Re: [Qemu-devel] [RFC 0/2] block/file-posix: allow -drive cache.direct=off live migration

[Stefan Hajnoczi <stefanha@redhat.com>]

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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

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