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тАО08-21-2007 06:03 AM
тАО08-21-2007 06:03 AM
I have a DLT7000 using fbackup for system backup to a single tape. fbackup reports that 200461040 blocks have been written to tape. It is my understanding that a DLT7000 has a 35/70 GB capacity. When I do the math, 200461040 x 512bytes = 102.6 GB. How can that much data fit onto a DLT7000?
Thanks,
Randy
Solved! Go to Solution.
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тАО08-21-2007 06:05 AM
тАО08-21-2007 06:05 AM
Re: What is the capacity of DLT7000 tape drive.
Take a look at this thread, following it through to the thread that Old School recommends:
http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=1153430
Pete
Pete
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тАО08-21-2007 06:11 AM
тАО08-21-2007 06:11 AM
SolutionI can confirm that a DLT7000 is supposed to hold 35GB native, which would be 70GB at full compression.
Pete
Pete
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тАО08-21-2007 06:13 AM
тАО08-21-2007 06:13 AM
Re: What is the capacity of DLT7000 tape drive.
Depending on what you were backing up, you could get this level of compression pretty easily.
Were these maybe text files with a lot of white space in them? Binary database files with a lot of preallocated pre-zeroed file space in them perhaps? Those are just two examples of files that would highly compress, but there are lots and lots of others.
Read up on Huffman encoding -
http://en.wikipedia.org/wiki/Huffman_encoding
for a lite explanation of an example compression method, and you'll get the idea of how this can be conceptually accomplished, at least through one type of compression method anyways.
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тАО08-21-2007 06:14 AM
тАО08-21-2007 06:14 AM
Re: What is the capacity of DLT7000 tape drive.
Now for the "gotcha": The amount of data written to the tape drive (as opposed to the tape itself) could be huge. Consider the case of writing the output of /dev/zero to your DLT7000 with compression enabled. I suspect that the compression ration for a bunch of NUL's would be enormous --- and much, much greater than your observed ~ 100GB.
The 70GiB value assumes a 2:1 compression ratio but the compression ratio is totally data dependent. I've seen data so random that the compressed and native capacity are all but identical; I've also seen many cases which greatly exceed your observed values; it just depends. Some of the most common cases where the data compress very well are those in which large sparse files are in use. Those sparse files resemble the /dev/zero case above and sparse files are quite common (at least initially) for database files.
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тАО08-21-2007 06:17 AM
тАО08-21-2007 06:17 AM
Re: What is the capacity of DLT7000 tape drive.
For lots and lots of white space the compression would be higher ? right ?
E.g. Assume a 100GB freshly formatted empty database. This most likely will fit on a 35/70GB tape with room to spare.
Another item. Is fbackup reporting disk blocks that it backed up ? If so the 102.6 GB may just be 102.6 / 2 if achieving 2:1 compression ?
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тАО08-21-2007 06:26 AM
тАО08-21-2007 06:26 AM
Re: What is the capacity of DLT7000 tape drive.
Its an attempt of a full backup of a production system with Oracle Database as it only use. So the tape will contain all the root files that the system needs, Oracle binaries and RMAN backup files of the database, control files and archive logs. I can't copy the actual database files, etc., due to fbackup seeing changes with the database up and skipping files.
Thanks,
Randy
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тАО08-21-2007 07:03 AM
тАО08-21-2007 07:03 AM
Re: What is the capacity of DLT7000 tape drive.
As Clay said, the tape stores 35 GB -- no more. Compression does NOT increase the density, but instead uses special codes to represent repeating patterns on the tape. Still only 35 Gb but as the bits are read off the tape, these special patterns expand to produce more data than the actual bits on the tape.
So you're getting 102 GB...tomorrow, it may be 101 GB or even 99 GB. And ss the database nears 100% capacity, your total capacity (for a single tape) may be only 60Gb or perhaps as little as 40 GB. So the ONLY number you can depend on is 35 GB.
Bill Hassell, sysadmin
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тАО08-21-2007 06:52 PM
тАО08-21-2007 06:52 PM