[Closed] Help/Advices about debugging zfs pool issues
Hi all,
UPDATE: I closed the post (the timebox I gave myself to understand the issue is now over). Thank you all for the help ^^
DISCLAIMER: The objective of this post is to understand how people would debug issues like these when real data is involved and get to the bottom of the problem. The objective is NOT to "restore service" but to understand what failed. The tone of the post is voluntarily not serious to keep it light.
I am playing a little with TrueNas Scale and ZFS. I was trying to use a second NVME disk via USB to do a replication once a day of the main pool, however I had issues with this secondary pool being SUSPENDED for "too many errors". This pool is not directly write/read by users/apps, but it is just there to be "replicated on" once a day.
Now, please, I know that using disks via USB is not advised. Also I am not interested in recovering the data, since there is nothing real on it. What I am doing is testing to see if the system is brittle, and if it is, how to debug if there is a real issue.
Now to the point. The pool is SUSPENDED. Good. Why? I mean, the real reason why. To see if the system can be used in real life it needs to be debuggable.
Let's start. The pool is SUSPENDED:
pool: tank-02
state: SUSPENDED
status: One or more devices are faulted in response to IO failures.
action: Make sure the affected devices are connected, then run 'zpool clear'.
see: https://openzfs.github.io/openzfs-docs/msg/ZFS-8000-JQ
config:
NAME STATE READ WRITE CKSUM
tank-02 UNAVAIL 0 0 0 insufficient replicas
xxx-xxx-xxx-xxx-xxx FAULTED 3 0 0 too many errors
errors: 4 data errors, use '-v' for a list
To which you may ask: why? Too many errors (the -v says nothing more). Well that doesn't help, does it. When you run zpool clear:
# zpool clear tank-02
cannot clear errors for tank-02: I/O error
Incredibly useful as you can see. dmesg to the rescue?
WARNING: Pool 'tank-02' has encountered an uncorrectable I/O failure and has been suspended.
Thanks? I guess. I know it it trying to safeguard data but again... why?
Before you ask:
- SMART checks are good
- Yes, I restarted the device. As soon as you try to use/mount/import you get to the same issues.
- Nothing else peculiar in dmesg. I mean the USB was
usb 2-4: USB disconnect, device number 12whatever the reason why. I mean, kick me if I know why TrueNas scale decided that having/sys/module/usbcore/parameters/autosuspendto2is a good idea but again, that is not the point. I need ZFS to reply to me what is the issue for its point of view.
I have read a lot online. Maybe it is the temperarure (usb enclosure heating up), maybe it is the cable, power, "it is the usb controller", or the chipset doing the usb -> nvme... However, therey are not saying what to check. People is guessing. I saw more tech behind reading tea leaves.
My question for you all is this: ZFS SUSPENDED one of my pools. It (seems to me) is refusing to fix it. Refusing to do anything with it and to tell me why. So, in a real world case, how to debug it? If I have to trust my data to it, I don’t want the only option to be “use many disks and just replace one and the cable when ZFS poo-poo”.
How to know the cause?
Thank you for the help.
PS: I am sure I am missing some very basic ZFS knoweldge on the topic, so please let me know what else can I do to make ZFS talk to me.
This takes a degree of understanding of what you are doing and why it fails.
I've done some research on this myself and the answer is the USB controller. Specifically the way the USB controller "shares" bandwidth. It is not the way a sata controller or a pci lane deals with this.
ZFS expects direct control of the disk to operate correctly and anything that gets in between the file system and the disk is a problem.
I the case of USB let's say you have two USB - nvme adapters plugged in to the same system in a basic zfs mirror. ZFS will expect to mirror operations between these devices but will be interrupted by the USB controller constantly sharing bandwidth between these two devices.
A better but still bad solution would be something like a USB to SATA enclosure. In this situation if you installed a couple disks in a mirror on the enclosure... They would be using a single USB port and the controller would at least keep the data on one lane instead of constantly switching.
Regardless if you want to dive deeper you will need to do reading on USB controllers and bandwidth sharing.
If you want a stable system give zfs direct access to your disks and accept it will damage zfs operations over time if you do not.
Hi,
Thanks for sharing. I agree with you 100% and I think everybody commenting here does. The whole point of the thread however was to understand if/how you can identify the location of the problem without guessing. The reality is I got to the conclusion that people... don't. Like you said people know ZFS is fussy about how does he speaks with the disks and the minimum issue it has it throws a tantrum. So people just switch things until they work (or buy expensive motherboards with many ports). I don't like the idea of not knowing "why", so I will just add on my notes that for my specific usecase I cannot trust ZFS + OS (TrueNas scale) to use the USB disk for backups via ZFS send/recieve.
I would like to add that I am not trying to
mirrormy main disk with a usb one. I just wanted to copy the zfs snapshots on the usb drive once a day at midnight. ZFS is just (don't throw stones at me for this, it is just my opinon) too brittle to use it this way too. I mean when I am trying toclean/recover the pool it just refuses (and there is no one writing on it).In my case there was no switching however. It was a single nvme drive in a single usb line in an enclusure. It was a separate stripe to just recieve data once a day.
Not without good logs or debugging tools.
I decided I cannot trust it so unfortunately I will take the usb enclosure with the nvme, format it with etx4 and use Kopia to backup the datasets there once a day. It is not what I wanted but it is the best I can get for now.
About better solutions for the my play-NAS in general, I am constrained with the ports I have. I (again personal choice - I understand people disagree with this) don't want to go SATA. Unfortunately, since I could not find any PCIe switch with ASM2812I (https://www.asmedia.com.tw/product/866yq74SPBqRdtgC/7c5YQ79xz8urEGr1) I am unable to get more from my m2 nvme pcie 3x4 (speed loss for me is not an issue, my main bottleneck is the network). It is interesting how you can find many more interesting attempt at it in the PIs ecosystem but not for mini PCs.
You need to know what to observe. You are not going to get the information you are looking for directly from zfs or even system logs.
What I suggest stands. You have to understand the behavior of the USB controller. That information is acquired from researching USB itself.
Now if you intend to utilize something like a USB enclosure you indeed would be better off with something like ext4. However, keep in mind that this effect is not directly a file system issue. It's an issue with how USB controllers interact with file systems.
That has been my experience from researching this matter. ZFS is simply more sensitive.
In my experience even for motherboards that have port limitations it's possible to take advantage of pci lanes and install a hba with an onboard SATA controller. They also make pci devices that will accept nvme drives.
Good luck with your experimentation and research.