Filesystem Management – Why Reserved Space for Root?

Let's see. The device size is 1,465,138,583½ kB = 1,500,301,909,504 B. The filesystem consists of 366,284,288 blocks of 4096 B each, which is 1,500,300,443,648 B. I don't know what the remaining 1,465,856 B (1.4 MB) are used for (additional copies of the superblock? I know there are a few kB of space at the beginning for the bootloader.).

The filesystem contains 91,578,368 inodes of 256 bytes each, which takes up 23,444,062,208 B (about 22 GB, hint, hint). Then there is 1,442,146,364 kB = 1,476,757,876,736 B for file contents. This accounts for 23,444,062,208 B + 1,476,757,876,736 B = 1,500,201,938,944 B. The remaining size is 98,504,704 B = 24,029 blocks which is in the right range to be the journal size.

As you can see, everything is accounted for. (Ok, almost everything, but we're talking megabytes, not gigabytes.)

This reserve is primarily for the core system partitions so that root can still log in if a regular user manages to fill the drive and clog up the works. The space is needed for temp files, copying, and general elbow room for shell commands.

None is strictly needed on simple user data volumes. 5% on large modern drives is way too much, I use 2% just to be safe but likely is still overkill.(100 MiB would likely be enough for emergency mode on many systems)

However, the secondary reason for reserved space is that it leaves gaps between the end of one file and the beginning of the next. A little space can help prevent fragmenting of frequently altered files, essentially the modifications can be kept within the same physical area as the original file. Media files are rarely modified, unless of course you are editing said media. On SSDs fragmenting doesn't matter, as all segments are accessed at equal speed.