Where would we be without RAID? Over at the Panasas Blog, Geoffrey Noer writes that the 25th anniversary of the landmark “Berkeley RAID Paper” is an opportune time to clear up misconceptions about erasure codes and the current state of RAID.
According to Noer, erasure codes aren’t “what comes next” after RAID; they’re inextricably linked. In fact, he argues that erasure codes should still be considered “RAID,” particularly if one agrees with the Berkeley paper that the concept of RAID applies to software-based protection of files as well as hardware-based device protection.
The second part of the confusion appears to be that some people consider RAID to be a concept that only applies to the protection of entire physical devices (i.e. hard drives). While the original RAID paper does not cover per-file RAID, it does explicitly state that although the examples in the paper assume a hardware-based approach, implementing the algorithms in software could be a superior approach depending on the circumstances. Fast forward twenty-five years and this is precisely what has happened. Software-based approaches are proving to be architecturally more flexible, enabling highly scalable RAID implementations. This is ideally achieved by applying RAID algorithms on a per-file basis, employing erasure codes to protect against physical device failure by spreading fragments of files across storage hardware elements. At Panasas, we believe this should still be called RAID as the purpose has not changed (protect against failures) and RAID algorithms (erasure codes) are still largely the same ones being applied to protect data. The difference is that the RAID algorithms are being more cleverly applied by performing them in software, on a per-file basis.
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