[Xen-devel] Re: [RFC] tmem ABI change... backwards compatibility unnecessary?

[Vasiliy G Tolstov <v.tolstov@xxxxxxxxx>]

Ð ÐÑÐ, 01/09/2010 Ð 07:36 -0700, Dan Magenheimer ÐÐÑÐÑ:
> Tmem users and Xen developers/distros --
> 
> (Please forward/repost as you see fit.)
> 
> After a great deal of discussion and review with linux
> kernel developers, it appears there are "next-generation"
> filesystems (such as btrfs, xfs, Lustre) that will not
> be able to use tmem due to an ABI limitation... a field
> that represents a unique file identifier is 64-bits in
> the tmem ABI and may need to be as large as 192-bits.
> So to support these guest filesystems, the tmem ABI must be
> revised, from "v0" to "v1".
> 
> I *think* it is still the case that tmem is experimental
> and is not used anywhere yet in production.  If I am
> wrong, PLEASE LET ME KNOW ASAP.
> 
> The tmem ABI is designed to support multiple revisions,
> so the Xen tmem implementation could be updated to
> handle both v0 and v1.  However this is a bit
> messy and would require data structures for both v0
> and v1 to appear in public Xen header files.
> 
> I am inclined to update the Xen tmem implementation
> to only support v1 and gracefully fail v0.  This would
> result in only a performance loss (as if tmem were
> disabled) for newly launched tmem-v0-enabled guests,
> but live-migration between old tmem-v0 Xen and new
> tmem-v1 Xen machines would fail, and saved tmem-v0
> guests will not be able to be restored on a tmem-v1
> Xen machine.  I would plan to update both pre-4.0.2
> and unstable (future 4.1) to only support v1.
> 
> I believe these restrictions are reasonable at this
> point in the tmem lifecycle, though they may not
> be reasonable in the near future; should the tmem
> ABI need to be revised from v1 to v2, I understand
> backwards compatibility will be required.
> 
> Comments or questions?  (If agreeable, positive public
> acks appreciated.)

Thank You, Dan. What needed to do, to use tmem v1? Does it needed to
recompile dom0 kernel, or only domU kernel affected by change?

-- 
Vasiliy G Tolstov <v.tolstov@xxxxxxxxx>
Selfip.Ru


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