On Fri, Apr 20, 2018 at 08:53:33AM -0500, Eric Blake wrote: > On 04/19/2018 10:05 PM, Stefan Hajnoczi wrote: > > >>> This patch series implements .bdrv_co_invalidate_cache() for > >>> block/file-posix.c > >>> on Linux so that shared storage live migration works. I have sent it as > >>> an RFC > >>> because cache consistency is not binary, there are corner cases which I've > >>> described in the actual patch, and this may require more discussion. > >> > >> Interesting, in that the NBD list is also discussing the possible > >> standardization of a NBD_CMD_CACHE command (based on existing practice > >> in the xNBD implementation), and covering whether that MIGHT be worth > >> doing as a thin wrapper that corresponds to posix_fadvise() semantics. > >> Thus, if NBD_CMD_CACHE learns flags, we could support > >> .bdrv_co_invalidate_cache() through the NBD protocol driver, in addition > >> to the POSIX file driver. Obviously, your usage invalidates the cache > >> of the entire file; but does it also make sense to expose a start/length > >> subset invalidation, for better exposure to posix_fadvise() semantics? > > > > bdrv_co_invalidate_cache() is currently only used by migration before > > using a file that may have been touched by the other host. I don't > > think start/length will be needed for that use case. > > > > Can you describe how will NBD use cache invalidation? Maybe this will > > help me understand other use cases. > > That's where things are still under discussion - no one has yet provided > a case that would benefit from a POSIX_FADV_DONTNEED over just a range > of the file [1]; on the other hand, it might make sense that if you know > an implementation has a limited cache, then having control over the > various posix_fadvise() flags over various ranges of the files may lead > to more optimum behavior. And posix_fadvise() does have the ability to > work over the entire file (offset 0 length 0) or a subrange (any offset > and nonzero length). So I'm also fine if .bdrv_co_invalidate_cache() > doesn't expose offset/length parameters, particularly if NBD can't come > up with an actual use case that would benefit. > > [1] https://lists.debian.org/nbd/2018/04/msg00020.html
Okay. .bdrv_co_invalidate_cache() is an internal interface. We can change it later to support NBD functionality, if necessary. Let's keep it without start/length for now. Stefan
signature.asc
Description: PGP signature
