SUMMARY: Additional info - DLT 7000 expected capacity

From: David Robson (
Date: Mon Aug 04 1997 - 09:39:09 CDT

I got this late reply which I have found usefull. I have experimented using
SBU's built in compression on one save group client and have already gained
another GB of capacity, so I've switched it on for all clients to see what
I get! ;-)

The therory bandied at work is that because the DLT has a limited buffer
capacity, any files larger than this capacity will not be compressed as
efficently, whereas SBU can compress the whole file in its entirety...

There is the risk the backups will take a little longer but I have no
evidence of that yet. I am still using the DLT compressed mode at the
moment, but switching it off may improve performance.
The only down side I know is that SBU documentation suggests that restore
would take longer - who cares as long as I have the data! ;-)

PS this still doesn't explain why I seem to get proportionly more data on a

Thanks to Kevin Warren <> for the info!

>Date: Thu, 31 Jul 1997 09:17:18 +0100
>From: Kevin Warren <>
>Organization: BT Laboratories
>To: David Robson <>
>Subject: Re: SUMMARY: DLT 7000 expected capacity
>David Robson wrote:
>> We have a SUN DLT 7000 with a documented (SUN) capacity of 70GB,
assuming 2:1
>> compression. The best I can get is 49GB using Networker 4.2.6 on an
Ultra 2200
>> running Solaris 2.5.1
>I'm sorry for not replying to this the first time - I actually don't
>recall the original. Anyway, sorry to reply after your summary but my
>following point was not mentioned:
>Using hardware compression switched on via the device driver you can
>expect at most 2:1 (100%) compression. As you, and I, have experienced
>the best we seem to get is 1.5:1. You report 49Gb from 35Gb native,
>with my Exabytes I see 11Gb from 7Gb native. Comparable I think you'll
>agree and from talking to other people this is expected.
>Now what you may be interested in is that with NetWorker's software
>compression I have seen compression ratios as high as 7:1. I only
>discovered this because I am now running NetWorker from an NT server
>(because the licence was much cheaper) and it refused to switch on
>hardware compression. Not knowing the internal jumper settings to force
>the use of hardware compression we switched on software compression.
>1Gb of data compressed to 139Mb, a compression ratio of 7:1, and this is
>standard, not a one-off.
>Switch on software compression via client setup - directive. A knock-on
>advantage of this method of working is that less data is sent across the
>network as the compression is done by the clients. The down side is
>that restores of files are slightly slower as the files have to be
>uncompressed as well as extracted from the NetWorker tape archive.
>Kevin Warren
>[Tel: 01473-646551]
>Disclaimer: These views are mine and do not represent the opinions of

David Robson
Davtin Systech Pty Ltd

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