Originally published on eve.gd on 3 January 2020, last updated 6 September 2026.
Read this post on the web (with images, PDF and citation details)
Urgh. I had a RAID 6 reshape on my NAS that was projected to take 28 days to complete, I kid you not. It was stuck at an abysmal 4MB/s transfer rate. Here’s how to unblock it.
First, follow all the advice on general raid speedups – assuming md2 is your RAID device and you need to replace sd[DEVICE] below with the correct block devices that constitute the array:
all the advice on general raid speedups
echo 50000 > /proc/sys/dev/raid/speed_limit_min
echo 100000 > /proc/sys/dev/raid/speed_limit_max
blockdev –setra 65536 /dev/md2
echo 32768 > /sys/block/md2/md/stripe_cache_size
echo 1 > /sys/block/sd[DEVICE]/device/queue_depth
This is all common advice that it is easy to find. But I was still stuck at the hideous transfer rate. I was worried there was a firmware fault with my drives. What unblocked it for me was:
echo max > /sys/block/md2/md/sync_max
Bingo. Up to 45MB/s now, even during heavy write usage from other processes.
This reference list has been generated, in many cases, through an automated process and may therefore contain errors or incomplete metadata. It is provided purely as assistance for finding the items mentioned within this post.
1. cyberciti.biz guide on Linux RAID resync speed <https://www.cyberciti.biz/tips/linux-raid-increase-resync-rebuild-speed.html>
https://www.cyberciti.biz/tips/linux-raid-increase-resync-rebuild-speed.html