Category: Computing

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.

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.

I’ve been using Photoprism to manage my large and growing photo library. We had simply outgrown using a single machine to manage the library, and Apple had burned us a couple of times by changing their native photo management system. I’m also not the type to trust someone else to keep and secure my photos, so I’m going to host it myself.

I have backups of those photo libraries which I’m working from, and unfortunately those backups seem to have replication of the photos. No problem right? Photoprism has the ability to detect duplicates and reject them. Sweet. However, it does rely on the photos being exactly the same binary.

My problems start when I have a bunch of smaller photos, which look ok – but are clearly not the original. In this particular case the original is 2000×2000, and the alternate version is 256×256 (see top of post for an example of two images). Great – just delete the small one, but with 1000’s of photos how do I know that one is a duplicate of another but resized?

There are other flags here too, the smaller resized version is missing a proper EXIF date stamp. So sure, I can just sort out the photos based on ones with valid EXIF data and then I have a bunch of others which don’t have data. But, what if one of those photos isn’t a resized version? Maybe it’s a photo of something that I only have a small version of?

Again, with 1000’s of photos to review, I’m not going to be able to reasonably figure out which ones are keepers or not. Good thing that doing dumb stuff is what computers are good at. However, looking at two images and determining if they are the same thing is not as easy as you might think.

The folks at imagemagick have some good ideas on comparing for differences, they even tackle the same issue of identifying duplicates but still end up relying on you creating your own solution based on some advice.

Since I had this problem, I did cook up some scripting and an approach which I’ll share here. It’s messy, and I still rely on a human to decide – but for the most part I get a computer to do some brute force work to make the problem human sized.

Continue reading “Comparing images to detect duplicates”