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Gas · medium

29. Storage layout and gas

Slots, packing and cold versus warm access.

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Storage is a map of 32-byte slots. Consecutive small variables share a slot if they fit. The first read of a slot in a transaction is cold (2100 gas); later reads are warm (100). Writing a zero slot to non-zero costs 20000.

solidity
uint128 a; uint128 b; // one slot
uint256 c; bool d;    // two slots

Cache storage reads in memory inside loops, and prefer `calldata` for read-only external arguments.

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Check your understanding

  1. 1. How large is one storage slot?

  2. 2. Which write is most expensive?

  3. 3. What should you do with storage reads in a loop?

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