Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!ogicse!cvedc!mcspdx!adphdw20!dtb
From: dtb@adpplz.UUCP (Tom Beach)
Newsgroups: comp.periphs.scsi
Subject: Re: High Capacity Tapes: Exabyte or DAT?
Summary: not true
Message-ID: <764@adphdw20.UUCP>
Date: 28 May 91 19:05:48 GMT
References: <29543@hydra.gatech.EDU> <9850023@hpcpbla.HP.COM> <1991May28.130950@anusf.anu.edu.au>
Organization: ADP Dealer Services R&D, Portland, OR
Lines: 26

In article <1991May28.130950@anusf.anu.edu.au>, mbl900@anusf.anu.edu.au (Mathew BM LIM) writes:
> One of our vendors has told me that the 5GByte figure on the capacity on the
> newer Exabyte drives is achived by on board data compression and that the actual
> capacity depends on the compressability of your data. So presumably the
> actual capacity of a tape is anywhere from 2.3GBytes to 5GBytes. Is this
> true? If it is, does anyone have any figures on what the "typical" capacity
> is when backing up a "typical" filesystem?

The new 5 GB Exabyte is REAL capacity e.g. not compressed.

The new DATs however DO use hardware compression to achieve 4 GB.
The new 4 GB DATs are compressed with a HP algorithm based on LZ-2
data compression. These drives presume a 2x compression from your
actual data.

For most files this is in fact conservative but if you have many
compressed files already in your filesystem problems arise. 

Tom

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|  Tom Beach : Sr Project Engineer : Mass Storage Technology             |
|  phone : (503) 294-1541                                                |
|  email : uunet : dtb@adpplz.uucp                                       |
|  ADP Dealer Services, ADP Plaza, 2525 S.W. 1st Ave, Portland OR, 97201 |
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