The meaning of fsync

Posted Jul 20, 2013 17:56 UTC (Sat) by giraffedata (guest, #1954)
In reply to: The 3.11 merge window closes by Karellen
Parent article: The 3.11 merge window closes

I thought your OS was "allowed" to overwrite your partition tables and FS journals completely in the event of a crash and still be POSIX-compliant.

The thing about adherence to any standard is that one specifies the very adherence with myriad conditions, most of them implied. So POSIX doesn't say, "if the system crashes, a read doesn't have to get back the same data that was written." Rather, the system designer says, "the system is POSIX-compliant as long as the system never crashes." And as I said, that condition is usually not actually spoken. There are tons of similar conditions: the superuser does not write directly to the disk; the disk drive never makes a mistake; cosmic rays don't change magnetic state; etc.

Of course, designers do whatever they can to reduce the conditions; few systems today are offered on a "if the power ever goes out, nothing in the POSIX standard applies" basis.

Fsync drives us into the awkward territory of robustness. Robustness is a system's ability to work when it is broken. That contradiction in terms is why any specification of fsync is bound to be fuzzy. It's like saying, "I will pay you back by Tuesday. If I don't, ..."