[ILUG] blocksize for a DLT drive?

[John P. Looney]

On Thu, Dec 05, 2002 at 10:33:59AM +0000, Ronan Waide mentioned:
> Hi Folks,
> 
> I have a machine here that I want to preserve the last-known-good
> state of in a fashion that Linux can read. As such, I'd planned on 
> 
> dd if=/dev/[disk] of=/dev/st0
> 
> where st0 is a DLT8000 drive. Thing is, it's been doing that all night
> and still is showing no signs of being finished - it's a 35GB
> partition and I assume it's moving one 512-byte block at a time. So my
> question is, what's a good blocksize for the DLT drive? The drive
> itself doesn't report a preferred size, in fact it says "block size:
> 0" when I ask it.

 I got stung on this before. I think the blocksize of GNU tar is 10k.
Solaris tar is 5k. That's what counts, not the underlying blocksize of
the device, as it's a character device, after all.

 What happened me was that I'd a DDS2 tape that had been used in work(for
sun boxes). I was doing a reinstall at home, borrowed the drives and
tapes, and did a backup. Tested the tapes, they were fine. Reinstalled,
and restarted the box, and neither tape was readable. I cursed and cursed.

 I tried to write more to the tape, and was able to read it back fine.
Ejected it, and couldn't read it.

 It turns out, that as the first program to write to the tape (solaris
tar) set the blocksize or something. Then, afterwards gnu tar would write
in the same 5k blocks. If you didn't eject, somehow the OS or something
knew the blocksize, so it was fine.

 I gave tar a -b 10 option and it could read the tapes fine. Alas, I'd
already destroyed the data on one of them learning this. 

 As for your question, tar seems to get on fine with a block size of 10k,
so try that, with dd and see does it speed it up a little. When DD'ing to
compact flash cards over USB, a blocksize of 64k seems to work best.

John