Recently, I've had some folks ask me offline what exactly would a “complete” Linux cluster stack look like. That's a good question, and this posting is intended to address that question.
So let's start with – what kind of cluster? For the purposes of this posting, I'm primarily talking about a full-function high-availability enterprise-style cluster, not primarily a load balancing cluster, and not a high-performance scientific (Beowulf-style) cluster.
A few caveats before proceeding – much of what I'll reference below will be relative to the Linux-HA[1] framework, but the concepts are easily translated to any other clustering framework one might have in mind.
It's also worth noting that not every application, nor every configuration needs every component. Adding unnecessary components adds complexity, and complexity is the enemy of reliability.
Many of the components (cluster filesystem, DLM) are primarily needed by cluster-aware applications. Note that at this time (early 2008) very few applications are cluster-aware.
The Full Cluster Stack Exposed!
Below is a picture of the full cluster stack – which I'll describe in more detail later. For the most part, the components higher up in the picture build on the components lower down in the picture. To simplify the drawing, I didn't add all the who-uses-whom lines that one might want to make a detailed study of this subject.
Cluster Comm - Intracluster communications
The most
basic component any cluster needs is intracluster communications.
There are a variety of different possibilities, but guaranteed packet
delivery is a requirement. Linux-HA has its own custom comm layer
for doing this. It's not perfect, but it works. At one time in the
past, we provided support for the AIS cluster APIs, and if you use
OpenAIS today, then you can still have a reasonable cluster using
Linux-HA and providing compatible support for the AIS protocols. As
will become clear, it's not a perfect configuration, but it's a
reasonable one. (Of course, like everything, it can always be improved even more)
Very large clusters (hundreds to tens of thousands of nodes) will likely require a different communication protocol, since most guaranteed delivery multicast protocols don't scale that high.
Nevertheless, in an ideal world, all cluster components and cluster-aware applications would sit on top of the same set of communications protocols.
Membership – who's in the cluster?
Looking to the right of the cluster comm box on our architecture chart, you'll see the membership box. The next basic function that a cluster has to provide is membership services. Membership closely related to communication – since a simplistic view of membership is just who we can communicate with. It is highly desirable that everyone in the cluster be able to communicate with everyone else. It's the job of the membership layer to provide this information to the cluster.
When your communication fails in weird ways, it's the job of the membership layer to present a view of the cluster that makes sense – in spite of the weird kinds of failures that might be going on.
If we eventually wind up with multiple kinds of communications methods, then we'll also have multiple ways of becoming a member.
Linux-HA (with or without OpenAIS) supports the AIS membership APIs.
Like I
mentioned for communication, in an ideal world, all cluster
components would use exactly the same membership information. However, it is important to note that the membership one uses must be computed using the communication method being used by the application. So, unless every cluster-aware application uses both the common communication method and the common membership, it risks getting its membership out of sync with respect to its communication and other components using other communication methods. In many cases, this can't be avoided. Methods for coping with such discrepencies are discussed in more detail at the end of this post.
Fencing
Fencing is the ability to “disable” nodes not currently in our membership without their cooperation Many of you will remember having discussed this in some detail in an earlier post[2]. As I explained in more detail there, fencing is vital to ensure safe cluster operation
Our current implementation is STONITH[3]-based - STONITH == Shoot The Other Node In The Head
Quorum
Quorum is a topic we've talked about extensively in a couple of earlier posts [2] [4]. Quorum encapsulates the ability to determine whether the cluster can continue to operate safely or not
Quorum is tied closely to both fencing and membership. In practice, as we discussed before, it is often highly desirable to implement multiple types of quorum. Linux-HA currently provides multiple implementations and can provide more through plugins. Like membership and communication, it is desirable for all cluster components to use the same quorum mechanism. All the interesting and legal ways that quorum can interact with fencing and membership and the communication layer are too detailed for this posting.
Cluster Filesystems
Cluster filesystems allow multiple machines to sanely mount the same filesystem at the same time. This is a great boon to parallel applications. Cluster filesystems typically don't use the network or another server involved when doing bulk I/O. Each node mounting the filesystem is normally expected to have access to the data. This typically requires a SAN.
Typically, this is done to improve performance, but convenience and manageability are common secondary goals Cluster filesystems are related to, but distinct from, network filesystems like NFS and CIFS. On Linux, there are several cluster filesystems available including
Red Hat's GFS
Oracle's OCFS2
IBM's GPFS
etc.
Normally, when they're being used for performance reasons, cluster-aware applications are required. You can't typically just run 'n' copies of your favorite cluster-ignorant application and have it work. The filesystem won't scramble the data, but your application typically will. It goes without saying that high-performance cluster filesystems run in the kernel – unlike all the other items we've talked about before. Because of the high-performance, it's common for a cluster filesystem to have its own communication and membership code – not using the typically userland communications and membership code. Since membership isn't high bandwidth or really low latency information, it is possible to feed membership from a user-space membership layer into the kernel. Of course, then the membership and the communications layer are out of sync. It is arguable whether this is an improvement or not.
Cluster filesystems typically need cluster lock managers – described in the next section.
DLM - Distributed Lock Manager
A DLM (Distributed Lock Manager) provides locking services across the cluster, and it's an interesting piece of code to implement them – particularly the error recovery.
To some degree, DLMs are analogous to System V semaphores but – cluster-aware. In addition, they provide much more sophisticated API and semantics. Although DLM APIs are fairly well understood, there is no formal standard, so switching from one to another can be annoying. Red Hat has a reasonable kernel-based DLM which they use with GFS. DLMs commonly have their own separate communications and membership code. The comments about getting membership from user-space and having them be potentially different from cluster filesytems also apply here.
Cluster Volume Managers
You might think that you really don't need a cluster-aware volume manager. Sometimes you might be right. More often, if you thought that, you'd be wrong. A cluster volume manager is just like a regular volume manager – only cluster aware. This is to keep different nodes from getting inconsistent views of the layout of a set of disks or volumes. The current cluster-aware volume managers are EVMS and CLVM. Only CLVM is expected to survive into the long term.
The big challenges for cluster volume managers are high-performance mirroring and snapshots. These operations are potentially very difficult to implement right and fast. Cluster-aware volume managers often have both kernel and user-space components. The membership inconsistency issues here are similar to those for cluster filesystems and the DLM.
CRM - Cluster Resource Manager
Every HA cluster has something like a CRM, but they may divide up these functions differently. Our CRM is a policy-based decision maker for what should run where – handling failed services and failed cluster nodes.
The CRM is similar to UNIX/Linux startup init scripts – it starts everything up – but across a cluster following some policies, and managing failures.
The Linux-HA CRM is arguably the best cluster resource manager around today – at least in terms of flexibility and power. It has usability issues, and can be extended, but those are solvable.
The Linux-HA CRM function is largely divided between the PE and TE – which are described below.
PE - Policy Engine
The Policy Engine is a key component of the CRM and does two distinct things.
It determines what should run where (cluster layout)
It creates a graph of actions of how to get from the current state of affairs to the new desired state
This graph of actions is then given to the TE (described below).
The system would have more flexibility if he PE were split into two parts for these two functions, and supported plugins for the cluster layout function.
It currently isn't aware of resource cost, nor of absolute resource limits and load balancing considerations, which complicate optimal placement. Those would be good things to add to it in the future. Having plugins for doing resource placement would also be a highly useful and desirable thing.
TE - Transition Engine
Receives a graph of actions to perform from the policy engine, then uses the LRM proxy to communicate with the LRMs to carry out the actions
Its main jobs are action sequencing, error detection and reporting
CIB - Cluster Information Base
The CIB manages information on cluster configuration and current status. The cluster configuration includes the configuration and policies as defined by the system administrator.
Its key difficulty is to keep a consistent copy replicated across the cluster, resolving potential version differences.
All the data it manages is XML, and the CIB has a minimal knowledge of the structure of this XML.
LRM - Local Resource Manager
In the Linux-HA architecture, a local resource manager runs on every machine and carries out the tasks given to it. Everything that gets done gets carried out by the LRM. Examples are:
start this resource
stop this resource
monitor this resource
migrate this resource
etc.
The LRM provides interface matching to the various kinds of resources through Resource Agents. The Linux-HA LRM supports several classes of Resource Agents.
The LRM is not at all cluster-aware. It can support an arbitrary number of clients, one of which is the LRM communications proxy (below).
LRM Communications Proxy
The LRM proxy communicates between the CRM and the LRMs on all the various machines. This function is currently built into the CRM. This architectural decision was based on expedience more than anything else.
To support larger clusters this needs to be separated out, made more scalable, and more flexible. This would allow a large number of LRMs to be supported by a small number of LRM proxies. In large systems, this would probably use the ClusterIP capability to provide load distribution (leveling) across multiple LRM proxies.
Init - Initialization and recovery
This code does really three things:
Sequences the startup of the cluster components
Recovers from component failures (restart or reboot)
Sequences the shutdown of all the various cluster components
This is currently provided by Linux-HA and bundled with the Linux-HA communications code. This likely needs to be separated out to a separate proxy function (process) in the future.
Infrastructure
The Linux-HA infrastructure libraries (“clplumbing”) does a wide variety of things. A few samples include:
Inter-Process Communication
Process management
Event management and scheduling
Many other miscellaneous functions
Surprisingly, these libraries amount to about 20K lines of code.
Quorum Daemon
The quorum daemon is an unusual daemon, because it's the only daemon we have that's intended to run outside the cluster proper. It is instrumental in solving certain knotty quorum problems – especially for:
2-node clusters (very common)
Split-site (disaster recovery) clusters
This was discussed extensively in previous postings [4] and [5].
Management Daemon
Provides Complete Authenticated Configuration and Status API. This includes both information contained in the CIB, and also information about the communications configuration and so on.
The management daemon is used by the:
GUI
SNMP agent
CIM agent
Clients are authenticated using PAM, and all communications is via SSL, so its clients can safely be outside cluster, or even outside a firewall. This daemon should provide different levels of authorization depending on the authenticated user, and should log its actions in a format suitable for Sarbanes-Oxley (SOX) auditing purposes.
GUI
Provides a Graphical User Interface providing configuration and status information. It also supports creating and configuring the cluster. Note that at the present time, there are a number of useful cluster configurations it cannot create.
CIM and SNMP agents
The CIM and SNMP agents provide CIM and SNMP management interfaces for systems management tools. The CIM interface supports status updates and configuration changes, whereas the SNMP interfaces only report status.
Disadvantages of this architecture
For a
variety of reasons, kernel space doesn't have access to user-space
cluster communications or membership.
As a result, both the DLM and most cluster filesytems implements their own membership and communications.
This is in contradiction to the “ideal world” statements earlier. This can result in some odd cases where one communication method is working in a particular case, but another method is not. This results in differences in membership – which can have bad effects.
Why this might not be quite as bad as it seems
One reason why one might not worry about this as much as one might, is because it's a problem which one can't make go away. A cluster system will always have to interface with software packages which do their own communication, and compute their own membership for a variety of usually good reasons. As a result, this is a problem which we can't make go away. Instead we have to deal with it effectively. There are basically two cases to consider:
The “Main” membership thinks that node X should not be in the cluster, whereas the “Other” membership thinks it should be.
The “Other” membership thinks that node X should not be in the cluster, whereas the “Main” membership thinks it should be.
Let's take these two cases one at a time:
Case 1:
If the main membership thinks node X is not in the cluster, then it will simply not start any resources on node X. This takes care of the problem.
Case 2:
If the “Other” membership discovers that a particular node should be dropped from its view of membership, and it can inform the CRM not to start its resources on that machine, then the local view of this membership from the perspective of the resources it deals with is effectively made to exclude these Other-errant nodes. In the Linux-HA CRM this is easily done having the Other-resources write node attributes to cause those nodes to be excluded, and the rules would then be written to exclude those nodes from consideration for running Other-related resources.
Although Case 2 isn't pretty, it works, and no amount of wishing and hoping is likely to ever make this kind of problem go away in the general case - particularly when one involves proprietary applications So, even if there is some membership discrepancy, it can is always possible to manage it appropriately assuming you can get a tiny bit of cooperation from the application.
References
[1]
http://linux-ha.org/
[2]
http://techthoughts.typepad.com/managing_computers/2007/10/split-brain-quo.html
[3]
http://linux-ha.org/STONITH
[4]
http://techthoughts.typepad.com/managing_computers/2007/10/more-about-quor.html
[5]
http://techthoughts.typepad.com/managing_computers/2007/11/quorum-server-i.html
