Optimal tape block size?

Platform: Unixware 2.1, Informix Online 7.22, Compaq Proliant 1500, Archive Python 28388 tape drive. 1 ) How does one determine the optimal block size to use when copying to a tape drive? Informix Online supplies an archive utility `ontape`, which requires a tape block size to be set. When copying to a remote tape drive, I see that `ontape` utilizes the `dd` command. I thought one could experiment with `dd` to try to determine the optimal block size setting, but I have some questions about the input block size vs the output block size. When the Unix vxfs file systems are created I choose 1024 as the file system block size. For the vxfs file system it seems that the "real" block sizes are 512, but the logical block size is 1024. The Informix raw slices use 2048 byte "pages". So: if I'm copying from a Unix file system do I use ibs=1024 or 512? And when copying from an Informix Online slice do I use ibs=2048? 2 ) This from `man dd` is of concern: Do not use dd to copy files between file systems having different block sizes. Does this mean that my ibs has to be set to the same size as my obs??? It seems to say this, but I see differing input and output block sizes all the time. Or is the tape not considered to be a "file system", and the `man` entry does not apply? 3 ) Does the following mean that the safest block size to use when copying out to tape is 512??? The default mode for I/O from any magnetic tape, such as QICtape, 9-track, or DAT is fixed-length blocks which are 512 bytes long. Reading from magnetic tape in any fixed-length block length, besides the block length that the media was written in originally, will cause an I/O error. In order to read a tape that was written using some block length besides the default of 512, use the tapecntl(1) command (qv) to either set the block length of the drive to match the block length of the media, or to set the drive into variable block length mode. dd does not always require block sizes that are in multiples of 512 bytes. Block size is device dependent. If input data blocks are not a multiple of 512, however, the read side will have no error messages, but the write side might have a ``write error'' message. dd transfers correctly if the input data block sizes are a multiple of 512. 4 ) Where does one *learn* about how to use `dd`. I see plenty of references to this command, but nothing that really explicates its workings. The man entry is not too good for this. Thanks for any clarification. -- * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Please reply to Newsgroup. ADDRESS ALTERED TO FOIL SPAMMERS: Remove "*NO-SPAM*" to reply. Cosmo Lee Multi-User Computer Systems Brooklyn, NY "JUST SAY 'NO' TO SPAM" * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *