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I had to replace one of the disks in my RAID setup, and the rebuild started after a hot swap as expected,... but boy was that going slow. After a day still only 50% done.
After a little searching I found a trick to increase the QNAP RAID rebuild speed ...
First thing I did, was look at the current speed by using:
cat /proc/mdstat
This resulted in the following output.
Expected to finish in 3243 minutes ... 54 hours!!!! 
[~] # cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] [multipath]
md0 : active raid6 sdc3[10] sda3[8] sdh3[7] sdg3[6] sdf3[5] sde3[4] sdd3[3] sdb3[9]
17572185216 blocks super 1.0 level 6, 64k chunk, algorithm 2 [8/7] [UU_UUUUU]
[==========>..........] recovery = 50.1% (1470051456/2928697536) finish=3243.0min speed=7495K/sec
md256 : active raid1 sdc2[2](S) sdh2[7](S) sdg2[6](S) sdf2[5](S) sde2[4](S) sdd2[3](S) sdb2[1] sda2[0]
530112 blocks super 1.0 [2/2] [UU]
bitmap: 0/1 pages [0KB], 65536KB chunk
md13 : active raid1 sdc4[2] sda4[0] sdh4[7] sdg4[6] sdf4[5] sde4[4] sdd4[3] sdb4[1]
458880 blocks [8/8] [UUUUUUUU]
bitmap: 0/57 pages [0KB], 4KB chunk
md9 : active raid1 sdc1[2] sda1[0] sdf1[7] sdg1[6] sdh1[5] sdd1[4] sde1[3] sdb1[1]
530048 blocks [8/8] [UUUUUUUU]
bitmap: 0/65 pages [0KB], 4KB chunk
unused devices: <none>
By setting a higher speed limit, with the following line, the time to completing drastically improved:
echo 50000 > /proc/sys/dev/raid/speed_limit_min
Displaying mdstat again shows and expected completion time in 633 minutes, which is about 10 hours.
[~] # cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] [multipath]
md0 : active raid6 sdc3[10] sda3[8] sdh3[7] sdg3[6] sdf3[5] sde3[4] sdd3[3] sdb3[9]
17572185216 blocks super 1.0 level 6, 64k chunk, algorithm 2 [8/7] [UU_UUUUU]
[==========>..........] recovery = 50.5% (1480883716/2928697536) finish=633.2min speed=38104K/sec
md256 : active raid1 sdc2[2](S) sdh2[7](S) sdg2[6](S) sdf2[5](S) sde2[4](S) sdd2[3](S) sdb2[1] sda2[0]
530112 blocks super 1.0 [2/2] [UU]
bitmap: 0/1 pages [0KB], 65536KB chunk
md13 : active raid1 sdc4[2] sda4[0] sdh4[7] sdg4[6] sdf4[5] sde4[4] sdd4[3] sdb4[1]
458880 blocks [8/8] [UUUUUUUU]
bitmap: 0/57 pages [0KB], 4KB chunk
md9 : active raid1 sdc1[2] sda1[0] sdf1[7] sdg1[6] sdh1[5] sdd1[4] sde1[3] sdb1[1]
530048 blocks [8/8] [UUUUUUUU]
bitmap: 0/65 pages [0KB], 4KB chunk
unused devices: <none>
