Oxicloud/benches/BLOB-PREFETCH.md

Blob download read-ahead benchmark

Measures the local backend's chunk read-ahead depth — the buffered(N) read-ahead in DedupService::stream_chunks fed by BlobStorageBackend::read_prefetch(). Reproduces the exact production reassembly combinator (stream::iter(hashes).map(get_blob_stream).buffered(N).try_flatten()) over a real LocalBlobBackend whose chunk files are scattered across the 256 hash-prefix dirs, then drains it and reports throughput. N = 1 is the old production default ("antes"); higher N is the change ("después").

Reproduce

cargo run --release --features bench --example bench_blob_prefetch
# tunables: BENCH_FILE_MB=192 BENCH_CHUNK_KB=256 BENCH_PREFETCH=1,2,4,8,16
#   BENCH_THROTTLE_MBPS=0,300,100 BENCH_REPS=5 BENCH_COLD=1

Results (4-core box, SSD-class storage, 192 MiB in 768×256 KiB chunks)

Median MB/s over 5 reps; vs N=1 is the read-ahead gain over the old default.

scenario	N=1	N=2	N=4	N=8	N=16
warm / unthrottled	1306	1460	1394	1357	1248
cold / unthrottled	456	489	469	478	473
warm / throttled@300MB/s	167	166	166	166	166
cold / throttled@300MB/s	138	140	135	135	136
warm / throttled@100MB/s	62	62	62	62	62
cold / throttled@100MB/s	57	58	57	57	57

(vs N=1 for the best column N=2: warm/unthrottled +11.8 %, cold/unthrottled +7.2 %; throttled rows ≈ 0 %. N=16 regresses warm −4.4 %.)

Conclusions

1. N=2 is the sweet spot, not 8. It wins or ties in 5 of 6 scenarios at the lowest fan-out: +11.8 % warm and +7.2 % cold on disk-bound reads, neutral when the consumer is the bottleneck. N=8 gives only +3.9 %/+4.8 %; N=16 regresses. So local now defaults to 2 (was 1); S3/Azure keep 8 (request-latency bound).

2. The win is disk-bound, not network-bound. Throttled (network-bound) rows are flat because buffered(N) here overlaps the per-chunk File::open (cheap on local disk), not the data read (which try_flatten polls sequentially). The disk-bound rows cover localhost/LAN downloads and the internal blob reads that drain as fast as the disk delivers — thumbnail render, transcode, ZIP export, content extraction — all via stream_chunks.

3. No cold regression on SSD. The trait doc's "slower cold" worry (concurrent opens → random I/O over scattered chunk files) is an HDD seek-thrash concern; on SSD-class storage cold reads improved at N=2. Operators on spinning disks can restore the old behaviour with OXICLOUD_LOCAL_READ_PREFETCH=1; NVMe arrays can raise it.