How does `resize2fs` change the size of a file system?

Question

According to https://unix.stackexchange.com/a/501454/674,

parted's resizepart does not care about the filesystem at all. It just changes the partition table to specify a new location where the partition now ends. It does not overwrite anything at or near that location. After modifying the partition table, it will signal the kernel that the partition table has been changed. The kernel will read the new table and apply it if possible.

How does resize2fs change the size of a file system? Does it also do it in a similar way to parted resizepart? Specially:

https://unix.stackexchange.com/a/231623/674 says it extends a file system, by

writing additional filesystem metadata to the newly available storage.

What filesystem metadata does it write to "the newly available storage"? Does writing to "the newly available storage" in the sense of writing zeros all over "the newly available storage"? Or does it change some value stored in the superblock of the file system only?

https://unix.stackexchange.com/a/231623/674 says it shrinks a file system

by moving both filesystem metadata and your data around.

Does it not write some "filesystem metadata" to somewhere?

Thanks.

Answer 1

What filesystem metadata does it write to "the newly available storage"?

Whatever it needs for that particular filesystem to make the space usable.

For, say ext4, the filesystem is split into block groups, with each group containing a number of data blocks, plus a number of inodes, plus the free/used bitmaps for both the blocks and the inodes. (and perhaps some other bookkeeping data.)

Then, there's the main data structures of the filesystem that are needed to know how large the filesystem is to begin with, etc. They also need to be updated.

Does writing to "the newly available storage" in the sense of writing zeros all over "the newly available storage"?

Probably not. Though mkfs.ext4 does discard the unused blocks on the filesystem, it's possible that the same is done for the part where the filesystem extends to.