Special failsafe feature - #6
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Special failsafe is a feature that allows your special allocation
class vdevs ('special' and 'dedup') to fail without losing any data. It
works by automatically backing up all special data to the pool. This
has the added benefit that you can safely create pools with non-matching
alloc class redundancy (like a mirrored pool with a single special
device).
This behavior is controlled via two properties:
1. feature@special_failsafe - This feature flag enables the special
failsafe subsystem. It prevents the backed-up pool from being
imported read/write on an older version of ZFS that does not
support special failsafe.
2. special_failsafe - This pool property is the main on/off switch
to control special failsafe. If you want to use special failsafe
simply turn it on either at creation time or with `zpool set` prior
to adding a special alloc class device. After special device have
been added, then you can either leave the property on or turn it
off, but once it's off you can't turn it back on again.
Note that special failsafe may create a performance penalty over pure
alloc class writes due to the extra backup copy write to the pool.
Alloc class reads should not be affected as they always read from DVA 0
first (the copy of the data on the special device). It can also inflate
disk usage on dRAID pools.
Closes: openzfs#15118
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
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Testing special_failsafe locally