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

To: "Xen-Devel (xen-devel@xxxxxxxxxxxxxxxxxxx)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx, tmem-devel@xxxxxxxxxxxxxx
Subject: [Xen-devel] [RFC] tmem ABI change... backwards compatibility unnecessary?
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Wed, 1 Sep 2010 07:36:51 -0700 (PDT)
Cc: Fitzhardinge <jeremy@xxxxxxxx>, Jeremy, kurt.hackel@xxxxxxxxxx, Vasiliy G Tolstov <v.tolstov@xxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, stephen.spector@xxxxxxxxxx, Chris Mason <chris.mason@xxxxxxxxxx>
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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

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.)


P.S. For general information on tmem, see:

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