I'm new to SP (in my trial period; I have the full evaluation version), and I have two questions.

First, I'm attempting to work out a backup strategy, and I'm trying to understand whether there is any disadvantage to selecting the "second and subsequent full backups are differentials" Advanced option.  It seems to me that if you're dealing with a non-infinite amount of storage space, repeated full backups are going to fill up that space rather rapidly.  Selecting this option would permit retention of many more "full" backups within the same space (I understand that any retention policy is always going to retain the original full backup).

So what's the catch?  Why isn't this the default option?  Am I missing something about Full backups - how many people can keep more than two or three Full images of a 100Gb+ system volume, even assuming they don't (as I do) also have two or three other multi-hundred Gb data volumes?  (This is why I've always used File and Folder backup software in the past - because I've never understood how to keep more than an image or two unless I bought my own tape deck!)

And also, how does this option compare with Continuous Incremental backup, practically speaking?  If I chose Continuous Incremental and used Image Manager to periodically merge those incrementals into effectively "weekly" incrementals, how would that differ from doing weekly full backups choosing the "second and subsequent full backups are differentials" options?

I'm liking what I've seen of this software so far, very much, and I like the level of support I've observed in the forum, but I have to say - I'm reasonably technically proficient, if not professional level, and I can understand the technicalities of the SP user's manual moderately well, but where I find it lacking is in helping me unravel the options - not to figure out how to do things, but to understand the objectives and to figure out why I should select a particular option over another (not that SP is so different in that regard - my other backup software is Retrospect!).  It took a significant amount of reading through the forums even to figure out the real difference between incremental and differential - the user's manual is totally obscure on that point.  It's very frustrating.  At least for people like me, who do read the documentation, the developers might save themselves some time and effort if they improved on that aspect.

My second question, speaking of Retrospect, is whether anyone has used it and SP together successfully.  I've had SP installed for a few days - no backups yet though - and my Retrospect backups are proceeding as scheduled, but with an error:  I'm finding that since I installed SP, Retrospect is no longer able to back up open files.  I did a little poking around, and it seems that Retrospect's Open File backup uses VSS, and this problem can arise when another program installed on the computer is already using Microsoft's VSS services, preventing Retrospect's access to them.  Could this be it?  Would SP be using VSS simply by virtue of being installed on the computer, even if a backup job is not actually running?

Anyway, thanks for whatever help you can offer - I am hopeful that once I get it figured out, I'll be very happy with SP!

"Second and subsequent full backups are differentials" is an excellent option for saving disk space on your backup destination drive.  When this feature was added it was anticipated that it would be widely used but it doesn't appear that it has, probably due to lack of exposure / information as you mentioned.  However, there are a couple of possible drawbacks.  First, the differential can take longer and be more resource intensive than a full backup.  This is because the existing full backup must be read (possibly from a network location) and compared to the current state as the differential full image is written.  Over time, the differential full may approach the size of the original full depending on the data usage of the volume being backed up so space saved may not justify the additional time and processing required.  However, in this case, you can take a manual full at any time which will become the new full against which subsequent differential fulls will be created.

The other drawback involves the retention policy which is based on the number of chains (full image + incrementals) you want to keep.  Using differential fulls may shorten the length of a chain due to an issue that has been resolved for the upcoming 3.3 release.  The scenario is when incremental tracking is interrupted (power failure, dirty shutdown etc.) the self healing advanced option will cause a differential to be created so the chain continues and tracking is reestablished.  This should be an incremental file but in the case of "second and subsequent full backups are differentials" it has been creating a new full differential instead of incremental.  This means that if you have one weekly full scheduled with a retention of 3 chains you would expect 3 weeks of backup image files but instead a new chain may start in the middle of the week so you have 3 chains but they only cover 2 weeks.  Or worse, if it happens twice in one week you only have 1 week of backup images.

The Continuous Incremental job type was added to address the issue of transporting image files to an offsite location.  In most cases it is not practical to transfer multiple full images which all other backup jobs require.  ImageManager should be run when continuous incrementals are used and it has more flexible options for retention of image files.  For example, with the retention policy for other backup job types, all incrementals are kept for the given number of chains.  With ImageManager, you can specify the number of days to keep incrementals independent of the daily and weekly collapse files that are kept.  But ImageManager requires keeping all monthly collapse files so after one year it could actually require more space than the differential full job type.

As you can see, these job types address different issues and depending on your objectives you may want one over the other.  I personally use the Differential Full job type on my home machines and think most users not transporting image files offsite would probably use it as well.

Thank you so much, that's an extremely helpful explanation.  Though I don't entirely understand the problem with the retention policy - are you describing a backup policy that uses Full Differentials plus Incrementals - so, for example, a Full Differential each week with daily incrementals?  And there's a dirty shutdown in the middle of the week, so the self healing advanced option does a differential, as it should, but instead of resuming incrementals for the weekly chain, it begins a new chain from that differential?  So if my retention policy calls for retaining three chains, which should be three weeks, I'll instead have only two weeks, since I have two chains in the week with the dirty shutdown?  I think that's what you're describing, and yes, that could be a real problem if you have a week with a bunch of thunderstorms or something!  I'm glad to hear that 3.3 should resolve it.

Yes, the problem with the retention policy is only with the Full Differential plus incrementals.