something that needs to be implemented
at driver level, since it includes SCSI commands. In this case, I
would need to format the tape as a 2-partition one... any clue about
if and/or how that works on FreeBSD?
Thanks again,
Tulio
Gary Corcoran wrote:
Tulio Guimar�es da Silva wrote:
Hello again,
I�m having some trouble putting a Sony SDX-700V SCSI AIT-3 unit
to work on FreeBSD 5.3-RELEASE.
...
Besides the speed, hardware compression seems to not being
funcional either. I already tried every 4 possible dip switch
setting for compression, but I am still not able to transfer a 180GB
archive to a (should-be) 260GB medium.
I can't help you with most of your problems, but regarding the
"compression"...
I would guess that if you have 180GB to backup, it's not all text. :)
When I last used a tape drive years ago, when writing to a 2GB tape
that would supposedly hold 4GB compressed, I could fit only about 1.9GB
before the tape was full. Turning off hardware compression, I could fit
2GB. The problem was that I was saving already compressed multimedia
files,
and the tape drive's "compression" just added overhead and took up
more space.
So unless you're backing up text or similar files, don't believe the
marketing hype about getting 2x the amount onto your tapes...
Gary
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