Just a small addendum to Christo's explanation (I can subscribe writing my
signature in blood to it):
this issue about compression generates a lot of confusion because there's
no standard commonly accepted viewpoint. one thing is looking at it client
side, a totally different thing is looking at it on the tape drive.
I would always do my math with the native cartridge capacities and not the
"potential" capacities.
the overly quoted compression ratios are, well, just indications, not to
be taken as written in stone.
the 3-1 ratio comes from the mainframe, the only place where you're ever
going to see that kind of ratio, the 2-1 is generally quoted for Open
Systems.
real world ratios are non predictable and will depend on your specifc
data, exactly as Christo has explained.
this applies to tape drive throughputs as well, obviously.
just to give a small example, on 3592 drives in a customer production
environment, I've gone from 40MB/s on rubbish data (basically non
compressible) to around 70MB/s on decent data.yeah, ok, this was in a
windoze environment and windoze has some serious issues in block size
towards tape, but you get the idea.
the catch is that if you listen carefully to the sales rep, he should
always say the magical phrase:"Up to 2-1, 3-1 compression". ..
Cordiali saluti
Gianluca Mariani
Tivoli TSM Global Response Team, Roma
Via Sciangai 53, Roma
phones : +39(0)659664598
+393351270554 (mobile)
gianluca_mariani AT it.ibm DOT com
----------------------------------------------------------------------------------------------------
The Hitch Hiker's Guide to the Galaxy says of the Sirius Cybernetics
Corporation product that "it is very easy to be blinded to the essential
uselessness of them by the sense of achievement you get from getting them
to work at all. In other words – and this is the rock solid principle on
which the whole of the Corporation's Galaxy-wide success is founded
-their fundamental design flaws are completely hidden by their
superficial design flaws"...
Christo Heuer <christoh AT ABSA.CO DOT ZA>
Sent by: "ADSM: Dist Stor Manager" <ADSM-L AT VM.MARIST DOT EDU>
16/04/2004 11.00
Please respond to
"ADSM: Dist Stor Manager"
To
ADSM-L AT VM.MARIST DOT EDU
cc
Subject
Re: LTO tape cartridge(200GB/400GB) stores data 500GB+.
Yes - If you use the client-side compression you will see most of your
cartridges showing close to or just over 200Gig - I have mix of clients
doing compression and others not doing.
What I've noticed is that on average I'm getting between 30% and 50%
compression.
In earlier years IBM would qoute a 3-1 compression ration - 10/30 5/15
etc.
I think they lowered this to a more concervative number of 2-1 - and my
tape
numbers reflect this:
LTOATS 282,437.4
LTOATS 344,472.0
LTOATS 570,294.9
LTOATS 383,550.0
LTOATS 387,271.4
LTOATS 457,437.0
LTOATS 359,432.9
LTOATS 329,021.7
LTOATS 333,663.8
LTOATS 456,539.2
As can be seen - somewhere between 280 and 570- giving an average capacity
of close to 400Gig.
On the other hand - if all my data was compressed before arriving at the
server it would have been very close to 200gig.
Cheers
Christo
----- Original Message -----
From: "Willem Roos" <wroos AT SHOPRITE.CO DOT ZA>
To: <ADSM-L AT VM.MARIST DOT EDU>
Sent: Friday, April 16, 2004 10:01 AM
Subject: Re: LTO tape cartridge(200GB/400GB) stores data 500GB+.
The whole compression issue has always confused the hell out of me (no
certification here :-). Does 200-400 mean
"200-native-,-400-if-we-hardware-compress-as-we-stream-to-tape"?
Sometimes the client may also compress? I think salespeople over the
years have greatly abused this x-2x tape cartridge capacity thing to
their advantage - you can always double up because nobody knows what
you're talking about anyway.
And you mean LZ (Lempel-Ziv) algorithm, don't you :-?
-------------------------------------------
Willem Roos - (+27) 21 980 4941
Per sercas vi malkovri
> -----Original Message-----
> From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU]
> On Behalf Of Christo Heuer
> Sent: 16 April 2004 09:26
> To: ADSM-L AT VM.MARIST DOT EDU
> Subject: Re: LTO tape cartridge(200GB/400GB) stores data 500GB+.
>
>
> It depends on the actual type of data - take a 2Tb oracle db
> that is mostly
> empty you will be getting a 90%+ compression ratio - so your
> LTO-2 cartridge
> will show the capacity used as 2000Gb.
> The typical ratio IBM used to qoute was 3-1 in recent years
> they changed
> this to 2-1,
> hence the 200-400 figure. The algorithm used for the comression is a
> modified ZL algorithm - similar to the algorithm used for pkzip etc.
> If you send already compressed data your tape usage will show
> 200Gig or
> less - if you were getting negative compression ratios - data already
> compressed can grow if comressed again.
> So - there is no clear-cut answer - work on the native
> capacity(200G), and
> everything else you get is a bonus.
>
> Cheers
> Christo
> ----- Original Message -----
> From: "Chandrashekar, C. R. (Chandrasekhar)"
> <cr.chandrashekar AT TIMKEN DOT COM>
> To: <ADSM-L AT VM.MARIST DOT EDU>
> Sent: Friday, April 16, 2004 7:50 AM
> Subject: Re: LTO tape cartridge(200GB/400GB) stores data 500GB+.
>
>
> > This is not a puzzle for me, Actually I want to know how
> much data it can
> > compress, Is there any one using same tape library. Which helps to
> estimate
> > the total storage capacity.
> >
> > Just to know how much percentage of compression in 3583L23
> library using
> > 3580ULTGen2 drive.
> >
> > Thanks,
> > c.r.chandrasekhar
> >
> > -----Original Message-----
> > From: ADSM: Dist Stor Manager
> [mailto:ADSM-L AT VM.MARIST DOT EDU]On Behalf Of
> > Christo Heuer
> > Sent: Friday, April 16, 2004 10:51 AM
> > To: ADSM-L AT VM.MARIST DOT EDU
> > Subject: Re: LTO tape cartridge(200GB/400GB) stores data 500GB+.
> >
> >
> > Hi,
> >
> > I see you are a Tivoli certified consultant (TSM) - maybe
> they missed that
> > part in
> > the certification exam - why don't you read up on
> compression - if this
> > puzzles you
> > I'm sure plenty other things will confuse the hell out of you.
> >
> > Christo
> > ----- Original Message -----
> > From: "Chandrashekar, C. R. (Chandrasekhar)"
> <cr.chandrashekar AT TIMKEN DOT COM>
> > To: <ADSM-L AT VM.MARIST DOT EDU>
> > Sent: Friday, April 16, 2004 6:51 AM
> > Subject: LTO tape cartridge(200GB/400GB) stores data 500GB+.
> >
> >
> > > Hi,
> > >
> > > Just for clarification, I'm using LTO-Ult tape cartridge
> having capacity
> > of
> > > 200GB/400GB, Tape library 3582L23 with two 3580-LTOG2 drives with
> firmware
> > > 38D0, and devclass was defined with device-type=LTO and
> Format=ULTRIM2C.
> > Now
> > > the tape is storing more then 500GB of data, Is it normal
> behavior.
> > >
> > > Thanks,
> > > CRC,
> > >
> > > C.R.Chandrasekhar.
> > > Systems Analyst.
> > > Tivoli Certified Consultant (TSM).
> > > TIMKEN Engineering & Research - INDIA (P) Ltd., Bangalore.
> > > Phone No: 91-80-51362222.
> > > Email:cr.chandrashekar AT timken DOT com.
> > >
> > >
> > >
> > >
> > >
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