After roughly 44.000 hours in service, I decided to slowly replace my trusty Western Digital Red 4TB drives with some spanking new Western Digital Red Pro 8TB ones. Partly because I am running out of storage and partly because some of the older drives started giving me elusive but terrifying dmesg outputs.
With Synology, this is usually a pretty pain-free process. You remove one old drive at a time, run an extended SMART check on the new drive and then repair/restore the storage pool with the replaced drive.
This causes three scrubs of your drives/pools for each md software RAID device (md2, md3 and md4):
With the number of drives in my ageing DS2413+, this takes about 14-ish hours per run. Except when it does not.
Sunday is SMART day. All drives run an automated extended SMART test at precisely midnight. This also takes about 14 hours. Except when it does not.
Turns out that running the SMART test during a pool restore is a mighty bad idea and slows both processes down to a crawl. I am not talking about a few performance hits, I am talking full-blown “this takes 3 times as long as it should” madness.
After about 38 hours I finally cancelled the SMART tests and the restore process finally reached acceptable speeds again.