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Add a new zfs property, "redundant_metadata" which can have values "all" or "most". The default will be "all", which is the current behavior. When set to all, ZFS stores an extra copy of all metadata. If a single on-disk block is corrupt, at worst a single block of user data (which is recordsize bytes long) can be lost. Setting to "most" will cause us to only store 1 copy of level-1 indirect blocks of user data files. This can improve performance of random writes, because less metadata has to be written. In practice, at worst about 100 blocks (of recordsize bytes each) of user data can be lost if a single on-disk block is corrupt. The exact behavior of which metadata blocks are stored redundantly may change in future releases. Illumos issue: 3835 zfs need not store 2 copies of all metadata MFC after: 2 weeks
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