Question & Answer
Question
Why would the data set size be different when dumping the same data or volume to DASD vs tape?
Cause
When dumping the same size data set or volume, the output data set will not necessarily be the same size when going to dasd vs. tape.
There are several factors, including blocksize of data being written to tape vs. dasd, the type of dataset being written, to name a few.
Answer
When the target data set is on DASD, the size of blocks being written, affect how efficiently the space on each track is utilized. Factors such as, how many blocks can be written to each track, influence how much space on the DASD volume will be used.
Dumping the same dataset to tape may result in a much different size target data set because DFSMSdss uses a default blocksize of 256K. This default can be changed. For this discussion, we will limit it to the default setting.
Using an example where a 500 byte data set is being dumped results in single 256K block to be written to tape. If a second 200 byte data set is also dumped to the same tape, it may appear as though it is written to a second 256K block. The actual layout on tape is one 500 byte data set, followed by another block with 200 bytes.
These same two datasets dumped to DASD if back to back, may only take up 2 tracks on DASD. It may seem that a lot more space is being used by tape vs. DASD, but in actuality, they may be close in actual space used. It depends on how this is being reported.
Additionally, there are added gaps on tape ( headers, trailer info, etc.. ) that also make a difference, but this can be negligible.
So one cannot reasonably expect to compare DFSMSdss dump output to use the exact same 'space' on DASD as it would when dumped to tape. More often than not, it would appear the tape usage is a bit larger. Again, this also depends upon the reporting mechanism being used.
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Document Information
Modified date:
02 May 2023
UID
ibm16987935