Operating System - OpenVMS

Re: Directory listing of large files on large tapes

 
AEFAEF
Advisor

Re: Directory listing of large files on large tapes

If IIRC: DIRECTORY is of no help if hardware compression was used. It gives the same size for a given save set whether or not it was compressed (or compacted, whatever). So, the compression is transparent. This makes sense: The tape drive decompresses during read and gives the result to the process. So DIRECTORY is looking at the uncompressed sizes.

I'm curious, though, as to what happens to the resulting block structure on the tape. Are the compressed blocks all separate (and of varying size, of course) or do they span across post-compression fixed-size tape-drive blocks?

You'd still want to keep the block size .LE. 32256 in case you want to copy the save set to a disk.

AEFAEF
GuentherF
Trusted Contributor

Re: Directory listing of large files on large tapes

All modern tape drives use a fixed block size which has nothing to do with the block size the a user/progam specifies. Within each block there is meta data and the block is filled with records. A tape mark for example is a record with a type of "tape mark" and zero length data. Most drives keep a directory of where each internal block is on tape and allow you quasi random access to these blocks. So a compressed user block can span over more than one on-tape-block. Some SCSI tape drives can return an estimate of bytes left on a cartridge. But the returned number is for uncompressed data.

Only encrypting and compressing a file to disk first an then copying to tape gives you reliable numbers.

Unfortunately OpenVMS BACKUP does not compress (undocumented feature) if encryption is used at the same time.

/Guenther
Hoff
Honored Contributor

Re: Directory listing of large files on large tapes

> Only encrypting and compressing a file to disk first an then copying to tape gives you reliable numbers.

Encrypted data doesn't compress.

Always compress before you encrypt.
Korendyk
Advisor

Re: Directory listing of large files on large tapes

Thanks to all for your comments, it was enlightening. I was able to easily adapt the current procedures to remember saveset sizes, so the procedures and processes used over the last 15 years remain essentially the same: guess at how much of the tape's capacity is being used and decide on whether to move on to the next tape. Older tapes are still handled properly, and so everyone is happy.

\bill