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fe957de892
This was an interesting issue found during a GitHub discussion with rmollway and thrasher8390. Blocks in the metadata-pair are relocated every "block_cycles", or, more mathy, when rev % block_cycles == 0 as long as rev += 1 every block write. But there's a problem, rev isn't += 1 every block write. There are two blocks in a metadata-pair, so looking at it from each blocks perspective, rev += 2 every block write. This leads to a sort of aliasing issue, where, if block_cycles is divisible by 2, one block in the metadata-pair is always relocated, and the other block is _never_ relocated. Causing a complete failure of block-level wear-leveling. Fortunately, because of a previous workaround to avoid block_cycles = 1 (since this will cause the relocation algorithm to never terminate), the actual math is rev % (block_cycles+1) == 0. This means the bug only shows its head in the much less likely case where block_cycles is a multiple of 2 plus 1, or, in more mathy terms, block_cycles = 2n+1 for some n. To workaround this we can bitwise or our block_cycles with 1 to force it to never be a multiple of 2n. (Maybe we should do this during initialization? But then block_cycles would need to be mutable.) --- There's a few unrelated changes mixed into this commit that shouldn't be there since I added this as part of a branch of bug fixes I'm putting together rather hastily, so unfortunately this is not easily cherry-pickable.