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DLT7000 Backup Compression - How much will it compress?

hi,
i have an external DLT7000 (35GB uncompressed/70GB compressed) backup drive, connected via SCSI to a pc running Windows 2000 Server.

i can select files and folders to backup and just before it backs them up it has a look to see how many GBs of data there is (uncompressed). it then starts the backup with hardware compression. i want it to look at the files i have selected for it to backup, and work out the compression of the particular files and tell me how much data it will be with compression.

for example, if i select a folder which has 40GB of data in it. i know this folder won't fit uncompressed on a DLT tape (35GB) so i turn on hardware compression. the compression depends on the type of files being backed up, some files can't be compressed, others can, so i have no way of knowing if that 40GB of data is going to fit on the tape or not. it may be able to fit it all on even with some space available, or may not be able to compress it at all.

is there a way or an application that can tell me before the backup job starts if it's going to fit on one tape or how many tapes i need?

i hope i have explained this well enough, feel free to query if not.

any help appreciated.
thanks
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ianmarchant
Asked:
ianmarchant
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2 Solutions
 
rindiCommented:
There won't be any possibility using hardware compresseion, as the backup software won't know the compression algorithm used by your DLT Device.

If you use software compression and your backup software (what software are you using?) does an estimation, as you say it does, it should give you an estimated space usage when done.

Generaly I prefer not to use hardware compression, as this can give you a problem if your tape drive breaks down, it can be difficult to read hardware compressed tapes on another drive.

With todays Server hardware software compression shouldn't cause a noticable performance decrease on it.
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Iamthecreator OMAdministrateur Systeme et ReseauxCommented:
u will not be able to see the compression ratio but Hardware compression is more efficient than software compression.So if u are not able to fit data using s/w compression then turn it off and try h/w
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Duncan MeyersCommented:
Use WinZip to zip up the directories that you will be backing up. The WinZip compression ratio will give you a guide to the compression that you can expect from the tape library. Note that WinZip compression will almost always be better than the drive's hardware compression because WinZip compresses by files whilst the tape drive compresses by blocks of data. Different tape drives also use different compression algorithms - DAT tapes usually have pretty ordinary compression ratios, while DLTs and LTOs are usually pretty good.

Setting off WinZip to zip up 40GB might be a trifle ambitious - you may want to zip up, say 5 GB at a time and then take an average of the compression ratios achieved.

Hope this helps...
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ianmarchantAuthor Commented:
i'm not using software compression, ie my NTFS volume doesn't have compression turned on if thats what you mean.
i'm using Microsoft Backup that comes with Windows 2000. I realise a lot of people say this application is rubbish and not to use it, but it seems to be able to do everything I need it to .... backup/restore/catalog/erase/incremental/differencal etc...etc. i have no problems with it.


>> If you use software compression and your backup software (what software are you using?) does an estimation, as you say it does, it should give you an estimated space usage when done.

it shows an estimation of the GBs I'm backing up once I tell it to do a backup... however, what I want to know is how much that data would compress. In theory, I may b able to select 40GB of data and get it on 1 tape, but then again may select a different 40GB of data and not be able to fit more than 35GB!


>> Generaly I prefer not to use hardware compression, as this can give you a problem if your tape drive breaks down, it can be difficult to read hardware compressed tapes on another drive.

i didn't know this! Does this mean if I get another DLT7000 drive it won't be able to restore from the tape and won't work? The DLT7000 drive I have is QUANTUM drive. Don't know any more details than that about it. How about if I got a DLT8000 drive would I be able to restore the files that I backed up on the DLT7000 on it?


> u will not be able to see the compression ratio but Hardware compression is more efficient than software compression.So if u are not able to fit data using s/w compression then turn it off and try h/w

yes, I'm only using hardware compression. Basically turning on the option in MS Backup "Use Hardware Compression, if possible"


> Setting off WinZip to zip up 40GB might be a trifle ambitious - you may want to zip up, say 5 GB at a time and then take an average of the compression ratios achieved.

very ambitious, that would also take a lot of time and wouldn't be automated. I also don't have enough space available to be able to zip it all up. It would take forever. However, all I want is a program to be able to work out how much the compression would be without actually doing it.... so I know before I start the backup how much it will compress it.

really this is quite a simple thing, at least I thought, surely a software program could be written to emulate the hardware compression in a DLT drive???

thanks for the help!
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rindiCommented:
>>  i'm not using software compression, ie my NTFS volume doesn't have compression turned on if thats what you mean.
>> i'm using Microsoft Backup that comes with Windows 2000. I realise a lot of people say this application is rubbish and not to use >> it, but it seems to be able to do everything I need it to .... backup/restore/catalog/erase/incremental/differencal etc...etc. i have no >> problems with it.

No, not ntfs compression. Actually I meant software compression used by the backup software.

>> i didn't know this! Does this mean if I get another DLT7000 drive it won't be able to restore from the tape and won't work?
>> The DLT7000 drive I have is QUANTUM drive. Don't know any more details than that about it.
>> How about if I got a DLT8000 drive would I be able to restore the files that I backed up on the DLT7000 on it?

It doesn't mean that it won't work on another drive. It just means that it doesn't have work, as this depends on tollerances of the hardware.

The built in backup software of Windows is OK, but basic. With commercial Software you get tools which can estimate if the selected data will fit on one tape or not (this is just a rough estimate, if it were exact, it would take almost as long to do the estimation as the actual backup would take!)

Generally if you know what your backing up you will also know the approximate compression.
Backing up programs will only get you a minimal compression, the same as Zip, mp3, mpeg files, etc (file types which already are using some sort of compression.
Normal Data, like word or excel files, will give you around 1:2 compression.
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Duncan MeyersCommented:
> Setting off WinZip to zip up 40GB might be a trifle ambitious - you may want to zip up, say 5 GB at a time and then take an average of the compression ratios achieved.

I'm not suggesting that you use WinZip to prepare data for backup. You asked if there was a way to determine if your data will compress enough to fit on one tape.  There are two ways to do this:

1. Try it. Either it fits or it doesn't. Given you have 40Gb to go onto a 35/70GB tape, you are likely to be OK.

2. Use WinZip compression ratios to give you an *indication* of how much your data will compress as detailed above. This addresses this question: "really this is quite a simple thing, at least I thought, surely a software program could be written to emulate the hardware compression in a DLT drive???"
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ianmarchantAuthor Commented:
sorry forgot to come back to this question.

thanks for the help.
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