Tape dataset size



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Tape dataset size

Postby Srini Srini » Sun Jun 06, 2010 5:56 pm

Hello,

How can we calculate the tape dataset size? Any formula to calculate it from block size and record length of the tape dataset?

Thanks
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Re: Tape dataset size

 

Re: Tape dataset size

Postby Robert Sample » Sun Jun 06, 2010 8:22 pm

What do you mean by "tape dataset size" -- length of the tape used by the data set, or the number of bytes used by the data set?
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Re: Tape dataset size

Postby Srini Srini » Sun Jun 06, 2010 8:57 pm

I meant the number of bytes used by the data set..

Is there a formula to calculate the number of bytes of the dataset.
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Re: Tape dataset size

Postby Robert Sample » Sun Jun 06, 2010 9:49 pm

The number of blocks times the block size will give you an approximate byte count. The count is approximate because the final block may not be full -- there could be as little as one record in the block. And if you site allows a tape disposition of MOD (not all sites allows this), there could be multiple short blocks on the tape. Most tape management systems will allow you to find out how many blocks of data were written to the tape.
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Re: Tape dataset size

Postby Srini Srini » Sun Jun 06, 2010 10:40 pm

Thanks. Does Record lenght have a role in deciding the size of tape dataset?
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Re: Tape dataset size

Postby dick scherrer » Sun Jun 06, 2010 10:50 pm

Hello,

A qualified yes. . . The size of 1million 100-byte records will be far less than 1million 4000-byte records.

However, this is included in the info Robert posted. Records live in blocks. . .

Suggest you talk with your storage management people as many tape management systems have info you can use.

If you have SAS, there is a way to do this with SAS also.
Hope this helps,
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Re: Tape dataset size

Postby Srini Srini » Sun Jun 06, 2010 11:05 pm

ok.. Thanks..
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Re: Tape dataset size

Postby steve-myers » Thu Jun 17, 2010 5:54 am

The problem is there is no truly meaningful answer to the question as posted for tape drives that do data compression, or for "virtual" tape. It is not widely known, but tape data compression, aside from possibly compressing the data as it is sent to the drive effectively wipes out your DCB parameters. The actual data is written in very long physical blocks, so there are fewer interblock gaps, a major contributor to tape drive usage, and especially with modern "streaming" tape drives, poor I/O performance. Another issue is a lot of tape usage these days is virtual tape. Several years ago my knuckles got rapped when I was trying to use real tape to measure real tape performance, rather than the "approved" virtual tape.

Still, knowing the number of blocks and their size can give you a rough estimate of the amount of data on the tape.
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