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As the name implies, ROSS is an object store. An object store really does not have a directory tree structure like conventional POSIX file systems do. (Grace's storage systems are POSIX file systems.) Instead, objects have names and live in buckets, essentially a one-level directory hierarchy. People often name objects using what looks like a POSIX-style file path. For example "directory1/directory2/directory3/directory4/filename" might be an object name. The RESTful APIs used to access ROSS can do a prefix search, listing just the objects that start with a particular prefix. For example "directory1/directory2/" would pull back all the objects that start with that prefix. So in a way one can mimic a POSIX file system with employing an object naming conventions convention that use uses the equivalent of POSIX path names. Unfortunately, prefix search is quite inefficient, making the simulated directory lookup slower than a native POSIX file system. Nevertheless, some tools exist that can mimic a POSIX-style file system using an object store behind the scenes. With a proper naming convention, it is fairly easy to backup and restore POSIX directory trees to and from an object store fairly efficiently. This is what makes ROSS an excellent backing storage system for Grace. (A backing storage system is the larger, but slower storage system that back ends a smaller, but faster storage cache in a hierarchical storage model. Grace's storage system should be considered cache.)
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Data in ROSS starts as triple replicated on 3 different storage nodes, but then transforms to erasure coding giving resilience without major costs. ROSS currently uses 12/16 error coding, meaning that for every 12 blocks of data stored it actually writes 16 blocks. Up to four blocks could be damaged before ROSS is no longer able to recover data if a fifth block were damaged. In contrast, most RAID systems only have a 1 or 2 drive redundancy. ROSS scatters the 16 blocks across the storage nodes in a data center, improving the performance of data retrievals and further improving the resilience of the system (less chance that a node failure blocks access). Data can be accessed from any of the storage nodes. Currently the two systems (UAMS and UARK) are isolated from each other, not allowing replication. But the plans to join the two are in progress. Eventually all of the content in ROSS, regardless of which campus it physically lives on, would be accessible from either campus. Of course, if data living on one campus is accessed from the other campus, the access will be somewhat slower due to networking delays and bandwidth limitations going between campus.
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