Many filesystems are logged and provide atomic operations. You can build multi-step transactions on top of them with various operations. And with CoW filesystems you can even get read consistency. Oh, and optimistic locking also is an option.

Are we talking about the POSIX interface to files or is there a new transactional API that allows for atomic writes (all pages are written to disk or none are), consistency, isolation (opening a file in another process which is currently being written won't show the pages that are being written), and makes the writes always durable (sync; not fsync)?

> Are we talking about the POSIX interface to files

Those give you these atomic ops and optimistic or cooperative locking: https://rcrowley.org/2010/01/06/things-unix-can-do-atomicall...

Multi-file transactions can be built by moving whole directories over symlinks to the previous version. The linux-specific RENAME_EXCHANGE flag can simplify this.

> or is there a new transactional API

CoW filesystems give you snapshots and reflink copies via ioctls. Under heavy concurrency this can provide cheaper isolation than locking.

> and makes the writes always durable (sync; not fsync)?

For durability fsync is sufficient if you do it on the file and the directory. To combine atomic and durable you can do the write, fsync, rename, fsync dir dance.

https://lwn.net/Articles/457667/

>For durability fsync is sufficient if you do it on the file and the directory. To combine atomic and durable you can do the write, fsync, rename, fsync dir dance.

Right. This allows for the correct behaviour but I wouldn't conclude that POSIX is a transactional API. I would regard this as being able to build a transactional API on top of it; but not transactional in itself. I mean, if you had to do the same dance in SQL (insert to dummy pkey and then update the entry) you would rightly dismiss it.

Which is why I said

> You can build multi-step transactions on top of them with various operations.

That's not really a true statement. I use plenty of databases that aren't transactional beyond a single record.

It is a true statement. You're not a member of the set of "most people":

> a basic property that most people consider necessary

I use non-ACID stores for some purposes, but don't consider them real databases.