gordan@xxxxxxxxxx wrote:
I'm assuming that versioning and locking can and should be combined.
You've admitted the necessity for keeping copies of files synchronized
and IO is always going to require some sort of lock to accomplish
this. By having the quorum remain aware of what the most recent
version of a given file is, whether that file is locked, and perhaps
where copies of the file reside, you could reduce the number of nodes
that must be consulted when a lock is needed.
True enough, but some care would need to be exercised to ensure that a
this doesn't lead to edge cases where a node thinks it still has a lock,
but all the other nodes have expired it (e.g. temporary network outage).
The DLM that GFS uses must already take this into account since it
appears to work just fine, and the GPL'd code for that DLM was
officially added to the Linux kernel with release 2.6.19, according to
Wikipedia. Not sure how portable that would be, but the source is
available...
I think you will also speed things up if you don't have to consult all
nodes for every IO operation. If all available nodes must be
consulted, then you introduce an implicit wait until a specified
timeout for every IO request if any single node is down. With the
quorum model, even before fencing takes place, almost half the nodes
can go incommunicado and the rest can operate as efficiently as they
did with all nodes in service.
Indeed quorum of (n/2)+1 nodes should, in theory, suffice for safely
granting a lock, but it would probably mean that the locks should be
refreshed several times more often than the default lock TTL, just to
account for scope of packet loss. Releases of locks should, of course,
be explicitly notified to the cluster.
Yes. Again, perhaps the DLM from RedHat's cluster project already
solves this?
Just brainstorming quorum theory, but if scalability of the quorum model
becomes an issue, perhaps the quorum could only be consulted to elect,
discover, and verify a mini-quorum of nodes (perhaps elected partially
based on the fact that they reside behind different switches). Then
once a node was aware of the identity of the mini-quorum, it would only
have to cummunicate with a single node for locking and file discovery
purposes. This assumes that hosts in the mini-quorum would refuse to
cooperate with a client if they could not confirm with the quorum that
they were still part of the mini-quorum or couldn't contact the rest of
the mini-quorum, of course, and would consult the quorum to elect a new
mini-quorum when either of these was not the case.
If some HA and fault-tolerant DHT implementation exists that already
handles atomic hash inserts with recognizable failures for keys that
already exist, then perhaps that could take the place of DLM's quorum
model, but I think any algorithm that requires contacting all nodes
will prove to be a bad idea in the end.
Not all nodes - only the nodes that contain a certain file. A single
ping broadcast to find out who has a copy of the file should prove to be
of insignifficant bandwidth overheat compared to actual file transfers,
unless you are dealing with a lot of files that are signifficantly
smaller than a network packet.
My point was that, as I understood your algorithm, a client would not
know which nodes contained a certain file until all nodes had been
contacted. So, while the actual bandwidth, even to consult thousands of
nodes, might be small relative to file transfer bandwidth, the client
can't assume it has a complete answer until it gets all the replies,
meaning requests to downed nodes have timed out. Meaning that if you
assume that at least one node will always be down, then the minimum time
to locate a node with the most recent copy of the file (and thus the
minimum time to begin any read) is always the timeout attached waiting
for the ping reply.
Having the entire quorum aware of which version of each file is the most
recent and where to find the file avoids this problem, again, until just
less than half the nodes become unreachable.
I might optimize the expunge algorithm slightly by having nodes with
low loads volunteer to copy files that otherwise couldn't be expunged
from a node. Better yet, perhaps, would be a background process that
runs on lightly loaded nodes and tries to create additional redundant
copies at some configurable tolerance beyond the "minimum # of copies"
threshold.
Not just lightly loaded nodes, but more importantly, nodes with most
free space available. :)
Yes, the algorithm to detect "loading" should probably consider as many
resource constraints as appears practical.
For file delta writes, an AFR type mechanism could be used to send
the deltas to all the nodes that have the file. This could all get
quite tricky, because it might require a separate multicast group to
be set up for up to every node combination subset, in order to keep
the network bandwidth down (or you'd just end up broadcasting to all
nodes, which means things wouldn't scale as switches should, it'd be
more like using hubs).
This would potentially have the problem that there is only 24 bits of
IP multicast address space, but that should provide enough groups
with sensible redundancy levels to cover all node combinations. This
may or may not be way OTT complicated, though. There is probably a
simpler and more sane solution.
I'm not sure what overhead is involved in creating multicast groups,
but they would only be required for files currently locked for write,
so perhaps creating and discarding the multicast groups could be done
in conjunction with creation and release of write locks.
Sure, these could be dynamic, but setup and teardown might cause enough
overhead that you might as well be broadcasting all the locks and
writes, and just expect the affected nodes to pick those out of the air
and act on them.
It's also possible that you could reduce the complexity of this
problem by simply discarding as many copies down to as close to the
minimum # as other nodes will allow, on write. However, I think that
might reduce some of the performance benefits this design otherwise
gives each node.
Also remember that the broadcasts or multicasts would only actually be
useful for locks and file discovery. The actual read file transfer would
be point-to-point and writes would be distributed to only the subset of
nodes that are currently caching the files.
Read would be point-to-point (perhaps multi-point to point for implicit
read striping across all known valid copies?), but it could still be
useful to use multi-cast for write, especially if the redundant copies
were behind a different switch than the node accepting the write. So
multi-cast setup could happen when a server obtained a write lock, and
teardown would be delayed until synchronization of redundant copies had
completed.
There would need to be special handling of a case where a node accepting
a big write is running out of space as a consequence and something has
to be dropped. Obviously, none of the currently open files can be
discarded, so there would need to be some kind of an auxiliary process
that would make a node request a "volunpeer" (pun intended) to take over
a file that it needs to flush out, if discarting it would bring the
redundancy below the required threshold.
I think this could be worked into the normal expunge algorithm with a
property like: "ANY request to expunge a file that reduces the file
count below the redundancy threshold will ALWAYS generate a volunpeer IF
at least one node exists with the disk space available".
It wouldn't require any special casing - the needed space will always
become available upon expunge if space for the migrating file exists
anywhere on the network. If all the files are expunged, or they can't
be even with this property of expunge, and the local disk still fills
up, then I think it would be reasonable for the FS to return a disk full
error.
Regards,
Derek
--
Derek R. Price
Solutions Architect
Ximbiot, LLC <http://ximbiot.com>
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