Networker

Re: [Networker] tape drive hardware compression a bad thing?

2003-01-29 13:58:41
Subject: Re: [Networker] tape drive hardware compression a bad thing?
From: Bokkelkamp Ernst <ernst.bokkelkamp AT SIEMENS DOT COM>
To: NETWORKER AT LISTMAIL.TEMPLE DOT EDU
Date: Wed, 29 Jan 2003 19:57:39 +0100
The only time it's a bad idea if you can not afford buying decent equipment
;-)

Once upon a time, many years ago, a company called Colorado Memory Systems
(bought by HP), and HP (IIRC) advised a conditional NOT to use hardware
compression because there were 1) tape drives without hardware compression
and 2) tape drives with hardware compression on the market. The backup
software on the market could either do software compression, using software
from Stac, or let the drive do the compression, sometimes even decompress
data written on a drive with hardware compression being read on a drive
without hardware compression, because both used the same algorithm from
Stac.

The condition given was that you should not use HW compression if you
intended to read the tapes on drives from other manufacturers. (at that time
mainly DAT and QIC drives).

Since then the price of tape drives have not dropped, but the capacity has
increased from 40MByte/tape to 200GByte/tape, and there compatibility has
increased tremendously, in particular with DLT, AIT and LTO. There are very
few reasons for NOT using compression because the drives we use have more
computing power then the average PC at that time and are clever enough to
stop compressing if the data is compressed already.

Software compression, on the client, is a bad idea under most conditions
because the algorithm used by our favorite tool is not that efficient. It
may have been good at the time that we were still running on yellow cable
with CSMA/CD or FDDI, but in todays world with switched 100TX/1000TX the
network overhead caused by not compressing is less then the overhead caused
on the client.

Btw. There is an oem version of networker that has replaced the compression
routines by a very efficient algorithm that wacks the hell out of DB
backups. I have been arguing the with one of my colleagues for months on to
compress or not compress, he pro-compression on Unix, me anti-compression on
WinNT. The argument stopped when we realised that we were comparing Legato
versus the OEM version. However, the rule of one cpu just for compression on
a SMP machine is still being applied.

Btw again. I can remember having gone through a similar discussion on the
CMS BBS many years ago when I was still using a CMS Jumbo 40, later Jumbo
120, to backup my two 150 MByte hard disks on a Intel 80386SX system with a
wacking 16Mbyte of RAM running Windows 3.1.

Btw again. The last time I used a 2GB DAT drive without HW compression was
over 6 years ago on a Compaq dual processor 486 system (IIRC running
Networker 4.1) ;-)

Bye
Ernie



-----Original Message-----
From: Jim Lane [mailto:JLane AT TORONTOHYDRO DOT COM]
Sent: Wednesday, January 29, 2003 6:42 PM
To: NETWORKER AT LISTMAIL.TEMPLE DOT EDU
Subject: Re: [Networker] tape drive hardware compression a bad thing?


Shaun: I know that, I was using compression for some years before your
guy told me not to. my whole purpose in starting this rant was to try
and get some perspective on why I'm being told it's a bad thing.

Jim Lane
Sr. Technical Consultant
Network Services
Toronto Hydro
office: (416)-542-2820
cell: (416)-896-8576

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