Martin Fick wrote:
--- On Mon, 1/12/09, Gordan Bobic <gordan@xxxxxxxxxx> wrote:
...
No need for fencing simply because you now use HA
translator. The assumption in this case is that the
servers can still talk to each other but that one
server's connection to the clients may have died.
That means that 50% of the scope for failure will still
wipe you out because you'll start splitbraining. Not the
way forward at all. A fencing setup will at least preserve
the data integrity.
Fencing won't help either without cooperation, see below...
The correct way to handle comms channel
failure between client and server is to have bonded
interfaces going via different physical paths. _ONLY_
dealing with the situation where both servers are alive and
connected to each other but we can only reach one due to an
obscure failure somewhere in the network (e.g. a failed
switch port or a failed NIC in the server) is a pretty
half-arsed edge case.
Why is that the correct way? There's nothing wrong with
having "bonding" at the glusterfs protocol level, is
there?
The problem is that it only covers a very narrow edge case that isn't
all that likely. A bonded NIC over separate switches all the way to both
servers is a much more sensible option. Or else what failure are you
trying to protect yourself against? It's a bit like fitting a big
padlock on the door when there's a wall missing.
That is somewhat what the HA translator is, except
that it is supposed to take care of some additional
failures. It is supposed to retransmit "in progress"
operations that have not succeeded because of comm
failures (I have yet to figure out where in the code
this happens though).
This is a reinvention of a wheel. NFS already handles this gracefully
for the use-case you are describing.
Why re-invent the wheel when the tools to deal with these
failure modes already exist?
Are you referring to bonding here? If so, see above
why HA may be better (or additional benefit).
My original point is that it doesn't add anything new that you couldn't
achieve with tools that are already available.
Any failures on the server side may still warrant a
fencing setup, but AFR is not yet setup to work
cooperatively with a fencing setup.
It doesn't have to be. If one server in AFR dies
nothing spectacular happens. Things time out and carry on. I
don't see what cooperation there would need to be. RHCS
does it's own heart-beating and fencing. Mix and match
as required.
Yes, if a server goes down you are fine (aside from the
scenario where the other server then goes down followed
by the first one coming back up). But, if you are using
the HA translator above and the communication goes down
between the two servers you may still get split brain
(thus the need for heartbeat/fencing).
And therein lies the problem - unless you are proposing adding a
complete fencing infrastructure into glusterfs, too.
But, even with the current write logging in AFR, there
are possible split brain scenarios which can not be
avoided even with heartbeat/fencing (yet). Anytime two
different clients try to write to the same area of the
filessystem and the network is segregated, there is a
chance that they each succeed and fail on opposite
servers causing split brain. There is nothing heartbeat
can do about this except attempt to mitigate the problem
by intervening. But heartbeat has no hooks to know when
this happens so by the time heartbeat intervenes,
"half writes" to each server may have occurred that
cannot be undone. That is the reason you really need
cooperation between AFR and some other tool (such as
heartbeat).
No, that's the whole point. You DON'T need that cooperation. If AFR is
server-side, if the server's disconnect, cluster/heartbeat will
disconnect, too, which will initialize fencing and failover,
hard-power-off the failed server, and everything lives happily ever
after. GlusterFS doesn't need to be aware. One node disappears. That's
about the size of it. When it comes back, if files were written to,
their timestamps will be newer, so on next read, they'll get synced to
the re-joined node.
AFR needs to be able write all or nothing
to all servers until some external policy machine
(such as heartbeat) decides that it is safe (because
of fencing or other mechanism) to proceed writing to
only a portion of the subvolumes (servers). Without
this I don't see how you can prevent split brain?
With server-side AFR, splitbrain cannot really occur (OK, there's a tiny
window of opportunity for it if the server isn't really totally dead
since there's no total FS lock-out until fencing is completed like on
GFS, but it's probably close enough). If the server's can't heartbeat to
each other, they can't AFR to each other, either. So either the write
gets propagated, or it doesn't. The machine that remained operational
will have more up to date files and as necessary those will get synced
back. It's not quite as tight as GFS in terms of ensuring data
consistency like a DRBD+GFS solution would be, but it is probably close
enough for most use-cases.
Gordan
