The tar utility has changed over time and older versions would generate what is viewed as incompatible files. Although most tar utilities will still support older version (before 1988), the may generate this one warning:

   tar: Record size = 8 blocks

I tried, a couple times, to create tar files and they worked just fine but I would get this warning... This is rather annoying so I looked in a way to avoid the warning. Why do you think we get it?

The answer is in the newest versions of POSIX the block size of a tar file was decided to be 10Kb. Internally, the block sizes are still 512 bytes, but when you generate a tar file, you must make sure that the final file is a multiple of 10Kb or you get this warning.

The warning is safe for disk files, however, it lets you know that your tar file may not be compatible with all tape drives available, so of which may even require larger blocks (i.e. 20Kb, 32Kb, etc.)

So... all you have to do is add a bunch of zeroes at the end of the file. If you file system is setup to have sparse files, it shouldn't be wasting any space on your hard drive. Although, you should keep the tarball compressed with gzip or bz2. That way the zeroes more or less disappear.