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

[Eric Blake]

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

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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