IBM Storwize V5010 Manual page 497

Figure 9-26 shows compression that uses a sliding window, where the first two repetitions of
the string "ABCD" fall within the same compression window, and can therefore be
compressed by using the same dictionary. The third repetition of the string falls outside of this
window, and therefore cannot be compressed by using the same compression dictionary as
the first two repetitions, reducing the overall achieved compression ratio.
Figure 9-26 Compression that uses a sliding window
Traditional data compression in storage systems
The traditional approach taken to implement data compression in storage systems is an
extension of how compression works in the previously mentioned compression utilities.
Similar to compression utilities, the incoming data is broken into fixed chunks, and then each
chunk is compressed and extracted independently.
However, there are drawbacks to this approach. An update to a chunk requires a read of the
chunk followed by a recompression of the chunk to include the update. The larger the chunk
size chosen, the heavier the I/O penalty to recompress the chunk. If a small chunk size is
chosen, the compression ratio is reduced because the repetition detection potential is
reduced.
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Chapter 9. Advanced features for storage efficiency