I currenly have a backup schedule of
Full, Diff, Diff, Diff, Diff, Full, Diff, Diff, Diff, Diff
If the sizes for these are
1GB, 2Mb, 4Mb, 6Mb, 8Mb, 1GB, 2Mb, 4Mb, 6Mb, 8Mb
how would it change if I stop doing the second full backup? Would I get
1GB, 2Mb, 4Mb, 6Mb, 8Mb, 10Mb, 12Mb, 14Mb, 16Mb
I'm weighing up the worth of the Full backup hitting the disks so hard...
Having not actually tested it, my understanding is that yes, that is roughly what you would get. As you continued accumulating more changes the differential would continue growing, and at roughly the same rate as it was before, until the next full backup is done.
Whether the extra full backup is worth it or not depends on how hard it is hitting the disks and if you can fit the full backup inside a maintenance window so its affect on users is minimal or not. As a general rule though, the extra full backup is worth it. I have seen cases where the differential backup can grow very large and time consuming.
I like Timothy's answer, and the other thing I'll add is that you are increasing your risk if you remove the 2nd full. You are now depending on the first full backup for a longer period of time, which means more chance for an issue with that file, tape, etc.
Having more regular backups ensures that your risk is lower since each is valid for a shorter period of time.
Paul wrote a script a while back that works out how many extents have changed since your last full backup, so instead of guesstimating how big your diffs are going to be you can know in advance & go for a full backup if it's over a certain percentage.
