RedHat Cluster, and some more

It’s been a long while since I’ve written. I get to have, once a while, a period of time dedicated for laziness. I’ve had just one of these for the last few weeks, in which I’ve been almost completely idle. Usually, waking up from such idle time is a time dedicated to self studies and hard work, so I don’t fight my idle periods too hard. This time, I’ve had the pleasure of testing and playing, for personal reasons, both with VMWare GSX, in a “Cluster-In-a-Box” setup, based on recommendation regarding MSCS, altered for Linux (and later, Veritas Cluster Service) and both with RedHat Cluster Server, with the notion of playing with RedHat’s GFS, but, regrettably, without the last.

First, VMware. In their latest rivalty with Microsoft over the issue of Virtualization of servers and desktops, MS has gained an advantage lately. Due to the lower prices of “Virtual Server 2005”, comparing with “VMware GSX Server”, and due to their excellent marketing system (from which we should all learn, if I may say!), Not a few servers and virtual server farms, especially the ones running Windows/Windows setups, had moved to this MS solution, which is as capable as VMware GSX Server. Judging by the history of such rivalries, MS would have won. They always have. However, VMware, in an excellent move, has announced that the next generation of their GSX, simply called “Server”, would be for free. Free for everyone. In this they probably mean to invest more in their more robust ESX server, and give the GSX as a taste of their abilities. While MS do not have any more advanced product than their Virtual Server, it could mean a death blow to their effort in this direction. It could even mean they will just give away their product! While this will happen, we, the customers, will earn a selection of free, advanced and reliable products designed for virtualization. Could it be any better than that?

One more advantage of this “Virtualization for the People” is that community based virtual images, of even the most complicated to install setups can and would be widely available. Meaning to shorten installation time, and allow for a quick working system for everyone. It will require, however, better knowledge and understanding of the products themselves, as merely installing them will not be enough. To survive the future market, you won’t be able to just sell an installation of a product, but should be able to support an out-of-the-box setup of it. That’s for the freelances, and the partially freelances of us…

So, I’ve reinstalled my GSX, and started playing with it. The original goal was to actually run a working setup of RHEL, VCS and Oracle 10g. Unfortunately, VCS supports only RH3 (update 2?), and not RH4, which was a shame. At that point, I’ve considered using RH Cluster Server for the task at hand. It grew to the task of learning this cluster server, and nothing more, which I did, and I can and would share my concepts about it here.

First – Names – I’ve had the pleasure of working with numerous cluster solutions. I’m thrilled each time I get to play with another cluster solution the naming conventions, and name changes vendors do, just to keep themselves unique. I hate it. So here’s a little explanation:
All clusters contain a group of resources (Resource Group, as most vendors call them). This group contains a set of resources, and in some cases, relations (order of startup, dependencies, etc). Each resource could be any single element required for an application. Example – Resource could be an IP address, which without you won’t be able to contact the application. Resource could be a disk device, containing the application’s data. It could be an application start/stop script, and it could be a sub-application – an application required for the whole group to be up, such as a DB for DB driven web server. The order you would ask them to start would be IP, disk, DB, web server (in our case). You’d ask the IP to be brought up first because some of the cluster servers can trick an IP based clients into some delay, so the client hardly feels the short downtime of application failover. But this is for later. So, in a resource group, we have resources. If we can separate resources into different groups, if they have no required dependency between them, it is always better to do so. In our previous example, lets say our web server uses the DB, but it contacts it using IP address, or using hostname. In this case, we won’t need the DB to run on the same physical machine the web server is using, and in such a case, assuming the physical disk holding the DB and the one holding the rest of the web application are not the same disk, we could separate them.

The idea, if I can try to sum it up, is to split your application to the smallest self-maintained structures. Each structure will be called resource group, and each component in this structure is a resource. On some cluster servers, one could group and set dependencies between resource groups, which allows for even more scalability, but that is not our case.

So we had resource groups containing resources. Each computer, a member in the cluster, is called a node. Now, let’s assume our cluster containing three nodes, but we want our application (our resource group) to be able to run on only two specific? In this case, we need to define, for our resource group, which nodes are to be associated with it. In RH Cluster Server, a thing called “Domain” is designed for it. This Domain containes a list of nodes. This Domain can be associated with Resource Group, and thus set priority of failover, and set the group of nodes allowed to deal with the resource group.

All clusters have a single point of error (unlike failure). The whole purpose of the cluster is to allow for non-cluster-aware application the high-availability you could expect for a (relatively) low price. We’re great – we know how to bring an application up, we know how to bring it down. We can assume when the other node(s) is/are down. We cannot be sure of it. We try. We demand few means of communication, so that one link failure won’t cause us to corrupt our shared volumes (by trying multiple access into them). We set a whole system of logic, a heartbit, just name it, to avoid, at almost all cost, a status of split-head – two cluster nodes believing they are the only ones up. You can guess what it means, right?

In RH, there is a heartbit, sure. However, it is based on bonding, in the event of more than one NIC, and not on separated infrastructures. It is a simple network-based HB, with nothing special about it. In case of loss of connection, it would have reset the inactive node, if it saw fit, using a mechanism they call “Fence”. A “Fence” is a system by which the cluster can *know* for sure (or almost for sure) a node has been down, or the cluster can physically take a node down (poweroff if needs), such as control of the UPS the node is connected to, or its power switch, or alternate monitoring infrastructure, such as the Fibre Channel Switch, etc. In such an event, the cluster can know for sure, or can assume, at least, that the hung node has been reset, or it can force it to reset, to release some hung application.

Naming – Resource group is called Service. Resource remains resource, but an application resource *must* be defined by an rc-like script, which accepts start/stop (/restart?). Nothing complicated to it, really. The service contains all required resources.

I was not happy with the cluster, if I can sum up my issues with it. Monitoring machines (nodes) it did correctly, but in the simple enough example I’ve chosen to setup, using apache as a resource (only afterwards I’ve noticed it to be the example RedHat used in their documentation) it failed miserably to take the correct action when an application failed (unlike a failure of a node). I’ve defined my “Service” to contain the following three items:

1) IP Address – Unique for my testing purposes.

2) Shared partition (in my case, and thanks to VMware, /dev/sdb1, mounted at /var/www/html)

3) The Apache application – “/etc/init.d/httpd”

All in all, it was brought up correctly, and switch-over went just fine, including in a case of correct and incorrect reset of the active/passive node, however, when I’ve killed my apache (killall httpd), the cluster detected failure in the application, but was helpless with it. I was unable to bring down the “Service”, as it failed to turn off Apache (duh!), so it did not release neither the IP address, nor the shared volume. In so doing, I’ve had to restart the service rgmanager on both nodes, after manual removal of the remains of the “Service”. I didn’t like it. I expect the cluster to notice failure in the application, which it did, but I expect it to either try to restart the application (/etc/init.d/httpd stop && /etc/init.d/httpd start) before it fails completely, or to set a flag saying it is down, remove the remains of the “Service” from the node in question (release the IP address and the shared storage), and try to bring it up on the other node(s). It did nothing of the likes. It just failed, completely, and required manual intervention.

I expect HA-Cluster to be able to react to an application or resource failure, and not just to a node failure. Since HA-Clusters are meant for the non-ideal world, a place where computers crash, where hardware failures occure, and where applications just die, while servers remain working, I expect the Cluster Server to be able to handle the full variety of problems, but maybe i was expecting too much. I believe it to be better in their future versions, and I believe it could have been done quite easily right now, as long as detection of the failed application occurred, which it has, but it’s not for me to define the cluster’s abilities. This cluster is not mature enough for real-life production sites, if and only because of its failure to react correctly to a resource failure, without demanding manual intervention. A year from now, I’ll probably recommend it as a cheap and reliable solution for most common HA-related tasks, but not today.

That leaves me with VCS and Oracle, which I’ll deal with in the future, wether I like it or not 🙂