Understanding cloud storage models

The roles of instance, volume, and object storage in the modern, virtualized data center

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This relaxed model allows object storage to provide extremely large namespaces across large distances with low cost and good aggregate performance. Many applications designed for cloud environments are written to use object storage in place of NAS, because of its advantageous scale and cost. For example, cloud-native applications will often use object storage to store images, static Web content, backup data, analytic data sets, and customer files.

It is also important to note that the relaxed consistency and course-grained updates of object storage make it a poor fit for a number of use cases. For example, it is a poor replacement for instance or volume storage (at least in its raw form). Instance and volume storage support strong consistency, small block updates, and write-heavy, random workloads, all of which are challenging to do using object storage.

NAS workloads that require strong consistency are also a poor fit for object storage, so NAS will most likely continue to exist in cloud environments alongside objects.

As with NAS, object storage is a software construct, not a hardware construct; object storage is accessed directly from the application itself via REST APIs. There are two major (and similar) object APIs in private cloud: Amazon’s S3 API and OpenStack’s Swift API.

Many vendors provide object storage implementations, such as OpenStack’s Swift, Amazon’s S3, Red Hat’s Ceph, and Cleversafe. All of these products speak S3, Swift, or both, often in addition to other APIs. Some existing file system vendors provide object interfaces along with their existing file interfaces, such as EMC Isilon.

All together now

Now we have our complete picture of storage models for the cloud and how they relate to storage models used in more conventional storage environments. Just as DAS, SAN, and NAS provide a set of tools to address a variety of physical and virtual use cases, instance, volume, and object storage together provide a flexible paradigm for the cloud. While all installations may not use all three types of storage, no one type of storage can address all of the necessary use cases on its own.

New cloud storage models

New cloud storage models.

The following table reviews the way each model is implemented, exposed to the VM and managed by an administrator.

Storage model Exposed to VM as Managed as Implemented by
Instance storage DAS DAS DAS, NAS, Volume
Volume storage DAS Volume NAS, SAN
Object storage REST API Objects, Buckets Swift, Ceph, Isilon

Instance and volume storage serve as a logical abstraction of the existing physical storage models of DAS and SAN. These virtual storage models exist on top of storage protocols in new data center environments. In contrast, object storage provides a new type of storage focused primarily on large scale, which addresses a set of use cases similar to those addressed by NAS in the physical world.

While these new abstractions are complicated at first, they provide new freedom to both data center and application administrators. Application administrators are no longer bound to data center infrastructure decisions; they are free to manage their data in the models that make sense to their applications. In turn, data center administrators can manage storage at the abstractions important to their users, while still utilizing best-in-breed storage implementations suited to their data center demands.

Just as the DAS, SAN, and NAS models have supported innovation in the physical data center, instance, volume, and object storage will enable continuing innovation in the modern, virtualized data center.

Brandon Salmon, Office of the CTO, Tintri, @Tintri, has been at Tintri since 2009. He is a systems guy who loves to think about user experience, which he picked up from his doctoral work at Carnegie Mellon on distributed file systems for the home. He designed and implemented Tintri’s initial algorithms for moving data between flash and disk, and he has worked on a number of areas since, most recently cloud technologies.

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