Home > Storage > PowerScale (Isilon) > Product Documentation > Data Efficiency > Next-Generation Storage Efficiency with Dell PowerScale Inline Data Reduction > Glossary of terms
Term | Description |
Compression | The process of modifying, encoding, or converting the bits structure of data in such a way that it consumes less space on the storage media. |
Deduplication | The elimination of duplicate or redundant data, thereby lowering the actual physical storage required. |
Inflation | Uncompression of data. |
Rehydration | Un-deduplication of data. |
Protection Group (PG) | OneFS embeds protection into each individual file. To do this, files are segmented into sections, or protection groups (PGs), which are independently protected. The PG tracks all the information about how that logical piece of the file is physically stored, and protected, on-disk. |
Cluster | In protection group terminology, a cluster is a division of the logical-block space with IFS MAXCONTIG size and alignment. |
Compression Chunk Size | The maximum amount of logical file data that is compressed as a single entity (128 KB for OneFS). When an I/O occurs to a compressed region, this also represents the smallest possible I/O, since the I/O must be expanded to include the entire compression chunk. |
Packing | The combining of multiple compression chunks together to reduce lost space. |
Write Amplification | The cost of additional file system writes to the storage device due to the compression implementation. |
Hardware offload | Compression is efficiently performed by a dedicated FPGA on a PCIe card, rather than using the node’s CPU cycles. |
Inline Compression | Data is compressed and decompressed on the fly as it is written to and read from to disk |
Post-process Compression | Data is compressed in a second pass after it has already been written to disk. |
Lossless compression | Reduction of a file’s size with no discernable loss of quality. Lossless compression rewrites the data of the original file in a more efficient way. |
Inline Deduplication | Data is deduplicated on the fly as it is written to disk. |
Post-process Deduplication | Data is deduplicated in a second pass after it has already been written to disk. |
Zero Block Removal | Blocks that contain only zeros are detected and prevented from being written to disk. |
Raw | Equivalent to ‘Protected physical’. Total footprint of data including protection overhead FEC erasure coding) and excluding data efficiency savings (compression and deduplication). |
Usable | Equivalent to ‘Preprotected Physical’. Data size excluding protection overhead and including data efficiency savings (compression and deduplication). |
Effective | Equivalent to ‘Logical data’. Data size excluding protection overhead and excluding data efficiency savings (compression and deduplication). |
Logical Data | Equivalent to ‘Effective’. Data size excluding protection overhead and excluding data efficiency savings (compression and deduplication). |
Dedupe Saved | Capacity savings from deduplication. |
Compression Saved | Capacity savings from inline compression. |
Preprotected Physical | Equivalent to ‘Usable’. Data size excluding protection overhead and including data efficiency savings (compression and deduplication). |
Protection Overhead | Size of erasure coding used to protect data. |
Protected Physical | Equivalent to ‘Raw’. Total footprint of data including protection overhead FEC erasure coding) and excluding data efficiency savings (compression and deduplication). |
Effective to Raw Ratio | Compression ratios stated as Effective to Raw are calculated including the data’s protection overhead. OneFS compression ratios are typically calculated and reported using this method. |
Effective to Usable Ratio | Compression ratios stated as Effective to Usable are calculated omitting protection overhead. Competitors’ compression ratios are often calculated and reported using this method. |
COW | Copy on write, to preserve data in snapshots. |
EC | Endurant cache. |
BAM | Block allocation manager. |
BSW | BAM safe write. |
Coalescer | OneFS non-volatile write cache. |