A DLT unit on /dev/sa0 is really slow

David Landgren david at landgren.net
Tue Aug 26 01:20:44 PDT 2003


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.

All of which makes me think that something is going unspecified 
somewhere. Something like a preferred block size?

'mt stat' produces the following:

Mode      Density              Blocksize      bpi      Compression
Current:  0x40                 variable       0        IDRC
---------available modes---------
0:        0x40                 variable       0        IDRC
1:        0x40                 variable       0        IDRC
2:        0x40                 variable       0        IDRC
3:        0x40                 variable       0        IDRC
Current Driver State: at rest.
File Number: 0  Record Number: 0        Residual Count 0

Can I manually configure a mode that would have better performance 
characteristics? Thanks for any tips you might have,


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