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What is the capacity of DLT7000 tape drive.

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Randy Hagedorn
Regular Advisor

What is the capacity of DLT7000 tape drive.

Hi,

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
8 REPLIES
Pete Randall
Outstanding Contributor

Re: What is the capacity of DLT7000 tape drive.

Randy,

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
Pete Randall
Outstanding Contributor
Solution

Re: What is the capacity of DLT7000 tape drive.

Now that I re-read your question, I see that you're not trying to get the full capacity, you're wondering how you got more than the full capacity. Hmmmmm . . . .

I can confirm that a DLT7000 is supposed to hold 35GB native, which would be 70GB at full compression.


Pete

Pete
TwoProc
Honored Contributor

Re: What is the capacity of DLT7000 tape drive.

Randy,

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.
We are the people our parents warned us about --Jimmy Buffett
A. Clay Stephenson
Acclaimed Contributor

Re: What is the capacity of DLT7000 tape drive.

The capacity of a DLT7000 DLT-IV tape is 35GiB, period. You cannot get more data on the tape than this.

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.
If it ain't broke, I can fix that.
Tim Nelson
Honored Contributor

Re: What is the capacity of DLT7000 tape drive.

The 70GB compressed is assuming 2:1 compression.

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 ?

Randy Hagedorn
Regular Advisor

Re: What is the capacity of DLT7000 tape drive.

OK .. to answer the question of what kind of data the tape contains.

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
Bill Hassell
Honored Contributor

Re: What is the capacity of DLT7000 tape drive.

And with that Oracle backup will come large data files that are probably unpopulated. Like most database, the data files start out empty, perhaps as a string of zeros, then records are written to various part of the files, but still with lots of free space full of zeros. Under these conditions, your DLT7000 will hold 500 BG to maybe as much as 2000 GB of data. But every day, as the database begins to store records, the total storage capacity of the DLT will seem to decline because there are fewer compressible records.

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
tkc
Esteemed Contributor

Re: What is the capacity of DLT7000 tape drive.

i believe if you compress or zip the file first before backing up, you won't get the 102.6GB number or even near to 70GB.