[Doc] Document blocks_per_chunk in the KV offloading guide

docs/features/kv_offloading_usage.md

@@ -68,13 +68,14 @@ vllm serve <model> \
 | --- | --- | --- | --- | --- |
 | `spec_name` | no | `CPUOffloadingSpec` | both | Set to `TieringOffloadingSpec` for multi-tier. |
 | `cpu_bytes_to_use` | yes | — | both | Total bytes of host memory reserved for the CPU tier across all workers (not per-worker). |
-| `block_size` | no | GPU block size | both | Offloaded block size in tokens; must be a multiple of the GPU block size. |
+| `block_size` | no | GPU block size | both | Offloaded block size in tokens; must be a multiple of the GPU block size. Mutually exclusive with `blocks_per_chunk`. |
+| `blocks_per_chunk` | no | `1` | both | Offloaded chunk size in GPU blocks; must be > 0. Alternative to `block_size` for models whose KV cache groups have different block sizes. |
 | `eviction_policy` | no | `lru` | both | Primary tier policy: `lru` or `arc`. |
 | `store_threshold` | no | `0` | single-tier | Min lookups before a block is offloaded. Values ≥ 2 are rejected by `TieringOffloadingSpec`. |
 | `max_tracker_size` | no | `64000` | single-tier | Max entries in the lookup tracker. |
 | `secondary_tiers` | no | `[]` | multi-tier | List of secondary tier configs (see below). |
 | `offload_prompt_only` | no | `true` | both | If `true`, only prompt (prefill) blocks are offloaded; decode blocks are skipped. |
-| `self_describing_kv_events` | no | `false` | single-tier | Opt-in. When `true` *and* KV cache events are enabled (`--kv-events-config` with `enable_kv_cache_events`), the connector emits self-describing block-granular `BlockStored`/`BlockRemoved` payloads (constituent block hashes, whole-chunk `token_ids`, per-block `block_size`, parent hash, LoRA + group/cache-spec metadata) instead of the placeholder fallback, so external KV-event consumers can index offloaded blocks. Inert unless events are enabled. Currently rejected by `TieringOffloadingSpec`. Full-attention groups only; sliding-window/SSM groups keep the placeholder fallback. In chunk mode (`block_size` > GPU block size), overlapping chunks re-announce shared per-block hashes, so consumers must reference-count (deduplicate) repeated store/remove announcements. |
+| `self_describing_kv_events` | no | `false` | single-tier | Opt-in. When `true` *and* KV cache events are enabled (`--kv-events-config` with `enable_kv_cache_events`), the connector emits self-describing block-granular `BlockStored`/`BlockRemoved` payloads (constituent block hashes, whole-chunk `token_ids`, per-block `block_size`, parent hash, LoRA + group/cache-spec metadata) instead of the placeholder fallback, so external KV-event consumers can index offloaded blocks. Inert unless events are enabled. Currently rejected by `TieringOffloadingSpec`. Full-attention groups only; sliding-window/SSM groups keep the placeholder fallback. In chunk mode (`block_size` > GPU block size, or `blocks_per_chunk` > 1), overlapping chunks re-announce shared per-block hashes, so consumers must reference-count (deduplicate) repeated store/remove announcements. |
 | `spec_module_path` | no | — | both | Python import path for a custom `OffloadingSpec` not in the built-in registry. Required only when `spec_name` is not built-in (advanced). |
 
 ## Secondary Tiers
@@ -179,7 +180,7 @@ Rather than embedding `host`/`port` in each `secondary_tiers` entry, set them on
 
 - `cpu_bytes_to_use`: a bigger CPU tier means fewer trips to slower secondary tiers and a higher hit rate. The value is total across all workers, not per-worker. Leave headroom for the rest of the host workload.
 - For single-tier (CPU-only) setups, set `cpu_bytes_to_use` larger than the aggregate GPU KV cache. Because offloading is immediate, a smaller CPU tier just mirrors what the GPU already holds and adds no hit rate.
-- `block_size`: larger offloaded blocks reduce per-block bookkeeping overhead but increase the granularity of lookups. Must be a multiple of the GPU block size.
+- `block_size` / `blocks_per_chunk`: larger offloaded chunks reduce per-block bookkeeping overhead but increase the granularity of lookups.
 - FS thread counts: tune `n_read_threads` and `n_write_threads` to the parallelism your storage can sustain. Reads are latency-sensitive on the prefill path, so prefer more read threads when prefill hit rates are high.
 - Sharing `root_dir` across runs: runs with the same model, `block_size`, parallelism layout, and dtype share files under the same `<digest>` subdirectory. Changing any of these produces a new subdirectory; old ones are orphaned but harmless. Delete them to reclaim disk.