Tape DDS4

[sam yu_1]

Dear all:

Why my DDS4 tape only can backup 17811360 kb?
isn't it 20~40 GB ?

My file contain about 6GB ghost file.
is that a problem?

PS. OS:nw5.1 ; software:Arcserve 660 + SP4

[CA486125]

Compression *assumes* a 2-1 compression ratio and varies a lot depending on what you store.

I would think that a ghost file would be almost uncompressible.

There are a number of good articles on the web on compression and how it works, I think you might find it very interesting.

[Eric de Lange_2]

Hello Sam,

I would do the following test:
run a backup with the hardware compression on the drive disabled. Then check the total amount that fit's on the tape.

If you get more data on tape with the hardware compression DISABLED then this is because the ghost file you are backing up is already compressed. The drive's compression chip can do little with this data. In fact you will lose some space on tape due to the overhead involved with this. if the above is the case, there is nothing wrong with the drive.

If it turns out that without hardware compression you get the same amount of data on tape then there might be a problem with the tape itself. Then clean the drive with a cleaning tape and try the same operation again with a new tape and see if this changes things.

Also, make sure that the software is configured to use a large blocksize. This may speed up the drives throughput.

Hope this helps,

-Cal.

[aezzet]

I'm running into the same situation where the DAT40 drive is basically storing less than the advertised 'uncompressed' capacity.

I wonder if the compression algorithm is actually expanding some of the files that are already compressed (such as digital photos in jpg or even raw but already compressed formats).

Since the software (colorado backup 2) doesn't bother to show how much space is actually used on the physical media, it's not easy to tell what's really going on. But, my 16GB backup job did not fit on a single DDS4 (20-40GB) cartridge.

-ae

[Eric de Lange_2]

Hello,

Some compressed data like MP3, ZIP and others may well end up taking more space on tape than their actual size. This is because of the compressions algorithm inabillity to further compress this data so it has to 'mark' this data somehow. This will cause extra space to be consumed.

The workaround is: do not use HW compression when storing your already compressed data on tape.

regards,

Cal.

[Jeff Technician 7]

Hi

The compression algorithm of DAT technology is rather flawed in that it compresses already comressed files types ( jpg, mp3 , zip , mpeg etc. ) by upto 30%!! This sounds crazy but unfortunately it is true. The amount of data that you get onto a tape ( and indeed the transfer rate that you achieve ) is dependant on the files you are backing up :)

The only drive which doesnt attempt to compress already compressed files is the Ultrium model.

Hope this helps

Jeff

Technician 7