From 050d720c43b6285fc0c30e1e97591f6b796dbd68 Mon Sep 17 00:00:00 2001 From: Matthew Ahrens Date: Wed, 19 Jun 2019 14:54:02 -0700 Subject: 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 Reviewed-by: Igor Kozhukhov Reviewed-by: Alek Pinchuk Issue #8270 Closes #8310 --- module/zcommon/zpool_prop.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'module/zcommon') diff --git a/module/zcommon/zpool_prop.c b/module/zcommon/zpool_prop.c index ac1c42b3f..539de5645 100644 --- a/module/zcommon/zpool_prop.c +++ b/module/zcommon/zpool_prop.c @@ -104,8 +104,6 @@ zpool_prop_init(void) /* default number properties */ zprop_register_number(ZPOOL_PROP_VERSION, "version", SPA_VERSION, PROP_DEFAULT, ZFS_TYPE_POOL, "", "VERSION"); - zprop_register_number(ZPOOL_PROP_DEDUPDITTO, "dedupditto", 0, - PROP_DEFAULT, ZFS_TYPE_POOL, "", "DEDUPDITTO"); zprop_register_number(ZPOOL_PROP_ASHIFT, "ashift", 0, PROP_DEFAULT, ZFS_TYPE_POOL, "", "ASHIFT"); @@ -143,6 +141,8 @@ zpool_prop_init(void) PROP_ONETIME, ZFS_TYPE_POOL, "TNAME"); zprop_register_hidden(ZPOOL_PROP_MAXDNODESIZE, "maxdnodesize", PROP_TYPE_NUMBER, PROP_READONLY, ZFS_TYPE_POOL, "MAXDNODESIZE"); + zprop_register_hidden(ZPOOL_PROP_DEDUPDITTO, "dedupditto", + PROP_TYPE_NUMBER, PROP_DEFAULT, ZFS_TYPE_POOL, "DEDUPDITTO"); } /* -- cgit v1.2.3