From owner-freebsd-questions  Sun May 27  9: 7:26 2001
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Date: Sun, 27 May 2001 09:07:22 -0700 (PDT)
From: Felix Hernandez <felix_hdez@yahoo.com>
Subject: Re: Slower tape drive when compression off
To: Dan Nelson <dnelson@emsphone.com>
Cc: Bill Moran <wmoran@iowna.com>, freebsd-questions@FreeBSD.ORG,
	Ian Dowse <iedowse@maths.tcd.ie>
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--- Dan Nelson <dnelson@emsphone.com> wrote:
> In the last episode (May 26), Felix Hernandez said:
> > root@oberon $$$ mt comp off
> > root@oberon $$$ dd if=junk of=/dev/rsa0 bs=10k
> > 51200000 bytes transferred in 20.322325 secs
> (2519397 bytes/sec)
> > 
> > root@oberon $$$ mt comp on 
> > root@oberon $$$ dd if=junk of=/dev/rsa0 bs=10k
> > 51200000 bytes transferred in 12.665153 secs
> (4042588 bytes/sec)
> 
> That's really weird.  A 10k blocksize is pretty
> small for DLT, though
> (it's the default tar blocksize).  

Yes!!! Right on the nail! It was a buffer size
problem.

root@oberon $$$ mt comp off
root@oberon $$$ dd if=junk of=/dev/rsa0 bs=10k
5000+0 records in
5000+0 records out
51200000 bytes transferred in 22.686704 secs (2256829
bytes/sec)
root@oberon $$$ dd if=junk of=/dev/rsa0 bs=64k
781+1 records in
781+1 records out
51200000 bytes transferred in 9.491773 secs (5394145
bytes/sec)
root@oberon $$$ dd if=junk of=/dev/rsa0 bs=256k
195+1 records in
195+1 records out
51200000 bytes transferred in 9.497948 secs (5390638
bytes/sec)

>Try with a 64K
> blocksize, and tell
> us your CPU speed.  

PIII Xeon 700 MHz.

>A DLT8000 should be able to
> stream data at 6MB/sec
> native, 12MB/sec if the data is compressible.  Maybe
> your hardware
> isn't capable of sending data to the DLT fast enough
> to keep it
> streaming.  Also see what a "dd if=/dev/zero
> of=/dev/rsa0 bs=64k" does;
> maybe your disks are the bottleneck.  

root@oberon $$$ mt comp off
root@oberon $$$ dd if=/dev/zero of=/dev/rsa0 bs=64k
^C28840+0 records in
28839+0 records out
1889992704 bytes transferred in 310.885445 secs
(6079386 bytes/sec)

For completion, I also tried another idea, that
confirms that the source of the problem was buffering:

root@oberon $$$ /usr/bin/time buffer -B -z10k < junk >
/dev/rsa0
     50000K
       22.44 real         0.01 user         0.73 sys
root@oberon $$$ /usr/bin/time buffer -B -z64k < junk >
/dev/rsa0
     50048K
        9.52 real         0.00 user         0.46 sys
root@oberon $$$ /usr/bin/time buffer -B -z256k < junk
> /dev/rsa0
     50176K
        9.50 real         0.00 user         0.41 sys


Thank you very much, Dan, Ian and Bill!

Felix Hernandez.


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