Byte count in Backup Exec 9.1

[Gazman]

Does anyone know what the byte count in the job monitor window is actually measuring? Does it show the bytes WRITTEN to tape (i.e. what's ON the tape) or does it show what has come IN to Backup Exec before any compression is applied?

I'm getting intermittent incomplete backups where sometimes the software asks for another tape at 24Gb and sometimes it works and 27Gb+ is written. I've tried new tapes and it makes no difference. Compression seems to be working (I'm using DDS4 40Gb tapes) as in all cases the backup asks for another tape when the byte count shows well in excess of 20Gb. All my tapes are set to overwrite (no append) so there isn't any info that it's trying to append to (as far as I am aware). Does BE erase a tape before writing or does it just write over what's on it already?

Anyone know what's going on here?

 

[Mthales]

Gazman - did you find anything out about this? cause I'm having similar problems of the byte count not seeming to match up with what I expect.

M

 

[Gazman]

Hi. No nothing at all. I'm now doing incremental backups, half the data today and half the next - taking care to keep the count below about 22GB! I back up critical files on both sessions but other "not so critical" stuff every other day.

Veritas told me that it was what was actually "written" to tape but I don't believe it. From the symptoms I was getting, the indicated figure has to be wrong.

If anyone reads this - we really need to know!

G.

 

[justin2000]

It's the bytes written to the tape.

 

[Gazman]

How can it be? In my case I have a 20/40Gb medium. I "should" be able (theoretically at least) to dump 40gB of compressed data to it. I can't get more than about 27Gb onto the tape. OK, I'd accept if I could write about 38Gb to it (allowing for some error) but I'm getting nowhere near the capacity of the tape so the figure in the byte count window must be wrong surely?

I think that maybe the software groups files together into a series of large blocks. It then calculates whether the tape can handle the next chunk of data and asks for another tape if it can't drop this entire group of files to the tape. This is the only explanation that I can come up with since I can only get around 27Gb onto these tapes. If this really IS the reason, then it's true - the byte count is what is on the tape. If files were written one by one, then the tape would fill up to the end surely?

It's interesting that if you backup to disk, this is exactly what happens. You end up with a series of very large files.

Veritas should make this clear as there are many, many users that have this issue.

 

[justin2000]

Huh? make what clear, that you can't get the supposed 2:1 compression from a tape - this has nothing to do with Veritas or any other software vendor - it is solely the responsibility of the hardware manufacturers.

Backing up a normal server you will never ever get 2:1 compression - you should be aiming for anything from 1.3:1 to 1.5:1 - though again this depends on what you are actually backing up.
If you were only backing up text files, you most probably will get the 2:1 compression, you may even get more.

The capacity of you tape is 20gb - anything above this is OK and you are getting compression.

BE, or any other software writes to the tape drive via the tape device driver. BE will write to the tape until it receives a notification that the tape is full (it receives this from the tape device driver) and then it pops up and asks for another tape.
Hardware compression is controlled only by the tape drive, nothing else.

Backup to Disk files are controlled by a setting in the Advanced tab - you can specify how big you want the B2D files to be - by default they are 1gb.
Consider a B2D file as a tape - it will write to the file until it hits the 1gb limit, and then automatically create a new one.
The difference with B2D files is - when the hard drive that is holding the B2D fills up - the job will fail saying there is no more space left.

Think about it this way -
If you compress a File in Windows NTFS - it will then give you 2 readings of the file size.
1. The uncompressed size of the file.
2. The compressed size of the file on disk.

In BE - the byte count you see in the Job Monitor and the Job Log is the actual uncompressed size of the files - ie. it's what it actually written to tape.
BE will not give you a size of the files on the tape though.

Hope that makes sense.