Re: when does BIO_FLUSH happen on a disk?

@Konstantin Belousov: Thank You for the fast and helpful explanation!

but does a finished write request from a disk / lower geli layer mean, 
that the data arrived on stable storage (in which case BIO_FLUSH seems 
unnecessary to exist) - or does fsync on UFS only guarantee ordering 
with respect to other operations on the same filesystem?

On 1/25/25 06:15, Konstantin Belousov wrote:
> The only case where UFS issues BIO_FLUSH, together with BIO_ORDERED flag,
> to prevent reordering of the requests before/after the flush, is when
> a journal segment is written.  This is done to ensure that metadata blocks
> are written after the conduit journal records are on the stable storage.
> 
> Generally, UFS uses softupdates technique and relies on the callbacks from
> the buffer write completions to update internal structures to indicate
> metadata write progress.  With such approach, there is no need to issue
> flushes.   For instance, when doing fsync(2) and scheduling writes required
> to flush the metadata and data, we only return from syscall when disk
> reported that all writes were done.
> 
> The case above (for the journal segment write) could be considered as a
> way to short-cut some additional code that would otherwise need to be
> written.