Veritas 8.6 29GB not fitting to one 40GB tape

[DannyBuk2005]

This is an odd one and hopefully you guys can point me in the right direction.

We have daily back-ups which we only backup changed data and some mail databases, this usually equates to about 28GB when we get to 27GB backed up it stops and asks for another tape....we are using 40GB uncompressed tapes.

However, when we do our full weekly back up which is usually about 41GB it doesnt ask to change tape until it reaches approx 29GB as you can imagine this is annoying as the daily ones would otherwise fit onto one tape.....any ideas?

sorry for the lack of setup information, i'm just taking a stab in the dark to see if someone knows what it is.

 

[mastrtonberry]

Wow, I'm having the exact same problem! My tapes are 20gb / 40gb compressed. I would imagine it has something to do with compression not being enabled or something. But when I check on that option it is set to Hardware Compression if available / software if not. I would imagine that that means it's turned on. I don't know. I will also await a response on this.

 

[stupman]

Is it a 20/40 or 40/80 tape?

 

[DannyBuk2005]

its a 40/80gb tape, so we dont need compression turned on

 

[stupman]

Best to turn hardware compression on in your backup program. If it is turned off, this behaviour is perfectly natural.

 

[DannyBuk2005]

we currently have it set to software compression, if we change it to hardware, will this slow down the backup?

 

[stupman]

Now actually, it will even speed it up. HW compression means that the compression will be done by the backup device. Hardware also most of times implicates faster than software.

 

[mastrtonberry]

Then what's with the 20/40 tapes that we're using doing the same thing? Backing up to 27gb then asking for another tape. It is set to Hardware on our server.

 

[Tony57]

Not a question, but as info. We have a 10 tape library 33/66 tapes) and using hardware compression, and when looking at media properties, I noticed a wide range of compression ratios, depending on what data is on what particular tape. Anywhere from .8 (bigger than original!) to 3.2, with the average being about 1.7 or so.

Compression should probably always be faster than not. Compressiion = less tape = less time. The difference between hardware vs software compression is that software compression uses server CPU cycles to compress the data, which might be significant, especially if your servers are approaching double digits (age wise). :^)

 

[Zelandakh]

in fact compression can be slower - the CPU has to compress the data before pushing it.

 

[DannyBuk2005]

Zelandakh - My thought exactly, oh well regardless...last night I attempted Hardware compression the back-up still required 2 tapes, I think it got to about 27GB then asked for another tape.

As I said these tapes are 40/80GB tapes so see no reason why it only backs up 27GB worth.

 

[Tony57]

The question being, is the time taken to compress the data then write the compressed data longer than the time taken to write uncompressed data?

 

[DannyBuk2005]

actually the question is....at the top of the page, why is it only writing 27GB to a 40GB tape....lol

 

[steveot]

Is the job set to overwrite or append?

 

[bkuepers]

Another thing to think about is compression ratios. With a 20/40 tape backing up thousands of 1K files you'll never get 2:1 compression. The larger the files sizes the better the compression ratio if you catch my drift.

Not sure what your situation is though.

 

[DannyBuk2005]

the jobs are set to overwrite...do you think its worth changing to append?

 

[justin2000]

Danny what kind of tape drive do you have? Is it by any chance a DLT4000?

Seeing that your backups only differ 1gb, why don't you just do a full backup everyday?

Something is just not adding up right here.

 

[DannyBuk2005]

Not sure of the tape drive, will have a look shortly....

Daily Backup = total 29GB - Requests that we change tapes when it reaches 27/28GB

Full Backup = total 44GB - Requests that we change tapes when it reaches 29GB

We should be able to get the daily back up onto one tape if it ran the same as the full back up.

 

[DannyBuk2005]

The Tape Drive is HP SureStore DLT40

 

[justin2000]

Not much you can do here - looks like you are getting better compression on the first 28/29gb it backs up on the full, than what you get with the daily backup.

The only thing I can suggest is to take out some directories you don't need to backup during the week.

Or get a 40/80 tape drive - which really shouldn't be that much any more - Maybe $1k.