Replacing a ZFS degraded device

It was no surprise that a new RAIDZ array built out of decade old drives was going to have problems, I didn’t expect the problems to happen quite so quickly, but I was not surprised. This drive had 4534 days of power on time, basically 12.5 years. It was also manufactured in Oct 2009, making it 14.5 years old.

I had started to backup some data to this new ZFS volume, and upon one of the first scrub operations ZFS flagged this drive as having problems.

The degraded device, maps to /dev/sdg – I determined this by looking a the /dev/disk/by-id/wwn-0x50014ee2ae38ab42 link.

On one of my other systems I’m using snapraid.it, which I quite like. It has a SMART check that does a calculation to indicate how likely the drive is to fail. I’ve often wondered how accurate this calculation is.

The nice thing is you don’t need to be using snapraid to get the SMART check data out, it’s a read only activity based on the devices. In this case it has decided the failing drive has 100% chance of failure, so that seems to check out.

Well, as it happens I had a spare 1TB drive on my desk so it was a matter of swapping some hardware. I found a very useful blog post covering how to do it, and will replicate some of the content here.

As I mentioned above, you first need to figure out which device it is, in this case it is /dev/sdg. I also want to figure out the serial number.

Good, so we know the serial number (and the brand of drive), but when you’ve got 4 identical drives, which of the 4 is the right serial number? Of course, I ended up pulling all 4 drives before I found the matching serial number. The blog post gave some very good advice.

Before I configure an array, I like to make sure all drive bays are labelled with the corresponding drive’s serial number, that makes this process much easier!

Every install I make will now follow this advice, at least for ones with many drives. My system now looks like this thanks to my label maker

I’m certain future me will be thankful.

Because the ZFS array had marked this disk as being in a FALTED state, we do not need to mark it ‘offline’ or anything else before pulling the drive. If we were swapping an ‘online’ disk we may need to do more before pulling the drive.

Now that we’ve done the physical swap, we need to get the new disk added to the pool.

The first, very scary thing we need to do is copy the partition from an existing drive in the vdev. The new disk is the TARGET, and an existing disk is SOURCE.

Once the partition is copied over, we want to randomize the GUIDs as I believe ZFS relies on unique GUIDs for devices.

This is where my steps deviate from the referenced blog post, but the changes make complete sense. When I created this ZFS RAIDZ array I used the short sdg name for the device. However, as you can see after a reboot the zpool command is showing me the /dev/disk/by-id/ name.

This worked fine. I actually had a few miss-steps trying to do this, and zpool gave me very friendly and helpful error messages. More reason to like ZFS as a filesystem.

Cool, we can see that ZFS is repairing things with the newly added drive. Interestingly it is shown as sdg currently.

This machine is pretty loud (it has a lot of old fans), so I was pretty wild and powered it down while the ZFS was trying to resilver things. When I rebooted it after relocating it to where it normally lives and the noise won’t bug me, it seems that the device naming has sorted itself out.

The snapraid SMART report now looks a lot better too

It took about 9 hours to finish the resilvering, but then things were happy.

Some folks think that you should not use RAIDZ, but create a pool with a collection of vdevs which are mirrors.

About 2 weeks later, I had a second disk go bad on me. Again, no surprise since these are very old devices. Here is a graph of the errors.

The zfs scrub ran on April 21st, and you can see the spike in errors – but clearly this drive was failing slowly all along as I was using it in this new build. This second failing drive was /dev/sdf – which if you look back at the snapraid SMART report, was at 97% failure percentage. It is worth noting that while ZFS and the snapraid SMART have both decided these drives are bad, I was able to put both drives into a USB enclosure and access them still – I certainly don’t trust these old drives to store data on them, but ZFS stopped using the device before it became unusable.

I managed to grab a used 1TB drive for $10. It is quite old (from 2012) but only has a 1.5yrs of power on time. Hopefully it’ll last, but at the price it’s hard to argue. Swapping that drive in was a matter of following the same steps. Having the drive bay labelled with the serial numbers was very helpful.

Since then, I’ve picked up another $10 1TB drive, and this one is from 2017 with only 70 days of power on time. Given I’ve still got two decade old drives in this RAIDZ, I suspect I’ll be replacing one of them soon. The going used rate for 1TB drives is between $10 and $20 locally, amazing value if you have a redundant layout.