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btrfs-progs: doc: add a warning when converting to a profile with lower #945

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btrfs-progs: doc: add a warning when converting to a profile with lower #945

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16 changes: 15 additions & 1 deletion Documentation/ch-balance-filters.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -79,10 +79,24 @@ convert=<profile>
Starting with kernel 4.5, the ``data`` chunks can be converted to/from the
``DUP`` profile on a single device.

.. note::
Starting with kernel 4.6, all profiles can be converted to/from ``DUP`` on
multi-device filesystems.

.. warning::
Btrfs doesn't detect bad/missing devices at runtime, thus if
there is a known failing device, or a device is deleted by
``/sys/block/<dev>/device/delete`` sysfs interface, btrfs will still
access and try to write into that failing/removed device.

In such case, one should not convert to a profile with lower
duplication (e.g. from ``RAID1`` to ``SINGLE``), as btrfs can
create new chunks on that failing/removed device, and cause
various problems.

And the proper action is to use ``btrfs replace`` or
``btrfs device remove`` to handle the failing/missing
device first. Then convert with all devices working correctly.

limit=<number>, limit=<range>
Process only given number of chunks, after all filters are applied. This can be
used to specifically target a chunk in connection with other filters (``drange``,
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