[Xen-devel] 2016 Xen hackathon notes - xenstored

["Luis R. Rodriguez" <mcgrof@xxxxxxxxxx>]

At the 2016 Xen Hackathon I raised the topic of the default xenstored
used. Here are my notes with some new additions and supported
documentation. It would seem we're moving to oxenstored as the default
on Linux distributions and FreeBSD now, if you have issues or concerns
with this please let us know.

Notes:
=====

Although we have had oxenstored be the the default *iff* you have
ocaml dev libs installled when compiling Xen from source both Linux
distributions (Debian, SUSE*, Gentoo) and FreeBSD were still using the
C xenstored as the default. This begged the question of why not make
the switch given Citrix has already been using oxensotred in
production for years with these known gains [0]:

  * 1/5th the size in terms of line of code in comparison to the C xenstored
  * better performance increasing support for the number of guests, it
    supports 3 times number of guests for an upper limit of 160 guests

At the 2014 summit Anil's presented work on a Xenstore 2.0 which
hinted also towards the future ability to provide git-like
capabilities for the xenstore, all still written in Ocaml [1]. There
are others who have worked on a C++ replacement lixs (Lightweight
XenStore) as well [2], such work revealed oxenstored had the CPU
pegged after just a few dozen guests. Such work hinted that the
oxenstored that should be considered for more serious work was the
Mirage OS xenstore [3], but that the C++ lixs was also performing
better than the current oxenstored.

Although there are questions about the future of the Xenestore we know
oxenstored performs better than cxenstored and since we are building
and using it by default already it begged the question why haven't
distributions made the switch to use it by default. Since Mirage OS
work seems promising we agreed to just set oxenstored as the default
in distributions and in the future hope that Mirage's work or other
contending efforts make it upstream to consider them as alternatives.

Given Mirage Xenstore would still require ocaml, it would be a good
stepping stone now to just use oxenstored by default more widely on
Linux distributions and FreeBSD. A concern was raised about expertise
over Ocaml, however it would seem that we will have no option but to
rely on the community / folks supporting upstream oxenstored for this.

[0] http://www.do-not-panic.com/2014/04/summary-of-gains-of-xen-oxenstored.html
[1] http://decks.openmirage.org/xendevsummit14#/
[2] 
http://events.linuxfoundation.org/sites/events/files/slides/xendevsummit14_0.pdf
[3] https://github.com/mirage/ocaml-xenstore

  Luis

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