Gluster linear scaling: How to choose wisely

We talk a lot about the linear scalability of Red Hat Gluster Storage, and we can generally back that up with empirical data. Indeed, homogeneously scaling out the storage nodes and network infrastructure can result in both capacity and throughput capabilities that are directly proportional. But it’s important to note that this is potential scalability, and how you use the volumes plays a vital role in the experience you have.

We architect optimal solution recommendations based on a few expectations:

  1. Most of the workload falls into a particular category—high throughput, small file, or latency sensitive, for example.
  2. When your capacity needs grow, so do your concurrent client demands.
  3. You’re using the glusterfs native client.

Let’s take a look at these points and how they affect your real scalability.

Architecting for workload

We know through thousands of test cycle results that there is a generally optimal server configuration that will apply broadly to a majority of workloads. This compiled knowledge is a huge benefit to you, the user, and it can greatly reduce your own time commitment in designing and testing fundamental system architectures. However, just up the stack from the server and network components are low-level configuration choices that you will make for every deployment. These choices are the big knobs—Particular to your workload there is likely one best choice for peak performance. And it’s important to note that these aren’t choices you can easily change later. Changes at these layers likely require moving data, potentially more than once, and data has inertia.

When you understand your majority workload, and preferably you isolate dislike workloads entirely, you will be positioned to make choices about server density (12, 24, or higher drive capacity), block-level configurations (e.g., HDD vs. SSD, RAID vs. JBOD, caching vs. not, block and stripe sizes), and Gluster volume geometry (e.g., replicated vs. dispersed, failure resiliency, arbiter bricks, tiering). Locked into these choices and the related workload, you’ll find it reasonably simple to integrate new nodes and bricks into the volume for predictable capacity and performance expansion.

Client concurrency

So you’ve built to your workload and everything is great. That is, unless your expectations aren’t aligned with a scale-out solution. Any single connection to the storage pool is bound by physics. One client communicates over one network link to one server to one file system and block stack. Sure, some design options allow for single-client concurrency to multiple stacks, but those come at a trade off, and each connection is still bound by physics and bottlenecked somewhere along the line. So if your need is to provide expanded throughput capabilities to a single or a small number of clients, you will likely find that horizontal scale-out won’t give you much performance benefit. There are some tricks we can use to architect for such a need, but it will never be an efficient solution.

To that end, an optimal design assumes you are operating at an appropriate client:server concurrency ratio. The best ratio will vary with your workload and the architecture decisions you make per the preceding discussion, but you can expect for most cases a ratio range of 12:1 to 48:1 to be appropriate for peak or plateau storage throughput capabilities. So if you build out a 12-node storage pool based on your capacity needs and then expect 4 client systems to use that storage concurrently, you’ll bottleneck on the server node I/O stack long before you saturate the aggregate system capabilities. But with an appropriate concurrent client count of say 150+ for your 12 server nodes, you may be operating at the peak capabilities of the system.

Client choice

Great! So you’re heeding all the advice here, and you’re going to deploy 12 Red Hat Gluster Storage nodes in an optimal architecture for 150 NFS clients. Well, hold on there a minute, buckaroo. We’re more than happy to support the NFS client, but you should know what you’re getting into.

When using the Gluster native client, data placement calculations are made on the client side. This means that each client is fully aware of the volume geometry and all server nodes participating, allowing it to determine how the data protection scheme is applied and which nodes and backend filesystems (bricks) each file will be written to. All client-to-server connections are then made efficiently based on this client-side intelligence. And because data placement among the distributed system is done pseudo-randomly, there is a statistically even distribution of work between the clients and servers and therefore predictable performance scalability.

When choosing NFS (or SMB), a client will make its connections to a single Gluster server. That server then has to apply the client-side intelligence for data resilience, conversion, and placement, and it will then make secondary network calls out to each participating server node for the file transaction. This inefficiency leads to a concurrency bottleneck far below the capabilities of the native client—You’ll still hit peak throughput at about the same client:server ratio, but that throughput will be well below what can be achieved on the same systems with the native client.

The one surprise that can come up with the NFS client is that if you do indeed require a lower client:server ratio, NFS can in some conditions outperform the native client at that concurrency level. YMMV on this, and you’ll still be far below the peak capabilities of the system, but it’s worth testing out if you’re absolutely determined to connect your 4 clients to your 12 Gluster nodes (but don’t say I didn’t warn you not to).

Oh yeah? Prove it.

Lucky for you, I did that already. Take a look at our published reference architectures and, in particular, our most recent Gluster Performance and Sizing Guide. And keep an eye out here for future publications as we continue to expand and refine our data.

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