Group: Forum Members
If you want to hold onto Full backups until you run out of space because you've got a destination drive dedicated to backups, then you could simply uncheck the Full retention policy entirely. In that case, Reflect will allow Fulls to accumulate unconstrained and only purge them when the "low disk space" purge is triggered.
If you'll be keeping the disk as full as possible with backups and EVER plan to use Differential or Incremental backups at all (rather than ONLY making Fulls), then I would keep "Run purge before backup" checked. The reason is that in that case, whenever creating a new Diff or Inc, Reflect will apply the Diff/Inc retention policy upfront, which can free up some space for the new one. If you did NOT do that, then you might run out of space during the Diff/Inc backup, which would trigger the "low disk space" purge. The downside to that is that you'll lose an entire backup SET if that happens, whereas if you allowed the Diff/Inc retention policy to run before the backup, you might have been able to free up enough space for the new backup by simply purging an old Diff/Inc.
Yes, you need to check the "Purge oldest backup sets if less than" checkbox. In terms of what the threshold should be, I remember finding that performance tended to suffer when free space dropped below 15-20% of capacity. But these days with disks up to 10TB, keeping 15-20% unused might seem overly wasteful. I definitely would use at least the default 5 GB, if not more. The exact choice is up to you. And here again, if this destination disk will be used ONLY for backups, then I guess you don't need to worry about keeping more space free for other data.