Object storage systems store objects and file systems store files. While an object and a file are typically the same thing, how they are stored is not the same; it is the difference between an object storage system and a file system. So why, then, do many object storage systems use a file system underneath?
Using a file system underneath an object storage interface doesn’t make it a file system, any more than an NFS interface on top of an object storage system does that. But why would a storage system being written to store objects store those objects on a file system, since many of the limitations that object storage systems are attempting to overcome are artifacts from the file system architecture?
The answer to this question is probably simplicity and time-to-market. A lot of object storage systems are built on commodity hardware and use a commodity Linux distribution, upon which they run their object storage system. That Linux distribution will come with a built-in file system, so it’s easier and quicker to build an object storage system on top of a file system. It could also be said that having a file system inside one node does not quite have the same limitations as having a single file system that spans dozens or hundreds of notes.
That is not to say that there are not efficiencies to be had by stripping out the file system and writing to disks natively. This is why it was interesting to learn that DDN’s Web Object Scaler (WOS) object storage system uses NoFS instead of a file system. They claim that this results in an immediate 15 to 20% reduction in storage overhead, reducing acquisition costs, rack space, and power and cooling costs. They also claim that it requires 4 to 10 times fewer IOPs for each read and write. That could have significant performance advantages. A DDN system can start as small as a two node cluster, which can then be grown to a cluster of 256 notes. Up to 32 clusters can be combined into a single namespace that can hold hundreds of billions of objects. These statistics apply to both their hardware versions and their software-only version, which DDN also announced now accounts for 10% of their object storage revenue. This is significant since they only started marketing this product a year ago.
Having a file system underneath an object storage system does result in storage and performance inefficiencies, so it is commendable that DDN’s object storage does not use a file system. Disk is never free and IOPs are never free, so a 15 to 20% reduction in file system overhead and a 4-10X improvement in IOPs are both significant. Other object storage vendors should take note.