On Tue, Nov 16, 2004 at 11:54:34AM -0500, Michael J. Pawlowsky wrote:
> A long time ago I ran tapetype.
> I left it with the results I received and everything worked fine for the
> past couple of years until today.
>
> The problem is that today I hit the 20Gb limit for data that I want to
> back up.
> tapetype did not take into account the HW compression of the drive.
>
> I'm suppose to be able to get 40Gb on a tape. They are DLT IV tapes.
> So I need to know what do I have to do, to get amanda to recognize that
> the tape drive uses hardware compression.
>
> Any links to the docs that might explain this would greatly be appreciated.
Michael,
It should be clearer in the docs that use of the tapetype program
(now amtapetype) is only of value if you are using software rather
than hardware compression (you should not use both although on
newer drives it doesn't hurt as it once did).
So, if you are using software compression and were using a
tape capacity value of approximately 20GB, amanda was doing
the right thing all along. And it was typically getting a
lot more than 20GB of data on to the tape. The 20GB limit
it would use was a post-compression value. Some of my DLE's
compress to less than 20% of their original value. If that
was true for all my data then over 100GB of data would fit
in the tape's 20GB capacity.
But remember, that DLT tape can only hold 20GB of actual data,
(0 and 1 bits). How much of your disk data those 20GB represent
depends on the ability of the compression routine (software
or hardware) and the compressibility of the data itself.
Relatively random data, eg. compiled programs, databases,
already compressed files, encrypted files, images, etc. will
not compress very much. Other data, notably text files, will
compress a great deal, often much more than 50%.
Your DLT tape does not hold 40GB of data. It holds 20GB of
data and your vendor estimates (actually guesses) that your
data will be a mixture of poorly compressible data and highly
compressible data so that it compresses on average 50%, thus
the 40GB number.
You may wish to use hardware compression, there are valid
reasons to do so. If you do, just replace the tape capacity
given to you by tapetype with your GUESS as to how much your
data will compress. If you think it will compress a lot,
GUESS 60GB or more. If you think your data will compress
poorly GUESS 25GB or 30GB. See, it will just be a GUESS,
not a measurement.
--
Jon H. LaBadie jon AT jgcomp DOT com
JG Computing
4455 Province Line Road (609) 252-0159
Princeton, NJ 08540-4322 (609) 683-7220 (fax)
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