Comment by lxgr

At least for ext4 and actually written data (i.e. not ftruncate’d files), I believe zeroes should really not occur.

Both extents and the file size are metadata as far as I understand, which would be atomically updated through the journal.

Data can be written before metadata (in data=ordered mode):

> All data are forced directly out to the main file system prior to its metadata being committed to the journal.