Jörn Engel wrote:

Does the allocation have to be persistent beyond lifetime of the file
descriptor?  It would be fairly simple to support the write guarantee
while the file is open (or rather the inode remains cached) and drop it
afterwards.

"The posix_fallocate() function shall ensure that any required storage
for regular file data starting at offset and continuing for len bytes is
allocated on the file system storage media."

I interpret "on the storage media" to mean that it is persistent.

-Eric