Not OP, but yes, that’s pretty much how it works. (ZFS scrubs do not defrgment data however).

Fragmentation isn’t really a problem for several reasons.

• Some (most?) COW filesystems have mechanisms to mitigate fragmentation. ZFS, for instance, uses a special allocation strategy to minimize fragmentation and can reallocate data during certain operations like resilvering or rebalancing.

• ZFS doesn’t even have a traditional defrag command. Because of its design and the way it handles file storage, a typical defrag process is not applicable or even necessary in the same way it is with other traditional filesystems

• Btrfs too handles chunk allocation effeciently and generally doesn’t require defragmentation, and although it does have a defrag command, it’s almost never used by anyone, unless you have a special reason to (eg: maybe you have a program that is reading raw sectors of a file, and needs the data to be contiguous).

• Fragmentation is only really an issue for spinning disks, however, that is no longer a concern for most spinning disk users because:

  • Most home users who still have spinning disks use it for archival/long term storage/media that rarely changes (eg: photos, movies, other infrequently accessed data), so fragmentation rarely occurs here and even if it does, it’s not a concern.
  • Power users typically have a DAS or NAS setup where spinning disks are in a RAID config with striping, so the spread of data across multiple sectors actually has an advantage for averaging out read times (so no file is completely stuck in the slow regions of a disk), but also, any performance loss is also generally negated because a single file can typically be read from two or more drives simultaneously, depending on the redundancy config.

• Enterprise users also almost always use a RAID (or similar) setup, so the same as above applies. They also use filesystems like ZFS which employs heavy caching mechanisms, typically backed by SSDs/NVMes, so again, fragmentation isn’t really an issue.