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How much compression in a tape dump?

3 posts in Backup and Recovery Last posting was on 2009-05-11 17:29:58.0Z
John Flynn Posted on 2009-04-17 16:03:52.0Z
From: "John Flynn" <jflynn@miqs.com>
Newsgroups: sybase.public.ase.backup+recovery
Subject: How much compression in a tape dump?
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Hi.

I'm on Solaris ASE 15.0.2 ESD #6 and I have just started experimenting with
the compression option in my dumps. I am using the "with compression="
syntax.

When I am dumping directly to a tape, how can I determine how much
compression has occurred? First I was surprised to see that the backup log
only seems to give me the size of the database that was dumped. Okay, maybe
the backup server has no access to the information about how many bytes were
actually written to the physical tape. But then I tried load with listonly
and headeronly and was again surprised that the size of the volumes/files on
the tape did not seem to get reported. But maybe I just don't know how to
read that output.

I assume if I'm dumping to disk I could simply look at the size of the
resulting file. How do I get this same info from a tape dump?

Thanks.
- John.


Bret Halford [Sybase] Posted on 2009-04-17 19:14:29.0Z
From: "Bret Halford [Sybase]" <bret@sybase.com>
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Subject: Re: How much compression in a tape dump?
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John Flynn wrote:
> Hi.
>
> I'm on Solaris ASE 15.0.2 ESD #6 and I have just started experimenting with
> the compression option in my dumps. I am using the "with compression="
> syntax.
>
> When I am dumping directly to a tape, how can I determine how much
> compression has occurred? First I was surprised to see that the backup log
> only seems to give me the size of the database that was dumped. Okay, maybe
> the backup server has no access to the information about how many bytes were
> actually written to the physical tape. But then I tried load with listonly
> and headeronly and was again surprised that the size of the volumes/files on
> the tape did not seem to get reported. But maybe I just don't know how to
> read that output.
>
> I assume if I'm dumping to disk I could simply look at the size of the
> resulting file. How do I get this same info from a tape dump?
>
> Thanks.
> - John.

I asked a resident guru, who suggested you might be able to use
"dd" to read the tape (directing read contents to /dev/null) to
get the file size in blocks. Each dump consists of 3 files - a short
header, the main body, and a short trailer.

-bret


John Flynn Posted on 2009-05-11 17:29:58.0Z
From: "John Flynn" <jflynn@miqs.com>
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Subject: Re: How much compression in a tape dump?
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Article PK: 48056

Does anyone have experience in using the compression option in ASE15.0.2
when writing to tape? Especially when appending multiple separate dumps to a
single tape?

Before I started playing with the compression option, I had a database that
barely fit on a 40Gb tape. Sometimes it fit and sometimes it didn't, which I
assume was because the exact size of the database and its transaction log
varied somewhat from day to day.

I then started using "with compression=1". But this doesn't seem to have
resolved the problem. Just today I got a message from Backup Server saying
"Volume already contains 41135442 kilobytes, which is larger than the
specified capacity of 40960000 kilobytes." It sure seems like the tape is
still filling up, which means that the compression must not be happening.

Unfortunately I still don't know a way to find out how many bytes are being
written to the tape. I've tried the "dd of=/dev/null" method, but it took
many extra hours to read off the dump file, which confuses me, and then I
didn't know how to interpret the final number that I got. When I dump to
disk with compression=1, I can tell that my final result is very much
smaller than the uncompressed dump. So I assume the same thing *should* be
happening when writing to tape.

Thanks.
- John.