Re: TAPESIZE : Compressed or Uncompressed ??

> When you set the size of the archive tape device via TAPADEV, do you specify > the "commpressed" size or the "uncompressed" size. For eg., our tape drives are > said to handle 10G worth of data, but this is compressed data ( from what I was > given to understand, the device driver compresses the data before storing it ). > Therefore 10G may actually translate to 30-40G uncompressed. How does Informix > translate all this. I'm pretty sure that Informix will calculate the size of the > backup without any compression factor. > My question is: What then should I set the TAPESIZE to. ( By the way, I am > using the simple down to earth ONTAPE ) If Informix ( ODS 7.23 UC1 ) prompts you > for the second tape based purely on the size in TAPESIZE, and the data is > actually compressed, wouldn't it be a better idea to set the tape size to the > actual uncompressed size ?? > I am going to try out this option today and see the consequences. I guess I > would have to do a full restore to see if my backup worked. You should definitely test this to be sure that it works in your environment, but in my case, I have two machines with DDS tape drives. With 90m tapes, these have a native capacity of 2 GB, but hardware compression could achieve up to a 4:1 compression ratio, giving a theoretical 8 GB capacity. I am a little conservative, so I assume no more than a 2:1 compression ratio and set TAPESIZE=4000000, which is slightly less than 4 GB. > SLIGHTLY OFF TOPIC: > If you had followed my previous chain on a similar topic, "Question on > the Archive TAPEDEV size ", of what would happen if we set the tapesize to > something much larger than what the tape can hold, then here is what happened. > We had set the size to 4 times the actual size ( since that was what the System > Admin tool recommended ), and the entire backup fit in one tape ( as opposed to > three ) !!! Well, that is what got us wondering on the tapesize itself. We do > not know if that backup is actually good. My understanding is that if Online encounters the physical end-of-tape prior to writing the number of bytes specified in TAPESIZE, the engine will crash. If no errors were written to online.log, I would bet that your archive was successful. I don't know that I'd always count on achieving the same compression ratio in the future, though. The actual compression ratio will vary with the characteristics of the data being compressed. > In the meantime, all opinions welcome. Now you've heard mine. Mark Collins mcollins@us.dhl.com The problem lies in how easily and dangerously we forget that manipulating things is not the same as understanding them. Words that come to mean everything may finally mean nothing; yet their very emptiness may allow them to be filled with a mesmerizing glamour.... People who have no clear idea what they mean by information or why they should want so much of it are nonetheless prepared to believe that we live in an Information Age, which makes every computer around us what the relics of the True Cross were in the Age of Faith: emblems of salvation.