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Writing to contract storage is one of the most expensive operations on Ethereum. Understanding the storage layout lets you reduce slots written and save users real money.

Contract storage is an array of slots, each 32 bytes (256 bits). State variables are assigned to slots in declaration order, starting at slot 0.

Multiple small variables that fit within 32 bytes are packed into the same slot. A uint128 and another uint128 share one slot, halving storage writes if updated together.

Packing only works for adjacent variables that fit. Poor ordering wastes slots. Group small types together rather than interleaving them with full-size uint256 variables.

Interleaving a small type between large ones prevents packing and uses extra slots.

Mappings and dynamic arrays do not pack. Their slot holds metadata, and actual values are stored at hashed locations. Each element write is its own storage cost.

Values marked constant or immutable are stored in the contract bytecode, not in storage slots. Reading them is far cheaper than reading a state variable.

Reading the same storage variable repeatedly inside a loop is wasteful. Cache it in a memory local variable once, operate on it, then write back once.

Writing a value identical to the current one still costs gas. Skip a storage write if the new value equals the existing one, or only update when something actually changed.

Clearing a storage slot back to zero gives a partial gas refund. Patterns that delete unused entries can reclaim some cost, though refund rules have changed across network upgrades.

Use a gas reporter to see the cost of each function. Optimize the hottest, most-called functions first; micro-optimizing rarely-used code is not worth the readability cost.

You learned to optimize contract storage:

• Storage is 32-byte slots assigned in declaration order
• Pack small adjacent variables to share slots
• Use constant/immutable for fixed values
• Cache storage in memory, avoid redundant writes, and measure gas

Smart storage layout directly lowers the cost of using your contract.