For raid5 on an array with more than 3 drive, if you attempt to write
a single block, it will:
- read the current value of the block, and the parity block.
- "subtract" the old value of the block from the parity, and "add"
the new value.
- write out the new data and the new parity.
If the parity was wrong before, it will still be wrong. If you then
lose a drive, you lose your data.
Wow, that really needs to be put somewhere in 120 point red blinking
text. A lot of us are used to uninitialized disks calculating the
parity-on-first-write, but if linux MD is forgoeing that
'dangerous-no-resync' sounds really REALLY bad. How about at least a
'Warning: unlike other systems this WILL cause corruption if you
forego reconstruction' on mkraid?
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