On Wed, 25 Mar 2009, Jeff Garzik wrote:
It is clearly possible to implement an fsync(2) that causes FLUSH CACHE to be
issued, without adding full barrier support to a filesystem. It is likely
doable to avoid touching per-filesystem code at all, if we issue the flush
from a generic fsync(2) code path in the kernel.
We could easily do that. It would even work for most cases. The problematic ones are where filesystems do their own disk management, but I guess those people can do their own fsync() management too.
Somebody send me the patch, we can try it out.
Remember, fsync(2) means that the user _expects_ a performance hit.
Within reason, though.
OS X, for example, doesn't do the disk barrier. It requires you to do a separate FULL_FSYNC (or something similar) ioctl to get that. Apparently exactly because users don't expect quite _that_ big of a performance hit.
(Or maybe just because it was easier to do that way. Never attribute to malice what can be sufficiently explained by stupidity).
Linus