Colossus under the hood: a peek into Google’s scalable storage system

With Colossus, a single cluster is scalable to exabytes of storage and tens of thousands of machines. In the example above, for example, we have instances accessing Cloud Storage from Compute Engine VMs, YouTube serving nodes, and Ads MapReduce nodes–all of which are able to share the same underlying file system to complete requests. The key ingredient is having a shared storage pool that is managed by the Colossus control plane, providing the illusion that each has its own isolated file system.

Disaggregation of resources drives more efficient use of valuable resources and lowers costs across all workloads. For instance, it’s possible to provision for the peak demand of low latency workloads, like a YouTube video, and then run batch analytic workloads more cheaply by having them fill in the gaps of otherwise idle time.

Let’s take a look at a few other benefits Colossus brings to the table.

Simplify hardware complexity

As you might imagine, any file system supporting Google services has fairly daunting throughput and scaling requirements that must handle multi-TB files and massive datasets. Colossus abstracts away a lot of physical hardware complexity that would otherwise plague storage-intensive applications.

Google data centers have a tremendous variety of underlying storage hardware, offering a mix of spinning disk and flash storage in many sizes and types. On top of this, applications have extremely diverse requirements around durability, availability, and latency. To ensure each application has the storage it requires, Colossus provides a range of service tiers. Applications use these different tiers by specifying I/O, availability, and durability requirements, and then provisioning resources (bytes and I/O) as abstract, undifferentiated units.

In addition, at Google scale, hardware is failing virtually all the time–not because it’s unreliable, but because there’s a lot of it. Failures are a natural part of operating at such an enormous scale, and it’s imperative that its file system provide fault tolerance and transparent recovery. Colossus steers IO around these failures and does fast background recovery to provide highly durable and available storage.

The end result is that the associated complexity headaches of dealing with hardware resources are significantly reduced, making it easy for any application to get and use the storage it requires.

Maximize storage efficiency

Now, as you might imagine it takes some management magic to ensure that storage resources are available when applications need them without overprovisioning. Colossus takes advantage of the fact that data has a wide variety of access patterns and frequencies (i.e., hot data that is accessed frequently) and uses a mix of flash and disk storage to meet any need.

The hottest data is put on flash for more efficient serving and lower latency. We buy just enough flash to push the I/O density per gigabyte into what disks can typically provide and buy just enough disks to ensure we have enough capacity. With the right mix, we can maximize storage efficiency and avoid wasteful overprovisioning.

For disk-based storage, we want to keep disks full and busy to avoid excess inventory and wasted disk IOPs. To do this, Colossus uses intelligent disk management to get as much value as possible from available disk IOPs. Newly written data (i.e. hotter data) is evenly distributed across all the drives in a cluster. Data is then rebalanced and moved to larger capacity drives as it ages and becomes colder. This works great for analytics workloads, for example, where data typically cools off as it ages.

So, there you have it–Colossus is the secret scaling superpower behind Google’s storage infrastructure. Colossus not only handles the storage needs of Google Cloud services, but also provides the storage capabilities of Google’s internal storage needs, helping to deliver content to the billions of people using Search, Maps, YouTube, and more every single day. When you build your business on Google Cloud you get access to the same super-charged infrastructure that keeps Google running. We’ll keep making our infrastructure better, so you don’t have to.

To learn more about Google Cloud’s storage architecture, check out the Next ’20 session from which this post was developed, “A peek at the Google Storage infrastructure behind the VM .” And check out the cloud storage website to learn more about all our storage offerings.