Re: TAPESIZE : Compressed or Uncompressed ??

>Hi all, > 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. > >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. > > In the meantime, all opinions welcome. > > >Thanks The TAPESIZE is based on the number of writes that the archive program (i.e. ontape) will attempt to write before trying to switch to the "next" tape. So it's the amount of data that you are attempting to "stuff" onto the backup media (tape). With compression algorthms, you can actually put more data onto the tape than the tape could hold in uncompressed mode. However, the actual compress will depend on the bit patterns being compress. As such, it is somewhat unpredicitable. I would never attempt to try to store as much as the tape manufacture states that you "can store up to". These estimates are generally over-optimistic. It might be best to plan on no more than 1/2 between the physical size of the tape and the estimated "up to" capacity. Madison Pruet