[pve-devel] [PATCH container 0/2] Linux LIO/targetcli support

Udo Rader udo.rader at bestsolution.at
Thu Jun 28 17:26:04 CEST 2018


On Fri, 2018-06-15 at 13:45 +0200, Dominik Csapak wrote:
> > > during my initial tests, it worked (mostly) but i found some
> > > strange
> > > behaviours:
> > > 
> > > when we execute a zfs request not in a worker (e.g. a content
> > > listing)
> > > and then create a lun in a worker, only that worker, and no
> > > future
> > > worker knows of it (because when we fork, we copy all data
> > > currently
> > > in
> > > variables)
> > > 
> > > this seems due to the $SETTINGS variable which get filled
> > > whenever we
> > > get info from the target
> > > 
> > > it seems this is a general problem also for the other zfs over
> > > iscsi
> > > providers, has anyone who uses them (@mir?) the same problems?
> > > how do you prevent/handle this?
> > > 
> > > a possible solution i guess would be to prevent filling the
> > > $SETTINGS variable when not in a worker, any other idea?
> > 
> > One of the things I found too about SETTINGS is that the other
> > providers somehow seem to use it as a "cache", in order to reduce
> > the
> > amount of calls between the PVE and the target.
> 
> this cache is in general a good idea, because if we have an
> api call that needs many infos in short time it makes it faster
> e.g. an clone call of a vm with 10 disks:
> 
> * get infos about 10 disks
> * create 10 disks
> * copy 10 disks
> 
> in such a case, a cache can be useful
> 
> i do not know how costly the connections to a target really is, so i
> cannot really say if this caching makes sense and we have to fix it,
> or to drop it altogether
> 
> > 
> > This has the dangerous drawback, that if some external process
> > modifies
> > the target (ie adds or removes another LUN), PVE will probably
> > never
> > pick up that change, because it relies on the assumption that the
> > target is only controlled by the PVE. I have not fully investigated
> > this though.
> 
> that is exactly the problem, but also happens within pve
> (due to the worker mechanic)
> 
> > 
> > Originally I always repopulated the SETTINGS variable before
> > running
> > any "LUN command", but after looking at the other implementations,
> > I
> > decided to do it as they do it.
> 
> if the time to get the information is not too high, i would in
> general
> prefer your initial approach. but since the other implementations
> also do this, i originally asked @Michael (or mir; the initial author
> of this plugin) this
> 
> > 
> > So the "easy" fix for this would probably be to revert that and to
> > repopulate the SETTINGS hash for each call.
> 
> another option would be to have a 'clear cache' sub and
> if we executed a command *not* in a worker we call it after
> the 'lun_command' call in ZFSPlugin.pm
> 
> or a 'cache' parameter which indicates the use of cache,
> which we can the use in a worker
> 
> any other opinions?

I've just submitted a new version of the patch that expires the
$SETTINGS "cache" after 15 seconds, so if I'm right, that should solve
your issues ("[PATCH V2 storage 0/3] updated Linux targetcli/LIO
support")

As per how expensive it is to query the portal too often, I can't
really say.

What I see is that quite a few operations are done against the portal
for only one action in the user interface, so completely dropping the
cache would certainly be "expensive".

The old iet implementation has a maximum number of LUNs, which is set
to 16K.

Reprocessing 16K of LUNs over and over again, both on the portal an on
the PVE side of life is certainly no good idea, so I hope that 15
seconds are a good compromise.

Regards

Udo

-- 
Udo Rader, GF/CEO
BestSolution.at EDV Systemhaus GmbH
Eduard-Bodem-Gasse 5-7, A-6020 Innsbruck
http://www.bestsolution.at/
Reg. Nr. FN 222302s am Firmenbuchgericht Innsbruck
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