Remove dedupditto functionality

If dedup is in use, the `dedupditto` property can be set, causing ZFS to
keep an extra copy of data that is referenced many times (>100x).  The
idea was that this data is more important than other data and thus we
want to be really sure that it is not lost if the disk experiences a
small amount of random corruption.

ZFS (and system administrators) rely on the pool-level redundancy to
protect their data (e.g. mirroring or RAIDZ).  Since the user/sysadmin
doesn't have control over what data will be offered extra redundancy by
dedupditto, this extra redundancy is not very useful.  The bulk of the
data is still vulnerable to loss based on the pool-level redundancy.
For example, if particle strikes corrupt 0.1% of blocks, you will either
be saved by mirror/raidz, or you will be sad.  This is true even if
dedupditto saved another 0.01% of blocks from being corrupted.

Therefore, the dedupditto functionality is rarely enabled (i.e. the
property is rarely set), and it fulfills its promise of increased
redundancy even more rarely.

Additionally, this feature does not work as advertised (on existing
releases), because scrub/resilver did not repair the extra (dedupditto)
copy (see https://github.com/zfsonlinux/zfs/pull/8270).

In summary, this seldom-used feature doesn't work, and even if it did it
wouldn't provide useful data protection.  It has a non-trivial
maintenance burden (again see https://github.com/zfsonlinux/zfs/pull/8270).

We should remove the dedupditto functionality.  For backwards
compatibility with the existing CLI, "zpool set dedupditto" will still
"succeed" (exit code zero), but won't have any effect.  For backwards
compatibility with existing pools that had dedupditto enabled at some
point, the code will still be able to understand dedupditto blocks and
free them when appropriate.  However, ZFS won't write any new dedupditto
blocks.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Igor Kozhukhov <[email protected]>
Reviewed-by: Alek Pinchuk <[email protected]>
Issue #8270 
Closes #8310 

1 files changed, 1 insertions, 11 deletions

diff --git a/man/man8/zpool.8 b/man/man8/zpool.8
index bdad81149..230004f1b 100644
--- a/man/man8/zpool.8
+++ b/man/man8/zpool.8
@@ -806,17 +806,7 @@ such that it is available even if the pool becomes faulted.
 An administrator can provide additional information about a pool using this
 property.
 .It Sy dedupditto Ns = Ns Ar number
-This property is deprecated.  In a future release, it will no longer have any
-effect.
-.Pp
-Threshold for the number of block ditto copies.
-If the reference count for a deduplicated block increases above this number, a
-new ditto copy of this block is automatically stored.
-The default setting is
-.Sy 0
-which causes no ditto copies to be created for deduplicated blocks.
-The minimum legal nonzero setting is
-.Sy 100 .
+This property is deprecated and no longer has any effect.
 .It Sy delegation Ns = Ns Sy on Ns | Ns Sy off
 Controls whether a non-privileged user is granted access based on the dataset
 permissions defined on the dataset.