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fcc4cbc3f3
This CL changes the chunking algorithm from "normalized chunking" to simple "regression chunking", and changes the has criteria from 'hash&mask' to 'hash<=threshold'. These are all ideas taken from testing and analysis done at https://github.com/dbaarda/rollsum-chunking/blob/master/RESULTS.rst Regression chunking was introduced in https://www.usenix.org/system/files/conference/atc12/atc12-final293.pdf The algorithm uses an arbitrary number of regressions using power-of-2 regression target lengths. This means we can use a simple bitmask for the regression hash criteria. Regression chunking yields high deduplication rates even for lower max chunk sizes, so that the cdc_stream max chunk can be reduced to 512K from 1024K. This fixes potential latency spikes from large chunks.
CDC Indexer
This directory contains a CDC indexer based on our implementation of FastCDC.
Run the sample with Bazel:
bazel run -c opt //cdc_indexer -- --inputs '/path/to/files'
The CDC algorithm can be tweaked with a few compile-time constants for
experimentation. See the file indexer.h for preprocessor macros that can be
enabled, for example:
bazel build -c opt --copt=-DCDC_GEAR_BITS=32 //cdc_indexer
At the end of the operation, the indexer outputs a summary of the results such as the following:
00:02 7.44 GB in 2 files processed at 3.1 GB/s, 50% deduplication
Operation succeeded.
Chunk size (min/avg/max): 128 KB / 256 KB / 1024 KB | Threads: 12
gear_table: 64 bit | threshold: 0x7fffc0001fff
Duration: 00:03
Total files: 2
Total chunks: 39203
Unique chunks: 20692
Total data: 9.25 GB
Unique data: 4.88 GB
Throughput: 3.07 GB/s
Avg. chunk size: 247 KB
Deduplication: 47.2%
160 KB ######### 1419 ( 7%)
192 KB ######## 1268 ( 6%)
224 KB ################### 2996 (14%)
256 KB ######################################## 6353 (31%)
288 KB ###################### 3466 (17%)
320 KB ########################## 4102 (20%)
352 KB ###### 946 ( 5%)
384 KB 75 ( 0%)
416 KB 27 ( 0%)
448 KB 7 ( 0%)
480 KB 5 ( 0%)
512 KB 1 ( 0%)
544 KB 4 ( 0%)
576 KB 2 ( 0%)
608 KB 3 ( 0%)
640 KB 3 ( 0%)
672 KB 3 ( 0%)
704 KB 2 ( 0%)
736 KB 0 ( 0%)
768 KB 0 ( 0%)
800 KB 1 ( 0%)
832 KB 0 ( 0%)
864 KB 0 ( 0%)
896 KB 0 ( 0%)
928 KB 0 ( 0%)
960 KB 0 ( 0%)
992 KB 0 ( 0%)
1024 KB 9 ( 0%)
For testing multiple combinations and comparing the results, the indexer also
features a flag --results_file="results.csv" which appends the raw data to the
given file in CSV format. Combine this flag with --description to label each
experiment with additional columns.