Archive Devices

Informix has somewhat of a split personality with regard to archive devices. The tb/ontape program can use any UNIX device as an archive destination, but it's really designed to archive to tape. Actually, tape is the only option that makes sense in many cases, especially if your disk space is limited. Archiving to tape reduces your exposure to catastrophic disk crashes. If you are archiving to disk and crash a raw disk because of hardware problems, you may well also crash the disk device you are using for the archive. With tape, you spread your exposure to mechanical problems across two totally separate types of devices, most often controlled by different controller boards. In a pinch, you can archive to a UNIX file, but it takes some manual intervention, especially if the archive is larger than one file.

For a system of any size, you can't beat a large DAT or DLT drive. These drives, although expensive, can give you tremendous data capacity and a high transfer speed. Most large tape drives can also use data compression to cram even more data onto an archive tape. The combination of fast reading and writing speeds and tremendous storage capabilities is hard to beat. Given the difference in price between large tape drives and drives of lesser capacity, one can often justify going to DAT or DLT simply by calculating the differences in media cost.

If you are using logging with your OnLine engine, it is preferable to have two separate tape drives, one for the archives and one for the logs. That way, your log tapes can continue their continuous backups to tape while the archive is going on. It's possible to get by on one tape drive if you are willing to do a lot of tape swapping. Tape drives seem to have a higher failure rate than other components, so having at least two drives available for OnLine makes sense.

If you plan on using multiple instances of Informix, remember that their archiving and logging needs should also be considered. The preferred but costly solution is to have two separate tape drives per instance, if the multiple instances will both be doing a lot of archiving and logging.

Archiving to Disk

If you do need to archive to disk on OnLine systems, it is very important to have enough disk space available for the archive. One thing to watch out for is the filesize limits that many UNIX operating systems impose on the user. Many UNIXes have a filesize maximum of 2 gigabytes. If your archive goes over this size, the archive will fail. To create an archive to disk on OnLine systems, create an empty file owned by the informix user with proper permissions. Then set the TAPESIZE parameter in the ON/TBCONFIG file to be less than the amount of free space available on disk. Then run the tb/ontape program. Informix thinks it is writing to a nonrewinding tape and does not realize that the archive is going to disk. Informix is very trusting with tape issues. If you are archiving to disk and reach the tape size limit specified in your tb/onconfig file, Informix will prompt you to swap tapes and hit ENTER. When you hit ENTER, Informix will assume that you really did swap the tape and will proceed to merrily write to the same disk file. For this reason, if you go over one "tape," you need to do a little bit of manual intervention to "change the tape". First of all, Informix looks only continuing until the archive is completed. Once the disk archives are created, move them off to tape using operating system utilities. Restoring the at the definition of the tape device at the beginning of the archive. You can't just change the tape device in the config file when it comes time to swap tapes. There are two ways to do it. One way is to make the archive device a link. When the archive program wants you to change tapes, change the link to point to another file. A second way is to rename the first "tape" file to another name and re-create the old-named file for the second tape, database is essentially the reverse of this procedure. If you decide to use this technique, don't depend upon it until you have archived and successfully restored several times. Also be aware that if you get into trouble with this, Informix will probably not support you, since you are not following their archive recommendations.

Again, note that IDS on Windows NT does support archiving to disk and this is fully supported by Informix in an NT environment.