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When a file is written to OneFS using inline data compression, the file’s logical space is divided up into equal-sized chunks called compression chunks. Compaction is used to create 128 KB compression chunks, with each chunk consisting of sixteen 8 KB data blocks. This setup is optimal since 128 KB is the same chunk size that OneFS uses for its data protection stripe units, providing simplicity and efficiency, and avoids the overhead of additional chunk packing.
For example, consider the following 128 KB chunk:
After compression, this chunk is reduced from sixteen to six 8 KB blocks in size. This reduction means that this chunk is now physically 48 KB in size. OneFS provides a transparent logical overlay to the physical attributes. This overlay describes whether the backing data is compressed or not and which blocks in the chunk are physical or sparse, such that file system consumers are unaffected by compression. As such, the compressed chunk is logically represented as 128 KB in size, regardless of its actual physical size. The orange sector in the illustration above represents the trailing, partially filled 8 KB block in the chunk. Depending on how each 128KB chunk compresses, the last block may be under-utilized by up to 7KB after compression.
Efficiency savings must be at least 8 KB (one block) for compression to occur, otherwise that chunk or file will be passed over and remain in its original, uncompressed state. For example, a file of 16 KB that yields 8 KB (one block) of savings would be compressed. Once a file has been compressed, it is protected with Forward Error Correction (FEC) parity blocks, reducing the number of FEC blocks and therefore providing further overall storage savings.
Compression chunks will never cross node pools, avoiding the requirement to decompress or recompress data to change protection levels, perform recovered writes, or otherwise shift protection-group boundaries.
In the illustration above, a 768 KB file (file 1) is written from a Windows client to an F810 cluster. After passing through the OneFS inline compression engine, the logical data footprint of that file is reduced from ninety-six to sixty 8 KB blocks, across six chunks. This is represented by the blue data blocks. Then, the file is FEC protected using twenty-six parity blocks, shown in green.