😱 Today I Almost Threw My Entire Rack into the Trash…

😱 Today I Almost Threw My Entire Rack into the Trash…

So, today I had one of those classic homelab panic moments. 😅 I was copying a huge dataset back onto my NAS VM, and the transfer was painfully slow, with occasional timeouts and hiccups. My personal, important files were even affected! At one point, I seriously considered just tossing my entire rack and servers into the trash…

Yesterday, everything was running smoothly, and I had some big learnings about my Proxmox cluster and ZFS setup. But today, out of nowhere, this massive slowdown hit. 🤯

After some deep thinking (and consulting ChatGPT, my trusty homelab oracle 🤓), it turned out that the culprit wasn’t the network, the VM configuration, or even ZFS compression. Nope — it was a small but critical ZFS setting controlling how writes are cached: the Sync Mode 🧠.

⚙️ The Culprit: ZFS Sync Mode

ZFS is amazing, but it can be brutally conservative when it comes to writes. By default, the Sync Mode is set to standard:

• standard: Every write waits until the data hits the physical disk. Very safe, very slow. 🐢
• disabled: Writes are cached and flushed asynchronously. ⚡ Fast, but risky if your power decides to quit mid-transfer.

In my case, standard caused massive slowdowns — like watching paint dry while copying 50GB of files. I initially suspected LZ4 compression (classic rookie move 😅), but it was really the sync setting all along.

🔍 How to Check Your Sync Mode

On your Proxmox host (or any ZFS system), you can check the sync mode with:

zfs get sync <DatasetName>

For example, my NAS VM dataset:

zfs get sync vmdata

Output might look like this:

NAME     PROPERTY  VALUE     SOURCE
vmdata   sync      standard  default

Here, standard = slow turtle 🐢, disabled = lightning-fast ⚡.

🔧 How to Fix It (and Gain Superpowers)

To boost your write performance, simply set sync to disabled on your dataset:

zfs set sync=disabled vmdata

Then verify:

zfs get sync vmdata

Expected output:

NAME     PROPERTY  VALUE     SOURCE
vmdata   sync      disabled  local

After this change, I went from ~100 MB/s to 3.2 GB/s 🚀 on my NAS VM. Yes… that’s thirty times faster. 😎

💡 Tip: This change is persistent across reboots. No need to worry about losing the setting when you restart the server. Just remember: with great power comes great responsibility — consider using a UPS if your data is critical!

After adjusting the ZFS cache, I ran a first test copying large files again — and everything worked beautifully. ✅ My NAS VM was back in full speed mode, no hiccups, no timeouts.

😂 Lessons Learned

• ZFS is awesome, but check the cache settings before blaming your hardware. 🖥️
• LZ4 compression is NOT always the culprit (rookie mistake!). 😅
• Sometimes the tiniest tweak makes your NAS feel like it’s on rocket fuel. 🚀

So yes, today I survived a potential homelab disaster thanks to understanding the ZFS sync cache. And yes, I will sleep a little better knowing my 50GB copy didn’t just crawl at 100 MB/s forever. 😎