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re: Unexpected RAIDframe behavior

> Parity Re-write is 79% complete.

OK, so this is really more about how parity checking works than
anything else (i guess.)

for RAID1, it reads both disks and compares them, and if one
fails it will write the "master" data.  (more generally, it
reads all disks and if anything fails parity check it writes
corrected parity back to it.)

this can lead to strange IO when rebuilds happen.  one general
way to avoid slowness here is to pre-zero disks before adding
them to a raid set, that way all the data read will have the
same parity and both disks will typically be mostly-read and
read at approaching the fastest speed.


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