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What separates object storage systems from SDS?

Although software-defined storage and object storage can work in similar ways, there are significant differences between the two.

Object storage is a highly scalable system for organizing and storing data objects (files) without the use of a traditional file system structure. Object storage systems ingest data as objects with unique keys or IDs into a flat directory structure using simple commands. Metadata is stored with the objects rather than in an extensive hierarchical journal or tree. Search and retrieval is performed via key or ID searching.

In an object system , metadata is mostly stored with the object file and it can contain an almost unlimited amount of data about the data, which makes object storage systems much more efficient in terms of the way they're managed and archived.

It could be argued that object storage is a form of software-defined storage, but the challenge is that a definition of the latter has been hard to pin down and made even murkier by the marketing hype surrounding SDS. Software-defined storage, in theory, is a model for storage infrastructure in which storage services, traditionally provided in the form of value-add software placed on an array controller, are moved off of array controllers and into a software layer that operates as part of the server hypervisor software stack.

While software-defined storage is touted as new and revolutionary, it is actually very much akin to how storage services were delivered in IBM mainframes using System Managed Storage (SMS) beginning in the late 1970s. At that time, the function of SMS was very similar to the current description of the functionality of object storage systems in that SMS enabled the management of data onto primary storage, and its later migration to less expensive storage (hierarchical storage management functionality), across virtually any commodity storage operating under the domain of the object system.

The key difference between object storage systems and software-defined storage today is that the latter typically provides storage only for data emanating from workload that has been virtualized using a specific brand of server hypervisor software. VMware and other hypervisor vendors appear to be seeking to create isolated storage repositories dedicated only to their own virtual machines and exclude the placement of other hypervisor workload data (or non-virtualized application data) in the dedicated storage. In my opinion, this is a strategy in sharp contrast to most object storage products, which are geared toward creating a unified, universal platform.

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Which makes more sense for your organization: object storage systems or SDS? Why?

Nice observations, sounds like Object is more "software defined" than SDS, with its ability to define policies for security, placement, and data life cycle management (S3 and other products have granular security, object level tiering, searchable metadata ..)  

another key advantage of Object is the self-service model, users can create and manage their own "buckets", while SDS is something managed by the IT or the Hypervisor, so yet again Object is more "software defined".

Note that new generation Object solutions provide even broader services (e.g. File/Block access, Hadoop integration, Tiering, Global distribution, Better performance ..) 

Maybe the problem is how the traditional IT and some "SDS" vendors who re positioned their existing products as SDS defined it, and we need a sound voice to help redefine SDS around the services it provides vs the way its deployed, just like SDN which focus on the service value.  

Well, Mr. Toigo makes sense with his definitions of software-defined storage (SDS) and object-based storage (OBS). SDS has been bandied about to describe just about ever "innovation" in data storage, but in reality definitions matter. You can't talk about these things in a coherent manner unless you are willing to accept that they have definitions. When "cloud computing" became a topic for general conversation, some people claimed that it could mean anything, despite NIST's effort to actually define it. Perhaps it is beter to rely on public organizations like NIST or industry organizations like SNIA to promulgate definitions in the area of storage technology.
Please also take a very close look at QuantaStor by OSNEXUS, one to watch in 2016!
Software-defined storage seems to me to be more about the base hardware itself and how things are arranged on it, while object storage is more about the user interface to the data. At a higher level of the OSI model, as it were.