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.