NAS Backups: Why Verification & Scrubbing Are Essential for Data Integrity

The peace of mind that comes with a successful backup is a powerful feeling. Seeing that progress bar hit 100% and knowing your files are (supposedly) safe is a small victory in the ongoing battle against data loss. But simply backing up essential files isn’t enough. That completed backup doesn’t guarantee your data is actually recoverable when disaster strikes.

Network-Attached Storage (NAS) devices have become increasingly popular for home and small business users looking for centralized storage and backup solutions. They offer convenience and control, but relying solely on a backup’s completion status can be a dangerous illusion. Silent data corruption, a phenomenon where data degrades without any immediate warning signs, can render your backups useless if left unchecked. This is especially true as storage capacities grow and data retention periods lengthen.

Understanding Silent Data Corruption

Let’s say you’ve been saving important data on your NAS for a couple of months. Unfortunately, silent corruption can rear its ugly head even in the most robust setup. The worst part? Improper shutdowns and file system errors aren’t the only causes of data corruption; an aged component, electromagnetic radiation, firmware bugs, and a bunch of errors can just as easily finish up corrupting your files.

As you may have already guessed from the name, silent corruption can poison your datasets while staying under the radar. By the time you become aware of its existence, your backup files might get turned into an unrecoverable mess. This makes snapshot and backup verification jobs borderline essential for NAS users. The issue isn’t limited to NAS devices, either. It affects any digital storage medium, including hard drives, SSDs, and even cloud storage.

Make sure to Configure Scrub Tasks and Verification Jobs

As a member of the TrueNAS faction, I adore the fact that it uses ZFS under the hood. While the Copy-on-Write facility and powerful RAID provisions are undoubtedly useful, it’s the scrub facility that makes ZFS great for combating silent corruption. When you transfer files to a dataset built on top of ZFS, the file system creates a hash value called a checksum for each data block. Think of it as a fingerprint that can uniquely identify each data block of your overarching storage pool.

Scrub tasks calculate new hash values for each data block and contrast them with the original checksum created at the same time as its parent dataset. When a file’s integrity is compromised, its checksum value changes, and scrub tasks flag these files as unrecoverable. If you’ve got redundant data in your NAS, these scrub tasks can even fix the corrupted files. TrueNAS supports this facility, and the same holds true for most distros that ship with ZFS or Btrfs.

Heck, even if you’re not on a NAS-centric distro, chances are your home server platform includes some form of this feature. Proxmox Backup Server, for instance, has verification jobs, which rely on SHA-256 hashes to scan your LXC and VM snapshots for inconsistencies. For folks who use a 3-2-1 PBS workflow as I do, you can schedule verification tasks on both nodes.

Plus, you’ll have to find the right frequency for scrub/verification tasks that match your specific backup configurations. Since they can tax the storage drive, you wouldn’t want to schedule them so frequently that they put your HDDs under unnecessary load. But you too shouldn’t set them so far apart that silent corruptions move unnoticed. I’ve got short scrubs scheduled every 2 weeks, while long tests are better left on a monthly basis.

Otherwise, You Can Strive Recovering Random Files Occasionally

Besides verification jobs, you can also go the old-fashioned way of checking everything yourself. If you’ve got small-sized datasets that are under 1TB, you can just restore them onto a spare drive to verify the data consistency manually. As for massive storage pools with several TBs of files, you can try to recover random files from your snapshots and backups. I’ve got everything from ebooks, projects, and movies to ripped ROMs and ISOs stored on my NAS units, and I’d pull standalone files at random. I’d repeat this process for both local and remote nodes. You can take it a step further by computing hashes for random files on each rig and comparing them – not just with each other, but also with the original document on your PC.

The same holds true for my Proxmox Backup Server instances, as I’ve connected them via Tailscale and synchronized their datastore contents using Rsync tasks. LXCs are fairly easy to restore on my cluster nodes, as they barely consume any space. Their VM counterparts can take a while, though PBS’ selective restoration helps me test the integrity of essential project files without waiting several minutes for the entire virtual machine to get deployed on a local Proxmox workstation.

You Can Even Follow These Principles for Cloud-Based Datasets

I’ve centered this article around a mostly self-hosted (since I rely on Tailscale) setup, but you can follow this advice when storing backups on typical cloud platforms. Manually checking random documents can help detect corrupted files, and I’d recommend doing that very often if your storage provider doesn’t support integrity scans or skips out on file versioning.

The best NAS for your needs will depend on your budget and requirements, but regardless of the hardware you choose, remember that a backup is only as good as its ability to be restored. Don’t fall into the trap of assuming your data is safe simply because a backup completed successfully. Regular verification is the key to ensuring your digital life remains protected against the silent threat of data corruption.

As NAS devices become more sophisticated, and as we entrust them with ever-increasing amounts of data, proactive data integrity checks will become even more critical. It’s a small investment of time and resources that can save you from a potentially devastating loss.