Why fsync() on OpenZFS can't fail, and what happens when it does By: Rob Norris

On OpenZFS, fsync() cannot fail - it will wait until the application’s changes are on disk before it returns. If there is a problem, such as a hardware failure, that causes the pool to suspend, then it will block until the pool returns. This could be seconds, hours, or never, depending on the nature on the failure.
Modern distributed systems can often cope with this type of failure by redirecting requests to another node, but they can only do this if fsync() returns an error instead of blocking.
In this talk I describe how OpenZFS implements fsync() and why it blocks when the pool fails. I then discuss a series of changes made to make it possible for fsync() to return failure - and what it means for applications when it does.