dd blocksize for DLT tapes

[Stuart Abramson]

I'm copying a big file (3 GB) to DLT tape with "dd", and it's taking forever. Is the problem that I am using the default blocksize, 512, and writing a lot of short blocks?

Should I increase the blocksize with "bs=". How do I know what to use?

[A. Clay Stephenson]

Hi Stuart,

I would bump the obs up to about 1000k as long as the obs is a multiple of your ibs everything will work fine.

Clay

[Stuart Abramson]

I don't know what my input block size is?

[A. Clay Stephenson]

Hi Stuart,

One other utility you might look into, if it is Hp to HP is ftio; it forks into multiple processes so that reads and writes are going on simultaneously and you can set the block size on the command line. Man ftio for details.

Clay

[A. Clay Stephenson · Accepted solution]

Ok Stuart,

The ibs by default is 512. You can do this:

dd if=myfile ibs=5k obs=1000k of=/dev/rmt/2m

This will work fine and if the last input block is a partial read then it will take care of it.

One thing to be aware of, is that dd may not copy the file with the exact output size; on some implementations it made pad the last output block with NUL's. Normally this does no harm. That's why I would suggest you use ftio -
it takes care of file ownerships and permissions and lengths.

Clay

[Carlos Fernandez Riera]

If your file is a raw device use rlvolxx instead lvolxx.

When using ibs and obs option whit the same size it is better do use bs (both ibs and obs).

Recommended bs for DLT is 68k .