Is Persistence doing full or incremental backups?
Persistence takes a single snapshot of the source directory, that it keeps updating every time you run the backup (during these updates, only files that have changed are backed up again). This means that you cannot retrieve older versions of your files, only the latest one backed up. Persistence is designed for large backups (say your entire iPhoto Library) and since its primary purpose is online backups, keeping older versions of large backups would take quite a bit of extra online storage space.
Mac OS X Leopard also comes with a built-in solution for "local" backups (Time Machine), that allows to retrieve older versions of backed up files. So even if you need incremental backups, Persistence is still a very good complement to Time Machine to add that extra "online safety net" to your backup solution.
What is this Maximum Data File Size?
One important optimization for online backups is to "pack" files together: if you have 5,000 files to backup, you don't want to upload 5,000 one by one. It's more efficient to group them into an archive (i.e. a "backup data file"), then compress it if necessary, and finally upload it.
The key is to figure out the right archive size: if too small, then it's as if you were uploading files one by one with an extra overhead. If too large, then considering that if a single file has changed in an archive, the entire archive needs updating, you can end up updating a lot more of the backup data than strictly necessary.
Thankfully, the Backup Assistant in Persistence recommends automatically an "optimal" data file size depending on the contents of the source directory, so you don't have to figure it yourself. You can of course specify a custom size if you wanted to.
Note that in Persistence 1.0b2 and earlier, the maximum data file size was set to a default of 25 Mb. This was much larger than needed in typical situations, and can result in large backup updates even when only a few files have changed.
If you originally created a backup with Persistence 1.0b2 or earlier, it is recommended that you use the Backup Assistant to reset it, duplicate it into a new one with an optimal maximum data file size, and finally delete the original one.
Can I use Persistence with Airport Disks or Time Capsule?
If you have a USB disk connected to an Airport Extreme Base Station, or have a Time Capsule, you can use them as destinations for Persistence backups. First, make sure file sharing is enabled using Airport Utility. Then create a new backup and when selecting the destination type in the Backup Assistant, choose "On another computer or server". In the next pane, select "AFP" as the protocol, and fill then other fields as follow:
- Address: the IP address for the AirPort Extreme Base Station or Time Capsule (typically 10.0.0.1)
- Username: if file sharing is protected with accounts, enter an account username, otherwise enter "guest"
- Password: if file sharing is protected with accounts, enter the corresponding password, otherwise enter the disk password
- Initial Path: the name of the volume (typically the same as the USB disk's name)
Can I access my backup data from another computer?
Keep in mind that Persistence is a backup solution, not a file-sharing solution. However, it is very easy to access the contents of a backup on another computer than the one it was generated from, as long you have a copy of Persistence also installed on this computer.
All you need to do is display the Backup Assistant, choose "Recreate a backup from orphan backup data", then follow the instructions. Once the backup has been added to the backup list, select it and simply go to the "Restoration" pane to get the files you're interested in.
All you need to do is display the Backup Assistant, choose "Recreate a backup from orphan backup data", then follow the instructions. Once the backup has been added to the backup list, select it and simply go to the "Restoration" pane to get the files you're interested in.
Is the backup data stored in a proprietary way?
The backup data generated by Persistence is compatible with open standard technologies (CPIO or CPGZ archives, split files, OpenSSL encryption…), and does not use proprietary formats, so you can still access your backup data without Persistence if you ever needed to. This may require some preliminary manipulations in Terminal because the backed up files can be open in the Finder.
Please note that this does not apply to Persistence Web: even if the backup data is generated the same way, you cannot download it directly from Amazon Web Services without going through the Persistence Web application.
Please note that this does not apply to Persistence Web: even if the backup data is generated the same way, you cannot download it directly from Amazon Web Services without going through the Persistence Web application.