Networker

Re: [Networker] Calculating optimal paralelism and target session values

2004-01-20 20:50:52
Subject: Re: [Networker] Calculating optimal paralelism and target session values
From: George Scott <George.Scott AT ITS.MONASH.EDU DOT AU>
To: NETWORKER AT LISTMAIL.TEMPLE DOT EDU
Date: Wed, 21 Jan 2004 12:16:02 +1100
All,

> To repeat myself; the problem with the DLT4000 is data underrun. It runs out 
> of data to write to the tape. The data dries out during compression of text 
> or text like data. The interface is not able to deliver uncompressed data 
> fast enough. If you want to be able to stream a DLT4000 at all times, you 
> must turn off hardware compression.

The problem with the DLT4000 is that it is a narrow SCSI device (even
those that have FWD SCSI interfaces are still narrow internally) which
limits the speed that you can force data into them.  When compressing
very compressible data they will get data underruns.

This was fixed in the DLT7000 by making them a wide device.

> The DLT7000 solves this problem by having a larger buffer and if neccessary; 
> writing data before it is completely compressed. It tries to keep the tape 
> streaming regardless. It means that less get written to the tape.

The problem with the DLT7000 is that the compression circuitry is not
fast enough - it can only process about 13 MB/s of raw data.  When
compressing very compressible data they will get data underruns.

The large cache in the DLT7000 helps to smooth out the peaks and
troughs in the incoming data rates (which is definitely good -
particularly for reducing shoe shining) but doesn't help much with
sustained data rates.  In other words: caching helps more with short
term problems than with long term ones.

(BTW, this is all MHO from observation and guess work in the absence of
any useful information from the manufacturer)

George.
--
George Scott           George.Scott AT its.monash DOT edu
Systems Programmer, IT Services, Monash University

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