Home > APEX > Storage > White Papers > Introduction to APEX File Storage for AWS > OneFS protection level
The OneFS cluster is designed to withstand one or more simultaneous component failures while continuing to serve data. To achieve this, OneFS protects files with either erasure code-based protection, using Reed-Solomon error correction (N+M protection), or a mirroring system. Data protection is applied in software at the file level, enabling the system to focus on recovering only those files that are compromised by a failure, rather than having to check and repair an entire file set or volume. OneFS metadata and inodes are always protected by mirroring, rather than Reed-Solomon coding, and with at least the level of protection as the data they reference. For more technical details about OneFS protection levels, refer to OneFS Data Protection.
APEX File Storage for AWS supports the protection level of +2n, which can tolerate the failure of two drives or two nodes. The reason we choose +2n for the APEX File Storage for AWS over the previous default of +2d:1n is to ensure high availability and reliability in the cloud environment. The overhead due to protection varies with cluster size. Table 3 shows the protection overhead of APEX File Storage for AWS with different cluster sizes:
Cluster size | Protection level | Erasure code | Protection overhead | Efficiency |
4 nodes | +2n | 2+2 (2 data blocks + 2 protection blocks) | 50% | 50% |
5 nodes | +2n | 3+2 (3 data blocks + 2 protection blocks) | 40% | 60% |
6 nodes | +2n | 4+2 (4 data blocks + 2 protection blocks) | 33% | 67% |
File system efficiency increases as the cluster size grows. Therefore, when capacity is your major requirement, deploying a cluster with more nodes to get a higher usable cluster capacity is recommended. “Usable cluster capacity” means the capacity available to store data after protection overhead. For example, if you are planning to provision 500 TiB total raw cluster capacity, you can achieve nearly 335 TiB usable cluster capacity on a 6-node cluster and only 250 TiB usable cluster capacity on a 4-node cluster.
Note: The final usable capacity of a OneFS cluster depends on the characteristics of the data and on file system features in use. For example, OneFS provides inline data reduction and small file efficiency features to help save storage capacity. Data capacity savings due to inline data reduction and small file efficiency are highly dependent on the data and can vary considerably. This variance means that accurate rates of savings are not predictable without comprehensive analysis of the dataset. The preceding usable cluster capacity estimation is for guidance only when implementing APEX File Storage for AWS. For more details about storage efficiency, see the white paper Dell PowerScale OneFS: Data Reduction and Storage Efficiency.