Tape Density vs Speed

[kenny chia]

Hi
I need to backup to a Quantum DLT8000, most of the files backed up are gzipped files.

Since the files itself are already compressed, would there be a speed increase should I choose a lower density ratio?

[Eugeny Brychkov]

Not sure. The best drive's perfomance is observed when host is able to supply drive with data stream which allows drive to keep 'streaming' - i.e. write data continuously without shoeshining (stopping, rewinding a little and continue writing). Check best SCSI HBA at http://www.hp.com/cposupport/information_storage/support_doc/lpg50063.html (AHA-29160lp ultra 3 wide). In your situation I would turn OFF hardware and software (in backup utility) compression(s) as soon as data is already compressed and selected the best density.
Anyway, you can try different settings by yourself and compare perfomance
Eugeny

[Leif Halvarsson_2]

Hi,

I have never reached the performance limit with DLT 8000 drives (12MB/s) when using hardware compression, if the files is already compressed it can be an idea to try with compression off.

[Michael Steele_2]

The general rule is already compressed data is slower to write when used with firmware compression on the drive. This is the /dev/rmt/cXtYdZBEST device vs. the /dev/rmt/0m device. Use the /dev/rmt/0m device.

Since drive compression is the usual default or suggested mode, if you have an application with the devices hard coded, like Veritas Netbackup, you'll have to account for this.

Also there are single and dual drive DLT8000s provided by Quantum. The 3800 and the 6802. I believe the dual drive writes to both sides while the single drive is one sided. The single drive holds 80 GB and the dual drive 160 GB. This is under 2 to 1 compression. Half that for non-compressed, so that's 80 GB and 40 GB.

You can easily calculate the time of backup if you know the collected sizes of your zip files. The single drive will sustain a 12 MB/second write rate while the dual will sustain a 24 MB/second write rate.

http://www.disuk.com/site/html/tapes/ufe_products/3800.html

http://www.disuk.com/site/html/tapes/ufa_products/6802.html