Category: Computing

PSA: DNS servers have no priority order

It is a common misconception that DNS servers that your system uses are managed in a priority order. I had this misunderstanding for years, and I’ve seen many others with the same.

The problem comes from the router or OS setup where you can list a “Primary” and “Secondary” DNS server. This certainly gives you the impression that you have one that is ‘mostly used’ and a ‘backup one’ that is used if the first one is broken, or too slow. This is false, but confusingly also sometimes true.

Consider this stack exchange question/answer. Or this serverfault question.  If you go searching there are many more questions on this topic.

Neither DNS resolver lists nor NS record sets are intrinsically ordered, so there is no “primary”. Clients are free to query whichever one they want in whichever order they want. For resolvers specifically, clients might default to using the servers in the same order as they were given to the client, but, as you’ve discovered, they also might not.

Let me also assure you from my personal experience, there is no guarantee of order. Some systems will always try the “Primary” first, then fall back to the “Secondary”. Others will round-robin queries. Some will detect a single failure and re-order the two servers for all future queries. Some devices (Amazon Fire Tablets) will magically use a hard coded DNS server if the configured ones are not working.

Things get even more confusing to understand because there is the behaviour of the individual clients (like your laptop or phone), and then the layers of DNS servers between you and the authoritative server. DNS is a core part of how the internet works, and there is lots of information on the different parts of DNS out there.

The naming “Primary” and “Secondary” come from the server side of DNS. When you are hosting a system and configure the domain name to IP mapping, you set up your DNS records in the “Primary” system. The “Secondary” system is usually an automated replica of that “Primary”. This really has nothing to do with what the client devices are going to do with those addresses.

Another pit-fall people run into when they think there is an ordering, is when they setup a pi-hole for ad-blocking. They will use their new pi-hole installation as the “Primary” and then use a popular public DNS server (like 8.8.8.8) as the “Secondary”.  This configuration sort of works – at least some of the time, your client machine will hit your pi-hole and ad-blocking will work. Then, unpredictably it will not block an ad – because the client has used the “Secondary”.

Advice: Assume all DNS servers are the same and will return the same answer. There is no ordering.

I personally run two pi-hole installations. My “Primary” handles about 80% of the traffic, and the “Secondary” about 20%. This isn’t because 20% of the time my “Primary” is unavailable or too slow, but simply that about 20% of the client requests are deciding to use the “Secondary” for whatever reason (and that a large amount of my traffic comes from my Ubuntu server machine). Looking deeper at the two pi-hole dashboards, the mix of clients looks about the same, but the “Secondary” has fewer clients – it does seem fairly random.

If your ISP hands out IPv6 addresses, you may find that things get even more interesting as you’ll also have clients assigned an IPv6 DNS address, this adds yet another interface to the client device and another potential DNS server (or two) that may be used for name lookups.

Remember, it’s always DNS.

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.

Getting started with ZFS

When ZFS first came out, it was a proprietary filesystem but it had some very interesting characteristics – at the time it’s ability to scale massively and protect your data seemed very cool. My interest in filesystems goes back to my C64 days editing floppy disks to create infinite directory listings and the like.  Talking about filesystems reminds me of when I was a COOP student at QNX, they had ‘QFS’ and meeting the developer helped de-mystify filesystems for me.

For some reason ZFS is also linked in my memory with the ‘shouting in the datacenter’ video. As best I can tell this is likely because both DTrace and ZFS both came out of Sun around the same time.

I finally decided to fully decommission my old server and the RAID5 array of 1TB drives. I’ve also recently been experimenting with NixOS, and I’ve really enjoyed that so far. I figured why not setup a dedicated backup server? This also presented a good chance to setup and play with ZFS which now has reliable open source versions available.

First I spent some time learning what I would consider ZFS basics. This video was useful for me. Also, these two blog posts were good starting points.

Since I’m using NixOS as my base operating system, I’ll be following the doc on setting up ZFS on NixOS. Now, while I’m not setting up my boot volume to be ZFS – it turns out you still need to do the same basic setup if you want ZFS capabilities in your NixOS.

You need to generate a unique ‘hostid’ – the doc suggests using

Now we need to modify the /etc/nixos/configuration.nix to include

Rebuild and reboot, then you can query available zpools

Now we create a pool, I think in this step we are actually adding a bunch of devices to a vdev, which is then wrapped in a pool. Using fdisk I’m able to identify the four 1TB drives which are all partitioned and ready to roll: sdd, sde, sdf, and sdg.

This process took a short while to complete, but after it was done running sudo fdisk -l /dev/sdd gave me this:

It seems new partitions were created and I now have a zpool

I don’t believe you can reasonably expand or shrink a RAIDZ vdev, this means you need to plan ahead for your storage needs. Also important to remember that the guidance is to not have ZFS volumes at more than 80% usage, beyond this level performance starts to suffer. Storage is cheap, and with pools I think you can have multiple vdev’s in a single pool, so while a single RAIDZ vdev has limitations I think ZFS offers some interesting flexibility.

Unexpectedly, it seems that the newly created ZFS is also mounted and ready to roll

That’s not where I want to mount the volume, so let’s go figure out how to move it.

Cool. I’ve got a ZFS filesytem. One snag, it isn’t mounted automatically after a reboot. I can manually mount it:

And digging into the NixOS doc, we find the configuration we need to add

This fixed me up, and ZFS is auto mounted on reboots.

One last configuration tweak, let’s enable scrubbing of the ZFS pool in our NixOS configuration

Setting up ZFS on NixOS is very easy. Why would you want ZFS over another filesystem or storage management system? I’ve been using snapraid.it for a while on my main server, and I like the data integrity that it brings beyond just a RAID5 setup. The snapraid site has an interesting comparison matrix. I will say that setting up ZFS RAIDZ was a lot less scary than any of my adventures using mdadm to setup a software RAID5.

What do I see as the key strengths of ZFS?

  • Data integrity verification and automatic repair – all files are check-summed, and with RAIDZ redundancy we can recovery from underlying data corruption.
  • Pooled Storage – something I need to explore more, but I think this will give me flexibility over adding more storage over time if needed.
  • Copy-on-write – this is about consistency of the filesystem, especially over power failure events.

Remember I started out with some old hardware I was repurposing? Those 1TB drives were all surprisingly in ‘good’ shape, but between 10 and 13 years of power on time (some of them have manufacture data of 2009). In my next blog post we’ll cover how ZFS handles failures as we see these ancient drives start to fail.