After purchasing a Synology NAS I inserted a bunch of drives and left ‘expanding’ overnight.
When I came back in the morning it had gone from 2% to 5% complete – about 95% less than what I was hoping.
I discovered that you can SSH into the box and check the status of the rebuild with:
cat /proc/mdstat
This showed that I had another 14 days left. Hmm, I don’t think so.
After some poking about I discovered it defaults to a strangely small 256 byte cache size.
There were a lot of posts on the internet about changing this value in /sys/block/md3/md/stripe_cache_size, but this failed with a permission problem. Maybe this applied to older versions.
Instead you can edit:
/etc.defaults/synoinfo.conf
And add the line:
md_resync_cache_size="16384"
It took a few minutes for this to be picked up and then the rebuild time decreased to 14 hours. Much more acceptable.
I tried some larger cache size values but they didn’t make any difference.
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