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[[!meta copyright="Copyright © 2011, 2013, 2014 Free Software Foundation,
Inc."]]
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[[!tag open_issue_documentation open_issue_hurd]]
[[!toc]]
# IRC, freenode, #hurd, 2011-11-18
<nocturnal> I'm learning about GNU Hurd and was speculating with a friend
who is also a computer enthusiast. I would like to know if Hurds
microkernel can recover services should they crash? and if it can, does
that recovery code exist in multiple services or just one core kernel
service?
<braunr> nocturnal: you should read about passive translators
<braunr> basically, there is no dedicated service to restore crashed
servers
<etenil> Hi everyone!
<braunr> services can crash and be restarted, but persistence support is
limited, and rather per serivce
<braunr> actually persistence is more a side effect than a designed thing
<braunr> etenil: hello
<etenil> braunr: translators can also be spawned on an ad-hoc basis, for
instance when accessing a particular file, no?
<braunr> that's what being passive, for a translator, means
<etenil> ah yeah I thought so :)
# Reincarnation Server
## IRC, freenode, #hurd, 2011-11-19
<chromaticwt> will hurd ever have the equivalent of a rs server?, is that
even possible with hurd?
<youpi> chromaticwt: what is an rs server ?
<chromaticwt> a reincarnation server
<youpi> ah, like minix. Well, the main ground issue is restoring existing
information, such as pids of processes, etc.
<youpi> I don't know how minix manages it
<antrik> chromaticwt: I have a vision of a session manager that could also
take care of reincarnation... but then, knowing myself, I'll probably
never imlement it
<youpi> we do get proc crashes from times to times
<youpi> it'd be cool to see the system heal itself :)
<braunr> i need a better description of reincarnation
<braunr> i didn't think it would make core servers like proc able to get
resurrected in a safe way
<antrik> depends on how it is implemented
<antrik> I don't know much about Minix, but I suspect they can recover most
core servers
<antrik> essentially, the condition is to make all precious state be
constantly serialised, and held by some third party, so the reincarnated
server could restore it
<braunr> should it work across reboots ?
<antrik> I haven't thought about the details of implementing it for each
core server; but proc should be doable I guess... it's not necessary for
the system to operate, just for various UNIX mechanisms
<antrik> well, I'm not aware of the Minix implementation working across
reboots. the one I have in mind based on a generic session management
infrastructure should though :-)
## IRC, freenode, #hurd, 2012-12-06
<Tekk_> out of curiosity, would it be possible to strap on a resurrection
server to hurd?
<Tekk_> in the future, that is
<braunr> sure
<Tekk_> cool :)
<braunr> but this requires things like persistence
<spiderweb> like a reincarnation server?
<braunr> it's a lot of works, with non negligible overhead
<Tekk_> spiderweb: yes, exactly. I didn't remember tanenbaum's wording on
that
<braunr> i'm pretty sure most people would be against that
<spiderweb> braunr: why so?
<Tekk_> it was actually the feature that convinced me that ukernels were a
good idea
<Tekk_> spiderweb: because then you need a process that keeps track of all
the other servers
<Tekk_> and they have to be replying to "useless" pings to see if they're
still alive
<braunr> spiderweb: the hurd community isn't looking for a system reliable
in critical environments
<braunr> just a general purpose system
<braunr> and persistence requires regular data saves
<braunr> it's expensive
<Tekk_> as well as that
<braunr> we already have performance problems because of the nature of the
system, adding more without really looking for the benefits is useless
<spiderweb> so you can't theoretically have both?
<braunr> persistence and performance ?
<braunr> it's hard
<Tekk_> spiderweb: you need to modify the other translators to be
persistent
<braunr> only the ones you care about actually
<braunr> but it's just better to make the critical servers very stable
<Tekk_> so it's not just turning on and off the reincarnation
<braunr> (there isn't that much code there)
<braunr> and the other servers restartable
<mcsim> braunr: I think that if there will be aim to make something like
resurrection server than it will be needed rewrite most servers to make
them stateless, isn't it?
<braunr> that's a lot easier and already works with non essential passive
translators
<Tekk_> mcsim: pretty much
<braunr> mcsim: only those you care about
<braunr> mcsim: the proc auth exec servers for example, perhaps the file
system servers that can act as root fs, but the others would simply be
restarted by the passive translator mechanism
<spiderweb> what about restarting device drivers, that would be simple
right?
<braunr> that's perfectly doable, yes
<spiderweb> (being an OS newbie) - it does seem to me that the whole
reincarnation server concept could quite possibly be a band aid.
<braunr> spiderweb: no it really works
<braunr> many systems do that actually
<braunr> let me give you a link
<braunr>
http://ftp.sceen.net/curios_improving_reliability_through_operating_system_structure.pdf
<braunr> it's a bit old, but there is a review of systems aiming at
resilience and how they achieve part of it
<spiderweb> neat, thanks
<braunr> actually it's not that old at all
<braunr> around 2007
## IRC, freenode, #hurd, 2013-08-26
< teythoon> I came across some paper about process reincarnation and
created a little prototype a while back:
< teythoon> http://darnassus.sceen.net/gitweb/teythoon/reincarnation.git/
< teythoon> and I looked into restarting the exec server in case it
dies. the exec server is an easy target since it has no state of its own
< teythoon> the only problem is that there is no exec server around to
start a new one
< youpi> teythoon: there could be another exec server only used to
(re)start the exec server
< youpi> that other exec server could even be restarted by the normal exec
server
< pinotree> what about a watchdog server?
< teythoon> youpi: yes, I had the same idea, i actually patched /hurd/init
to do that, it's just not yet working
< pinotree> make it watch other servers (exec included), and make exec
watch the watchdog only
< teythoon> pinotree: look at my prototype, there is a watchdog server
< braunr> teythoon: what's the point of reincarnation without persistence ?
< teythoon> braunr: there is no point in reincarnation w/o persistence of
course
< teythoon> my prototype does a limited form of persistence
< teythoon> the point was to see whether I can mitm a translator and
restart it on demand and to gain more insight into the whole translator
mechanism
< braunr> teythoon: ok
< teythoon> braunr: see the readme, it retains state across reincarnations
< braunr> teythoon: how ?
< teythoon> braunr: the server can store a checkpoint using the
reincarnation_checkpoint procedure
< teythoon>
http://darnassus.sceen.net/gitweb/teythoon/reincarnation.git/blame/HEAD:/reincarnation.defshttp://darnassus.sceen.net/gitweb/teythoon/reincarnation.git/blame/HEAD:/reincarnation.defs
< teythoon> uh >,< sorry, pasted twice
< braunr> oh ok
## IRC, freenode, #hurd, 2014-02-01
<pere> btw, can hurd upgrade the kernel without reboot?
<teythoon> no
<teythoon> but since most functionality is not within the kernel, the more
interesting question is, what parts of the hurd can be replaced at
runtime
<pere> ok. what is the answer to that question?
<teythoon> no hurd server can be restarted transparently, i.e. w/o its
clients noticing that
<teythoon> however, if a server is not in use, it can be easily restarted
<teythoon> transparently restarting servers would be nice
<teythoon> and i believe it is even possible on mach
<braunr> teythoon: how ?
<teythoon> one has to retain two things, client-related state and the port
right
<braunr> doesn't that require persistence ?
<teythoon> it does
<teythoon> but i see no reason why it should not be possible to implement
this on top of mach
<braunr> maybe
<teythoon> the most crucial thing is to preserve the receive port, and to
replace the server without race-conditions
<teythoon> receive rights can be transfered using the notification
mechanism
<antrik> braunr: restarting servers doesn't exactly require
persistance. you only need to pass the state from the old server to the
new one, rather than serialising it for on-disk storage. it's a slightly
easier requirement...
<antrik> (most notably, you don't need any magic to keep the capabilities
around -- just pass them over using normal IPC)
<teythoon> antrik: i agree, but then again, once this is in place, adding
persistence is only a little step
<antrik> teythoon: depends. if it's implemented with persistence in mind
from the beginning, it might be a fairly small step indeed; but
otherwise, it could be two entirely different things
<antrik> this also depends on the kind of persistence you want
<antrik> I must say that for the kind of persistence *I* would like, it is
indeed quite related
<teythoon> well, please elaborate a little :)
<teythoon> what do you have in mind ?
<antrik> busy right now... remind me some other time if I forget :-)
<teythoon> sure
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