> 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. .mrg.
