We provide four metrics to help you understand the usage of a particular backup. It's impossible to provide a full picture, but we believe our metrics associated with each backup can be helpful.
The Mapped metric tells you how much data exists within that backup. If you were to delete every other backup, attach the backup as a drive, then delete every other drive, this would be your total storage usage. It is important to understand that your total usage is not the sum of all "mapped" values. The same data can be mapped in multiple backups (and drives).
Unique to Slicie, we provide the Added metric, which tells you how much data within the backup was added immediately prior to this backup.
Unique to Slicie, we provide the Modified metric, which tells you how much data within the backup was overwritten immediately prior to this backup.
If you backup your drive, then modify 1 GiB of data that existed in the past, then backup your drive again; that new backup will show 1 GiB for the Modified metric. The same process applies to the "added" metric, but you would want to add 1 GiB of brand new data. This isn't easy to reproduce in practice because your filesystem does not necessarily overwrite file data by modifying the same existing data on the drive. Our servers come with XFS by default, which also has a "write log," which will be modified when you write any data. Filesystems like ext3/ext4 (like your boot partition by default) have a metadata journal that is modified as well.
Unique to Slicie, we provide the Freed metric, which tells you how much data within the backup was discarded immediately prior to this backup. A "discard" is a mechanism your server has to signal to the underlying storage that space on a storage device is no longer in use. This was originally intended to help the performance of SSD drives by freeing room for garbage collection algorithms, but we use it to allow you to scale storage usage down.
There are several utilities for discarding data, which allows you to give space back to Slicie. It is important to keep in mind that space that is mapped within a backup will not be freed because it still exists in the backup.
The safest utility is "fstrim" in Linux, which is configured to run automatically on your server by default. You can manually run it with "fstrim /". Running fstrim more often than daily does not provide much additional value.
The other definitely not safe command for non-experts is "blkdiscard". Plenty of people have destroyed their data with that command. Fortunately, on Slicie, you will probably have so many backups it's not a big deal if you do. If you are an expert, keep in mind Slicie works with blocks that are 64KiB in size.
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