Module compressor 

L2 Blob Compressor.

Stateless, deep module encapsulating compression of blob payloads spilled to the L2 (on-disk) tier of [BlobCache]. The purpose is to shrink durable blob footprints without harming hot-path latency or wasting CPU on payloads that are already incompressible.

Design

The module is intentionally small and side-effect free:

• All operations are static — no internal state, no allocator pinning, no threading concerns. Inputs are &[u8] slices, outputs are owned Vec<u8>.
• Compression is best-effort: when the input is small, when the content type already represents a compressed media format, or when the zstd output fails to shrink the input meaningfully, the original bytes are returned inside Compressed::Raw and no encode is performed (or its result is discarded).
• The compressed variant carries the original byte length so decompression can pre-allocate exactly and verify the encoded length on decode.

Wiring into [BlobCache] L2 put/get is performed in a follow-up slice; this module is additive and has no callers in this commit.

Structs

CompressOpts

Configuration knobs for L2BlobCompressor::compress.

L2BlobCompressor

Stateless compressor for L2 blob payloads.

Enums

CompressError

Errors produced by the compressor.

Compressed

Storage-ready representation of a blob payload after the compressor has inspected it. Raw is byte-equivalent to the input; Zstd carries an encoded payload plus the original byte length for verification.

Constants

DEFAULT_MAX_RATIO

Default cutoff ratio above which the compressed bytes are discarded. 0.95 means we require at least a 5% reduction to keep the encoded form.

DEFAULT_MIN_BYTES

Default minimum payload size eligible for compression. Sub-kilobyte payloads rarely benefit and the framing overhead can exceed any savings.

DEFAULT_ZSTD_LEVEL

Default zstd compression level — favours encode speed over ratio. The L2 tier is meant to amplify capacity, not to be the smallest possible store.