Many thanks to Kentvv for the hint on those 2 parameters !!!
it makes the reorg of the RAID come down from 8 days to 22H on my DS508 going from 3 to 4 disks of 1TB
From my experience here is how it goes :
those 2 parameters can be manually and dynamicly modified by doing a simple echo command redirected to the name of the file
I have entered 0 for speed_limit_max as suggested by Kentvvv and entered 40000 for speed_limit_min
In firmware 722 (full re-installed), values are by default : min = 1000 and max = 200000, and the reorg tends to dramaticly aligned on the min value... I was around 1200 KB/s...
After dynamicly changed min and max, I get almost immediately 11500 KB/s on around 50% of the reorg, with even +23000 KB/s in the second half of the reorg...
I already did a RAID increase in the firmware 640 in the past months and the timing was also 22H... so I guess something has disappeared in the 722 release...
Also don't underestimate the duration of the second operation : the filesystem resizing !
Here is the final statistics I can give everyone (based on my experience and the command and parameters given by kentvv) :
DS-508, going from 3 to 4 disks of 1 TB in RAID5, firmware 722, free space less than 1GB on the current space
to know default values, see content of following files (under root user, do a cat on the following files in /proc/sys/dev/raid) : speed_limit_min and speed_limit_max
before : min = 1000 and max = 200000 (those values are KB by seconds)
to see the current value of progress of your reorg of data : cat /proc/mdstat -> in my case, rate was around 1200 KB/s and time expected was around 15000 min (8 days)
modifying values to :
min = 40000, go into the directory, and type : echo 40000 > speed_limit_min
max = 0, go into the directory, and type : echo 0 > speed_limit_max
to see the changes, issue a command : cat /proc/mdstat, you will notice the difference almost immediately -> in my case 11500 KB/s, then 22000 to 25000 KB/s later
So after 22H, the reorg is done... now starts automatically the resize operation of the filesystem to take benefit of your new disk
This final step will take around 2 hours : when you start, the progress rate is increasing in a way you may expect 15mn would be enough to complete but... it would be too easy !!! In fact, when around 50% of progress, the progress rate will increase much more slowly.. who knows why... and at the end it represents around 2 hour for this step...
So globally, adding a 4th disk of 1 TB ends represents around 25 hours of unavailability of your shares !
Final hint for all : regarding the duration of that kind of critical operations, I encourage everyone to use a UPS... I can't imagine what would result from a power cut during the reorg...a mess for sure...