Retrospect not compressing data.

[cmika]

I am using the newest version of Retrospect (5.0.238) on the MacOS X (10.3.4) using an Exabyte VXA-2 tape drive (firmware 2101) with v23 tapes. According to the exabyte website, the storage capacity is 80GB uncompressed/160GB compressed (assuming 2:1 ratio). I understand that compression is not perfect, but I don't seem to be getting any compression at all.

 

I've checked, and the options summary for the backup sets report that hardware compression is turned on, which means that if available, compression is being used.

 

I know there's a compression chip in the VXA-2 drive, but it doesn't seem that it is being used. How do I ensure that compression is being used? I have about 100GB of data, and it's not fitting on one tape, which it should.

 

What am I doing wrong here? Is compression not available? Is it not turned on on the drive (possible, but the Exabyte website says that it's turned on by default)? Any suggestions would be greatly appreciated.

[scstr]

Neighbor, you're asking for 25% compression. I have never gotten that with a VXA-1 drive or with a DDS-2 drive. I got about 10-20% over my history, which now covers a decade.

 

I've just gotten a VXA-2 drive. Unless the nature of hardware compression has changed, I don't expect to do any better.

 

Bottom line: I wouldn't expect much more than 90 GB to fit on a single tape -- definitely not more than 95 GB, which would be heroic.

 

Suman

[cmika]

10-20%? Then how do they advetise that their tapes hold 80G uncompressed, 160G compressed?

 

 

 

Last night, the tape ran with 75.7G written, 17.0G remaining. I'm not asking for true 2:1 compression, I realize that that's impossible. But even 10% compression would be at least 88G.

[scstr]

"cmika", you've missed out. 2:1 is standard compression marketing for all tape vendors. If you compress a purely text file, you can actually get 2:1. That's why they can say it. But that doesn't work for any other kind of file.

 

Now all tape vendors are stuck doing this same marketing that's not real world. Don't pay heed to it.

 

There's something wrong with your phrasing or math. 75.7G written + 17.0G remaining is 92.7G, which is more than the 88G mentioned in your final sentence. That is "even 10% compression" ... actually a bit more than 15%

 

Suman

[ghoffman]

You don't say much about your backup host and network, but if it is less than top of the line, you could be suffering from data underrun to the tape drive. That is, the tape drive is wasting tape (writing blank tape) waiting for the host computer to send it more data.

 

Go to Exabyte's web site and get the VXA Tool from the Support -> Downloads area. Read the ReadMe file.

 

Use the VXA Tool to set your preference for Capacity (over Speed):

./vxatool Tape1 -P C

 

This enables the full effect of Variable Speed Operation that is one of the hallmarks of the VXA technology. No guarantees this will solve your problem, but it should have at least some positive effect on capacity.

[cmika]

The network isn't an issue. The backup host is an XServe running MacOSX. Retrospect is backing up an attached RAID unit (connected via SCSI) directly to the tape drive (connected via FireWire).

 

I'll try your suggestion, thanks.

[natew]

Hi

 

One thing to note - Retrospect 5.0 hasn't been tested with Panther. Its hard to know what to expect there.

 

Try this:

Create a new backup set in Retrospect and disable hardware compression. This is your only chance to toggle this setting. Then turn on software compression in your script. Retrospect will compress the data before it goes to tape so we know for sure it is getting compressed.

 

Thanks

Nate

[scstr]

First, Nate is right in that Dantz has only tested Retrospect 6.0 with Panther. If memory serves, it took Dantz 3-6 months after the release of Panther to fully certify and release and support version 6.0 for Panther. So "cmika" has *none* of that backing him up, pun intended. ;-)

 

Second, "ghoffman" mentions a recipe for using VXA Tool to set the preference for Capacity (over Speed). That instruction is correct. I have used previous vxatool versions for my single-ended VXA-1 drive.

 

However, for the 1x10 Firewire VXA-2 drive, the Exabyte instructions say to not use that option except under the supervision of tech support. While I doubt there are serious consequences, you could just give them a buzz.

 

Suman