(Why not) write from multiple shards into the same file?

[niekbouman]

The documentation of Seastar's file api suggests that writing from multiple shards to the same file simultaneously is not supported, at least not through the dup mechanism:

seastar::file::dup()
...
Note
Use on read-only files.

Assuming that we would ensure that there is enough space in the file (by pre-allocating space), and by ensuring that each shard performs aligned writes in non-overlapping regions,
then is this actually supported by the OS (for the async-IO_DIRECT mode that Seastar uses)?

In case if it would be supported by the OS, then why not by Seastar's file api?

[avikivity]

It should work. Note performance on large machines may be bad.