Note: Incremental backups are a feature of UpdraftPlus Premium, launched in December 2018.
What are incremental backups?
An incremental backup is contrasted with a full backup. A full backup includes everything; an incremental backup is a later backup which leaves out anything which has not changed since the last full backup. The main benefit of incremental backups is that they are smaller (because they leave out any data that is already in the full backup and has not changed since) and quicker and less resource-intensive to create (for the same reason).
In order to restore to a point in time when an incremental backup was taken, you need to have the full backup, and then all the incremental backups (whether one or more) since the full backup. Thus, they take longer to restore. So, whilst incremental backups are quicker, they generate more backup archives. Read more at Wikipedia. Few users should find that a significant problem (in ordinary use, a user doesn’t have to know or care how many archive files UpdraftPlus has created, since UpdraftPlus takes care of all their management).
What do I need to know about how UpdraftPlus creates incremental backups?
Incremental backups have been implemented in UpdraftPlus in a way that is consistent with our overall philosophy. In particular, we do not use proprietary formats for storing data, and the feature works across all remote storage back-ends (e.g. Dropbox, FTP, etc.). No proprietary features of storage back-ends are used. Increments are stored in standard zip files which (as well as restoring in UpdraftPlus) you can unzip manually using any zip tool of your choice.
Database backups are usually smaller, more easily compressible, and quick to make, relative to the size of files and the resources needed to back them up. As a result, in the case of UpdraftPlus, incremental backups are available for backups of your files, and all database backups are always full backups. Since UpdraftPlus allows you to use separate schedules for your files and database backups, if you want more frequent “restore points” for your database, simply schedule more frequent database backups.
When you configure a time interval for your files backup, you are then able to choose an interval for incremental backups. When you choose an interval, UpdraftPlus will check every time this interval occurs if an incremental backup can be taken. If an original (i.e. full) backup is currently in progress, then it will do nothing. If it has finished, then UpdraftPlus will make an incremental backup (i.e. of all the files that have changed since the full backup finished).
When setting your retention settings for how many backups to retain, you should understand that increments added to a set still leave that set being counted as one set. If it is kept, it is one set that was kept; if it is deleted, then the original backup and all subsequent increments are deleted.
When restoring a backup that includes increments, you will be shown a list of times when the increments were created, just before you confirm that you wish the restoration to proceed. You can choose which point you wish to restore up to.
Note that, because WordPress media libraries can grow to enormous sizes which require a lot of resources to process (to list all files and compare two separate lists), UpdraftPlus does not keep track of every deletion event in your media library (by default, the folder wp-content/uploads). This means that whilst a full backup is always a perfectly accurate record of what files exist (and restoring it will always restore exactly what was present at the time of the backup), restoring an incremental restore point may leave behind media files on disk that were actually deleted in between the full backup and the incremental backup. They will not appear in the WordPress media library in the admin dashboard because their database record has gone, but the files can still exist on disk. This is a trade-off between the outcome and the resources needed to achieve it. The continued existence of a previously-deleted file after restoring an incremental backup is almost always entirely harmless; but if it could cause you a problem, then you should manually check for such files after restoring.