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

[Eric Blake]

On 04/19/2018 02:52 AM, Stefan Hajnoczi wrote:
> file-posix.c only supports shared storage live migration with -drive
> cache.direct=off due to cache consistency issues.  There are two main shared
> storage configurations: files on NFS and host block devices on SAN LUNs.
> 
> The problem is that QEMU starts on the destination host before the source host
> has written everything out to the disk.  The page cache on the destination 
> host
> may contain stale data read when QEMU opened the image file (before migration
> handover).  Using O_DIRECT avoids this problem but prevents users from taking
> advantage of the host page cache.
> 
> Although cache=none is the recommended setting for virtualization use cases,
> there are scenarios where cache=writeback makes sense.  If the guest has much
> less RAM than the host or many guests share the same backing file, then the
> host page cache can significantly improve disk I/O performance.
> 
> 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?

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

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