Re: Virtual tape

Parallel save to tape is available [from BRMS] to spread save activity across multiple [virtual] tape devices. Two forms of parallel save are available; one is a serial format for the data [labels] across the different media devices and the other a parallel format where the data for the labels is spread across different media devices. Searching tokens PARALLEL SAVE in InfoCenter returns among others, the following link:
http://publib.boulder.ibm.com/infocenter/iseries/v5r4/topic/rzaiu/rzaiusavinglibraries.htm

If there is not a matching physical drive for each virtual, only the serial format is reasonable. Outside of a utility like BRMS, listing the order of the labels to enable consecutive restore would be prudent, if fewer physical devices might be available than the number of virtual devices used for the save.

As for storage, the virtual tape device provides no data compaction because there is no hardware, but the Data Compression DTACPR() parameter enables software compression which can make the saved library or object significantly smaller [in its offline size] than its online size, but at the cost of the CPU to effect that compression. The offline size for an object is presented in DSPOBJD output. Other than the ability to save multiple libraries on a virtual tape versus only one on library into a save file, because the compression is in software, there should be little difference between the two. See the DTACPR parameter on SAVLIB for example:
http://publib.boulder.ibm.com/infocenter/iseries/v5r4/topic/cl/savlib.htm

Regards, Chuck

John McKee wrote:

My question was well answered. I knew that both virtual tape and save files took up space. Obviously, for decent performance, a separate ASP is almost mandatory. What I had wondered was whether multiple processes could be used to speed up backup. I was thinking that critical libraries could be backed up to a SAVF and later transferred to tape with the system being available for users. Short answer is >maybe<, as availability of DASD is still a factor, as I thought surely it would be. Years ago, I wrote a CL that prioritized
the libraries to be backed up. Once the high priority libraries
were safely on tape, other libraries were kept locked and users were allowed back in. As each remaining library was backed up, the lock on
the library was released. That did cut downtime considerably, on
that ancient system. <<SNIP>>

John McKee wrote:

To back up 100G of DASD to virtual tape, would you need 100G of
DASD for the virtual tape? Or possibly a better question: Does
virtual tape utilize data compression, thus not requiring quite as
much space as if data was simply copied disk to disk?

How would the size be calculated?

How does virtual tape compare to using a save file or multiple save
files?