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5a3e4a1cc0
managed_bytes is implemented as chain of blob_storage objects. Each blob_storage contains 24 bytes of metadata. But in the most common case -- when there is only a single element in the chain -- 16 bytes of this metadata is trivial/unused. This is regrettable waste because managed_bytes is used for every database cell in the memtables and cache. It means that every value of size >= 7 bytes (smaller ones fit in the inline storage of managed_bytes) receives 16 bytes of useless overhead. To correct that, this patch adds to managed_bytes an alternative storage layout -- used for buffers small enough to fit in one contiguous fragment -- which only stores the necessary minimum of metadata. (That is: a pointer to the parent, to facilitate moving the storage during memory defragmentation).