I’d like to understand how (modern) filesystems are implemented and having trouble to fully understand superblocks and their backups. I reference ext4 and btrfs, but the questions may also apply to other filesystems.
Ext4 stores a couple of superblocks (my fs for example has one primary and seven backup SBs). I do understand that, since the superblock defines important characteristics of the filesystem, an inline backup makes sense. But I don’t get why so many backups, so:
- Why store so many superblocks? What’s the benefit of having 7 backup SBs versus e.g just one?
According to ext4 documentation ext4 stores a “write time” within the SB (last write since epoch). That would imply that every write-transaction also consists of a write to the SB. Given my system, having 7 backup SBs, each write-transaction would consist of 8 SB writes. That seems a ridiculous amount of non-sequential metadata writes for a single transaction, leading to the question:
- Am I correct that on ext4 SBs are written that often?
The same questions basically apply to btrfs where SBs are distributed among static address blocks (primary block at 0×10000) and only in SSD mode (due to wear leveling concerns) only one is written per commit.
- Is there a benefit for btrfs to store the primary superblock at 0×10000 instead of 0×0?
The documentation of btrfs also states:
Note that btrfs only recognizes disks with a valid 0×10000 superblock; otherwise, there would be confusion with other filesystems. This is even more confusing since a broken SB at 0×10000 would lead to an invalid filesystem, even if there’re other valid SBs at other locations.
- How does btrfs benefit from superblock backups, if the filesystem is invalid on a broken primary superblock?
