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Chris LuandGitHub
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* Re-check an EC shard map a failed read has disproved A read that fails against a cached location drops that shard from the map, which leaves it one short of complete -- and a map one short is trusted for seven more minutes. So a moment's trouble between volume servers cost minutes in which every read of that shard skipped the direct fetch and paid for a Reed-Solomon recovery instead, at DataShards times the memory and the peer load. Mark the map when a read disproves it, and re-check a marked map on the same eleven-second footing as one that never had enough shards to begin with. The mark clears on refresh, so it buys one prompt re-check rather than a master lookup per read. The tiers move into a helper; they were three overlapping conditions in one expression, and the reading of them was not obvious. Rust keeps the entry rather than dropping it -- a dead peer fails fast on the next attempt, and it was the freshness window, not the entry, hiding a shard that had moved. Claude-Session: https://claude.ai/code/session_01SM5ARdPvFcnvGWNpBPgNRN * Invalidate the location of an EC shard whose own read failed Recovery fans out to the other shards, so the one whose direct read just failed is the only location nothing ever invalidates: a shard that moved to another server was reconstructed on every read until the map's own window expired, up to thirty-seven minutes for a map still complete. Mark the map there too. The entry stays -- a moved shard's old holder fails fast, and the next refresh is seconds away. Claude-Session: https://claude.ai/code/session_01SM5ARdPvFcnvGWNpBPgNRN * Consume the stale mark before the lookup, not after A read that fails while the master is answering has disproved the very map that answer is about to install, and clearing the mark on the refresh's return swallowed it. Clear it where it is acted on instead. A lookup that then fails loses the mark, which costs nothing: the refresh time is only advanced on success, so the next read looks up regardless. Claude-Session: https://claude.ai/code/session_01SM5ARdPvFcnvGWNpBPgNRN * Judge the shard map and consume its mark in one critical section Reading the mark and clearing it were two separate acquisitions, so a mark raised between them was cleared by a refresh that had not seen it. In Go that gap was a few instructions; in Rust the mark was read when the read first snapshotted the volume and cleared at the decision point, with the local interval reads in between. Take both under one hold. Rust needs a mutex rather than an atomic to do it, and no longer carries the mark through the snapshot. Claude-Session: https://claude.ai/code/session_01SM5ARdPvFcnvGWNpBPgNRN * Put the stale mark back when the lookup does not answer for it Consuming the mark up front assumed the lookup would supersede it. A lookup that fails, or comes back with fewer than DataShards holders, supersedes nothing: the map is unchanged, its refresh time unadvanced, and with the mark gone the map a read had disproved is trusted for its full window again on the strength of a lookup that never landed. Put the mark back on both branches. Claude-Session: https://claude.ai/code/session_01SM5ARdPvFcnvGWNpBPgNRN