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Date: Sat, 26 May 2001 18:11:54 -0400
From: Bill Moran <wmoran@iowna.com>
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To: Felix Hernandez <felix_hdez@yahoo.com>
Cc: freebsd-questions@FreeBSD.ORG
Subject: Re: Slower tape drive when compression off
References: <20010526191056.97775.qmail@web9502.mail.yahoo.com>
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Felix Hernandez wrote:
> 
> Hi,
> 
> I have a Quantum DLT8000 tape drive (40/80 GB), and
> I'm puzzled by the following fact: writing the tape in
> compressed mode (mt comp on) is faster (4 MB/s) than
> in uncompressed mode (mt comp off, 2 MB/s). I use tar
> (1.13) for my backups, and the tape is attached to an
> IBM Netfinity 7100 running FreeBSD 4.1. I have already
> tried a large and fixed blocksize (mt blocksize 10240,
> tar -b 20), but it didn't help. Do you know why this
> happens? How can I fix it? I don't want to use
> compression, since the data is already gzipped, and
> recompressing it wastes 5 GB.

I'm a little confused.
You say your compressing the data with tar and then using compression on
the tape drive as well?
If that's the case, I don't know what the issue is.
If you're piping the data through raw and compression is faster, it's
probably because it's measuring the data rate before it compresses the
data, and then gets about 50% compression. The slow performer in any of
these cases is going to be the tape itself.

-Bill

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