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Explain the internal working of Redis's hash table resizing (rehashing)?

Redis's core keyspace (and large Hash-type values) are backed by a hash table, and as entries are added or removed, the table needs to grow or shrink to keep lookups close to O(1) — but a naive full rehash (allocate a new table, move every entry, free the old one) would briefly block every other operation on a table with millions of entries, which conflicts directly with Redis's single-threaded, low-latency design.

flowchart TD A[Load factor crosses a threshold, growth or shrink needed] --> B[Allocate a second, differently-sized hash table] B --> C[Rehashing flag set: both old and new tables now active simultaneously] C --> D[Each subsequent command incrementally moves one bucket's worth of entries from old to new table] D --> E{All buckets migrated?} E -- No --> F[Old table still holds unmigrated entries; lookups check both tables during this period] F --> D E -- Yes --> G[Old table freed, new table becomes the sole active table]

This incremental rehashing spreads the cost of a full table resize across many individual commands instead of pausing everything for one large operation, which is what lets Redis resize its hash tables even under heavy load without a noticeable latency spike. During the migration window, any lookup for a key has to check both tables (since it might not have been migrated yet), which is a small, bounded per-operation cost rather than one large blocking one.

Why doesn't Redis perform a full hash table rehash in one blocking step?
What must a lookup do while incremental rehashing is in progress?

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