A DLT unit on /dev/sa0 is really slow

Dan Nelson dnelson at allantgroup.com
Tue Aug 26 07:15:03 PDT 2003

In the last episode (Aug 26), David Landgren said:
> I have an external DLT tape (a COMPAQ DLT VS 40/80 5133 according to
> camcontrol). It seems glacially slow to me. Last night I backed up
> many gigabytes of files from a remote machine over NFS. It took over
> five hours to do it. My overworked Sun E-450 will do this in a shade
> over three hours.
> What is more, the remote machine and my FreeBSD box have gigabit
> Ethernet adapters, and both machines, as well as the switch they are
> connected to all report 1000baseT full duplex. The sun, on the other
> hand, is running 100baseT.
> When I run a "tar t" to list the contents of the archive, the output
> is really slow. It's taken about half an hour so far to list the
> first 50 or so files (and none are exceptionally big, 10Mb on
> average).
> When I listen to the tape drive, I hear it start and stop frequently,
> with lots of pauses. I would have expected it to spin continuously.
> It is, after all, a *streaming* device.

DLTs will only stream if they are fed data at a fast enough rate;
otherwise they shoeshine.  Try running with a 64k blocksize instead of
tar's default 10k ( tar cb 128 ) .
	Dan Nelson
	dnelson at allantgroup.com

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