- The HDDs are in an external USB case.
- RAID5 with 3 HDD (10TB)
- Software RAID5 with mdadm and Debian Linux
Adding a new disk
When you add a new HDD to an existing RAID, a sync is started. In my case I added a 10TB disk to a RAID5. The sync started and as estimated time I got something in the range of days. The estimated time is listed in finish=5384 min.
This number goes up and down a little bit, but overall result is that the sync will need days. After checking the status again after a while, it still showed days: finish=3437min.
The main problem here Is the rate at which mdadm can sync the data. The value is between 30000K and 43000K. That’s not much given the size of the RAID. There are several tips available on the internet. What help me was to set the stripe_cache_size.
You set the size of stripe_cache_size for each RAID device (mdX). In case your RAID is md0:
echo 32768 > /sys/block/md0/md/stripe_cache_size
The speed increased to 100000K/sec. That’s close to 3x faster than before. Time went down drastically.