MFC r266771: MFV r266766:

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

1 files changed, 49 insertions, 2 deletions

diff --git a/cddl/contrib/opensolaris/cmd/zfs/zfs.8 b/cddl/contrib/opensolaris/cmd/zfs/zfs.8
index b9760c4..d0f21d3 100644
--- a/cddl/contrib/opensolaris/cmd/zfs/zfs.8
+++ b/cddl/contrib/opensolaris/cmd/zfs/zfs.8
@@ -18,7 +18,7 @@
 .\" information: Portions Copyright [yyyy] [name of copyright owner]
 .\"
 .\" Copyright (c) 2010, Sun Microsystems, Inc. All Rights Reserved.
-.\" Copyright (c) 2013 by Delphix. All rights reserved.
+.\" Copyright (c) 2014 by Delphix. All rights reserved.
 .\" Copyright (c) 2011, Pawel Jakub Dawidek <pjd@FreeBSD.org>
 .\" Copyright (c) 2012, Glen Barber <gjb@FreeBSD.org>
 .\" Copyright (c) 2012, Bryan Drewery <bdrewery@FreeBSD.org>
@@ -30,7 +30,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd April 23, 2014
+.Dd May 27, 2014
 .Dt ZFS 8
 .Os
 .Sh NAME
@@ -1193,6 +1193,53 @@ affects only files created afterward; existing files are unaffected.
 .Pp
 This property can also be referred to by its shortened column name,
 .Sy recsize .
+.It Sy redundant_metadata Ns = Ns Cm all | most
+Controls what types of metadata are stored redundantly.
+ZFS stores an extra copy of metadata, so that if a single block is corrupted,
+the amount of user data lost is limited.
+This extra copy is in addition to any redundancy provided at the pool level
+.Pq e.g. by mirroring or RAID-Z ,
+and is in addition to an extra copy specified by the
+.Sy copies
+property
+.Pq up to a total of 3 copies .
+For example if the pool is mirrored,
+.Cm copies Ns = Ns Ar 2 ,
+and
+.Cm redundant_metadata Ns = Ns Ar most ,
+then ZFS
+stores 6 copies of most metadata, and 4 copies of data and some
+metadata.
+.Pp
+When set to
+.Cm 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
+.Po which is
+.Cm recordsize
+bytes long
+can be lost.
+.Pc
+.Pp
+When set to
+.Cm 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 practice, at worst about 100 blocks
+.Po of
+.Cm recordsize
+bytes each
+.Pc
+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.
+.Pp
+The default value is
+.Cm all .
 .It Sy refquota Ns = Ns Ar size | Cm none
 Limits the amount of space a dataset can consume. This property enforces a hard
 limit on the amount of space used. This hard limit does not include space used