Why Your Proxmox Migration Failed (Hint: It Wasn't Proxmox)

There's a familiar moment happening in a lot of IT meetings right now. Someone pulls up a spreadsheet. Licensing costs are circled in red. Broadcom gets mentioned, usually with a sigh. And then someone asks the question that sounds obvious, almost inevitable: "Why don't we just move everything to Proxmox? It's basically free VMware." On the surface, it makes sense. Proxmox looks ready. The feature list checks out. Plenty of people are running it in production without issue. Meanwhile VMware vSphere has gone from default choice to uncomfortable budget conversation. So the decision gets made. VMs are converted. Hosts are built. The cutover happens. And then the weird stuff starts. Databases fall over under load. Latency jumps for no obvious reason. Entire nodes feel sluggish even though monitoring says everything is "fine." Before long, the conclusion lands: "Proxmox is unstable." "Open source isn't enterprise-ready." "We should've stayed on VMware." Except that's not what actually happened. What failed wasn't Proxmox. What failed was an assumption VMware spent 15 years teaching us to make. ## VMware Didn't Make Storage Easy — It Made It Invisible VMware's biggest accomplishment wasn't virtualization itself. It was insulation. VMFS, SAN integrations, vSAN, and years of engineering work smoothed over the sharp edges of storage. Locking behavior, alignment issues, cache pressure — all of it lived behind a clean abstraction layer. You didn't need to think about it most days. That was the point. That didn't make admins lazy. It made the platform successful. But it also meant a lot of teams learned what buttons to click, not why things worked. You sized hosts. You followed best practices. You trusted the system to absorb the complexity. Proxmox doesn't play that role. Instead, it hands you the raw materials: ZFS, Ceph, LVM, XFS. Powerful tools. Fewer guardrails. Much more honesty. If something goes wrong, you're going to see it — and feel it. That difference alone is enough to derail migrations that look perfectly fine on paper. ## ZFS and the Memory You Thought You Had One of the most common post-migration stories sounds almost boring at first. A VM crashes. There's plenty of RAM installed. The host isn't overloaded. Nothing obvious is wrong. Proxmox gets the blame. Dig a little deeper and ZFS is usually sitting right there, doing exactly what it was designed to do. ZFS loves memory. It uses ARC to turn spare RAM into faster reads and better performance. And if you don't tell it otherwise, it will take what it can get. A lot of it. If you migrated from ESXi and sized your hosts the same way, that's where things start to unravel. On VMware, that memory belonged almost entirely to guests. On Proxmox with ZFS, the filesystem is now competing for it too. When pressure hits, the Linux OOM killer doesn't care that your SQL VM is "important." It just sees a system under stress and makes a decision. Usually the wrong one, from your point of view. Yes, newer ZFS versions are better about releasing memory. Yes, ARC is now more reclaimable than it used to be. That doesn't change the core lesson: if you don't cap it intentionally, you're trusting adaptive behavior during the exact moments when predictability matters most. That's not a Proxmox bug. That's storage doing storage things. ## Ceph on 1GbE: Technically Possible, Practically a Disaster Then there's Ceph. Or rather, how people try to run it. Ceph is distributed storage. That means your disks are constantly talking to each other over the network. When everything is healthy, it's manageable. When a disk fails, that chatter turns into a firehose. On a 1GbE network, that firehose becomes a brick wall. The cluster doesn't necessarily go down. That's the deceptive part. It stays "up." Health checks might even look okay. But latency skyrockets, rebuilds crawl, and every VM feels like it's wading through mud. From the outside, it looks like Proxmox fell apart. In reality, the network did. Ceph documentation has been blunt about this for years: 10GbE is the minimum. Not a suggestion. The floor. And in real production environments, 25GbE is what actually gives you breathing room when things break — because things always break. Running Ceph on 1GbE because "it works" is like discovering your car can technically drive with the parking brake on. You'll get moving. You just won't like what happens next. ## The Migration Mistake Nobody Notices Until It's Too Late Most migrations follow the same script. Convert the disk. Import the VM. Boot it up. If it starts, call it good. That's how you end up with systems that feel inexplicably slow. If you move virtual disks without paying attention to sector alignment — especially when crossing from legacy 512-byte layouts into 4K-backed storage — every write can turn into extra work. One logical write becomes multiple physical operations. IOPS drop. Latency climbs. Nothing crashes. Nothing throws an error. Performance just quietly degrades. And because the hypervisor changed at the same time, the blame lands there. Not on the invisible storage mismatch quietly dragging everything down. VMware absorbed a lot of that pain for you. Proxmox doesn't. It assumes you know what you're doing. If you don't, it won't stop you — it'll just let you learn the hard way. ## "Free" Was Never the Real Cost Here's the part nobody likes to say out loud: removing VMware licensing doesn't remove complexity. It relocates it. With VMware, you paid money to avoid thinking about certain problems. With Proxmox, you save money and take those problems back in-house. Memory behavior. Network saturation. Disk geometry. Failure domains. Rebuild dynamics. That trade can be fantastic. Plenty of teams are making it work. But they're not doing it by pretending Proxmox is a drop-in replacement for vSphere. If you want set-it-and-forget-it storage, buy a SAN. If you want performance, buy RAM and tune ZFS. If you want hyperconverged infrastructure, build a real network and accept what that implies. What you can't do is assume nothing else needs to change. ## This Isn't a Proxmox Problem. It's a Reality Check. VMware didn't make engineers worse. It made certain mistakes harder to see. Proxmox removes that cushion. That's uncomfortable, especially when migrations are rushed and expectations are unrealistic. But the failures people are seeing aren't proof that Proxmox isn't ready. They're proof that abstraction hid more than we realized. The teams that slow down, relearn the fundamentals, and design intentionally will be fine. The ones that rush, convert disks, and hope muscle memory carries them through will keep blaming the wrong thing. Your migration didn't fail because Proxmox is bad. It failed because Proxmox stopped lying about how your infrastructure actually works — and that can be a rude awakening.