illumos gate

From Matt Ahrens's bug report at Delphix:

Add a new zfs property, "redundant_metadata" which can have values "all" or
"most".  The default will be "all", which is the current behavior.  Setting to
"most" will cause us to only store 1 copy of level-1 indirect blocks of user
data files.

From the new man-page entry:

redundant_metadata=all | most
         Controls what types of metadata are stored  redundantly.
         ZFS  stores an extra copy of metadata, so that if a sin-
         gle block is corrupted, the amount of user data lost  is
         limited.   This  extra copy is in addition to any redun-
         dancy provided at the pool level (e.g. by  mirroring  or
         RAID-Z),  and  is in addition to an extra copy specified
         by the copies property (up to a total of 3 copies).  For
         example   if   the   pool  is  mirrored,  copies=2,  and
         redundant_metadata=most, then ZFS  stores  6  copies  of
         most metadata, and 4 copies of data and some metadata.

         When set to all, ZFS stores an extra copy of  all  meta-
         data.   If a single on-disk block is corrupt, at worst a
         single block of user data  (which  is  recordsize  bytes
         long) can be lost.

         When set to most, ZFS stores an extra copy of most types
         of  metadata.   This  can  improve performance of random
         writes, because less metadata must be written.  In prac-
         tice,  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.

         The default value is all.