Combining --enable-prefix-caching with the DFlash drafter crashed engine
init with "block_size must be divisible by hash_block_size" in
HybridKVCacheCoordinator. The drafter's attention KV page is ~2x the
target's, so vLLM's page-size unification scales the target's mamba+attn
block 2240->4480 to match; the (align-mode) mamba block then differs from
cache_config.block_size and resolve_kv_cache_block_sizes backs off to
hash_block_size = LCM (4480), which the drafter group (still 2240) is not
divisible by. But the GCD (2240) divides every group and is the correct
finer hash granularity; the back-off's `block_size != cache_block` test is
a buggy proxy for "non-align mamba".
patch_prefix_align.py makes the back-off align-aware (only back off when
mamba_cache_mode != "align"), so resolve uses the GCD. Prefix caching is on
by default (a win for multi-turn / long-context, neutral for single-turn
c=1); set PREFIX_CACHE=0 to disable.
Validated on GB10: READY, DFlash accept ~7.7 tok/step on code, ~13x
warm-prefix TTFT (2.30s->0.18s), KV pool ~422k tokens (no regression).
See docs/FINDINGS.md section 1a.
Entrpi
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