Date: Sat, 26 May 2001 15:58:55 -0700 (PDT) From: Felix Hernandez <felix_hdez@yahoo.com> To: Bill Moran <wmoran@iowna.com> Cc: freebsd-questions@FreeBSD.ORG Subject: Re: Slower tape drive when compression off Message-ID: <20010526225855.16007.qmail@web9506.mail.yahoo.com> In-Reply-To: <3B102A2A.8AF59769@iowna.com>
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--- Bill Moran <wmoran@iowna.com> wrote:
> 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.
The original data is already compressed, so
compressing it again only makes it larger. I just
tried an experiment, suggested by Ian Dowse, in which
I only use dd, so we can discard tar as the source of
the problem:
root@oberon $$$ dd if=/dev/urandom bs=10k count=5000 >
junk
5000+0 records in
5000+0 records out
51200000 bytes transferred in 41.783845 secs (1225354
bytes/sec)
root@oberon $$$ mt comp off
root@oberon $$$ dd if=junk of=/dev/rsa0 bs=10k
5000+0 records in
5000+0 records out
51200000 bytes transferred in 20.322325 secs (2519397
bytes/sec)
root@oberon $$$ mt comp on
root@oberon $$$ dd if=junk of=/dev/rsa0 bs=10k
5000+0 records in
5000+0 records out
51200000 bytes transferred in 12.665153 secs (4042588
bytes/sec)
("junk" cannot be compressed, since it is completely
random -- it only gets larger after gzipping it)
The output of "mt status" is:
Mode Density Blocksize bpi
Compression
Current: 0x41 variable 0
IDRC
---------available modes---------
0: 0x41 variable 0
IDRC
1: 0x41 variable 0
IDRC
2: 0x41 variable 0
IDRC
3: 0x41 variable 0
IDRC
---------------------------------
Current Driver State: at rest.
---------------------------------
File Number: 0 Record Number: 0
Thank you for your quick reply. I hope someone can
help figure this out.
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