# Syncthing Stuck at 100% CPU While Scanning/Preparing to Sync
**URL:** https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676
**Category:** Support
**Created:** [November 14, 2025, 12:38pm UTC](https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676 "2025-11-14T12:38:55Z")
**Posts on this page:** 17
**Page:** 1
### Author:  [@SyncOrSwim](https://forum.syncthing.net/u/SyncOrSwim)
#### Post date: [November 14, 2025, 12:38pm UTC](https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676/1 "2025-11-14T12:38:55Z")
Hi,
I had Syncthing disabled for a day while migrating my TrueNAS OS from a bare-metal host to a Proxmox VM. Once everything was set up, and I re-enabled Syncthing, I noticed the TrueNAS VM’s CPU was pegged at 100% for an unusually extended period.
Checking `top` in the terminal showed Syncthing consuming 100% CPU. In the Syncthing GUI, one of my two shared folders was stuck in “scanning” and “preparing to sync” status. Expanding the folder revealed it was “out of sync” with about 500 MiB of data, processing very slowly.
I left it running overnight (8+ hours), but it was still hovering around 450 MiB out of sync with no meaningful progress and CPU still at 100%.
This is the first time I’ve encountered this behavior. The out-of-sync files are legitimate changes, but the scanning/sync preparation is taking far longer than I’ve ever seen before.
The good news: since this is a Proxmox VM, the 100% CPU usage isn’t overheating physical hardware. Still, I’d like to understand what’s causing the extreme slowness and high CPU load, and how to resolve or work around it.
Any insights or suggestions?
Thanks in advance.
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### Author:  [@calmh](https://forum.syncthing.net/u/calmh)
#### Post date: [November 14, 2025, 12:41pm UTC](https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676/2 "2025-11-14T12:41:15Z")
Take a support bundle and we can figure out what it’s doing.
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### Author:  [@marbens](https://forum.syncthing.net/u/marbens)
#### Post date: [November 14, 2025, 12:54pm UTC](https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676/4 "2025-11-14T12:54:40Z")
[Creating a Support Bundle — Kastelo Docs documentation](https://docs.kastelo.net/syncthing/support-bundle).
Skip the “Enable Debugging” step if using 2.0.0+.
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### Author:  [@tomasz86](https://forum.syncthing.net/u/tomasz86)
#### Post date: [November 14, 2025, 5:19pm UTC](https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676/6 "2025-11-14T17:19:24Z")
> [@SyncOrSwim](#):
>
> I imagine it’s safe to post it here, considering the documentation says it’s redacted.
Just for the record, it is safe in terms of Syncthing (i.e. no-one can access your device using anything from the support bundle), however the logs and the config do include device names, folder names, file paths, etc. so it depends whether you consider those private or not.
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### Author:  [@tomasz86](https://forum.syncthing.net/u/tomasz86)
#### Post date: [November 14, 2025, 5:30pm UTC](https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676/8 "2025-11-14T17:30:44Z")
Not really, unless you send the file via a private message, but then only that person will have access to it. In this specific case, you probably only need to upload the `*.pprof` files from the bundle, and those, as far as I’m aware, don’t contain any private information.
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### Author:  [@marbens](https://forum.syncthing.net/u/marbens)
#### Post date: [November 15, 2025, 1:45am UTC](https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676/11 "2025-11-15T01:45:03Z")
Your build has build user `root@buildkitsandbox` (which if you search on the forum, you get multiple reports of problems), and uses `modernc-sqlite` as its database engine (which is not as efficient as `mattn-sqlite`).
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### Author:  [@SyncOrSwim](https://forum.syncthing.net/u/SyncOrSwim)
#### Post date: [November 15, 2025, 3:17am UTC](https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676/13 "2025-11-15T03:17:12Z")
I switched to an official `dnf` repo install of Syncthing, and it’s running into the same issue.
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### Author:  [@calmh](https://forum.syncthing.net/u/calmh)
#### Post date: [November 15, 2025, 7:23am UTC](https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676/14 "2025-11-15T07:23:52Z")
The profile shows it scanning, where it spends most the of the time grabbing items from the database, and serving a an API request from the GUI which is a bigger database query. I suppose it’s possible the dnf build also has the same issue, whatever build that might be.
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### Author:  [@SyncOrSwim](https://forum.syncthing.net/u/SyncOrSwim)
#### Post date: [November 15, 2025, 1:43pm UTC](https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676/16 "2025-11-15T13:43:04Z")
Are there any solutions for my problem?
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### Author:  [@marbens](https://forum.syncthing.net/u/marbens)
#### Post date: [November 15, 2025, 1:59pm UTC](https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676/17 "2025-11-15T13:59:39Z")
Is it the local Linux machine that’s having the problem or the NAS?
If the latter, then your comment about the `dnf` repo
> [@SyncOrSwim](#):
>
> I switched to an official `dnf` repo install of Syncthing, and it’s running into the same issue.
is somewhat confusing to me, as (at least to me) the implication here is that you did this on the device with the problem. 😅
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### Author:  [@marbens](https://forum.syncthing.net/u/marbens)
#### Post date: [November 15, 2025, 2:50pm UTC](https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676/18 "2025-11-15T14:50:49Z")
Also, based on what filenames I’m seeing in the folder, and the folder label, this looks like a home folder that may contain the Syncthing database. That must be ignored, or it will most likely go into a loop of see database update → scan → update database → see database update → scan → update database.
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### Author:  [@calmh](https://forum.syncthing.net/u/calmh)
#### Post date: [November 15, 2025, 3:32pm UTC](https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676/19 "2025-11-15T15:32:10Z")
If you’re seeing the same on 1.30 as 2.0 it’s certainly not a database thing.
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### Author:  [@marbens](https://forum.syncthing.net/u/marbens)
#### Post date: [November 15, 2025, 4:56pm UTC](https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676/23 "2025-11-15T16:56:55Z")
What item is out of sync?
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### Author:  [@SyncOrSwim](https://forum.syncthing.net/u/SyncOrSwim)
#### Post date: [November 15, 2025, 4:57pm UTC](https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676/24 "2025-11-15T16:57:51Z")
When I click the “1 items”, it shows nothing:
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### Author:  [@tomasz86](https://forum.syncthing.net/u/tomasz86)
#### Post date: [November 15, 2025, 5:24pm UTC](https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676/25 "2025-11-15T17:24:32Z")
> [@SyncOrSwim](#):
>
> And just to be clear, I do have an ignore pattern set on both ends of the shared folder since it’s a sync job for my Linux home folder, and it has always worked just fine.
>
> ```plaintext
> !/Documents
> !/Downloads
> !/Music
> !/Pictures
> !/Videos
> *
>
> ```
If you only sync these five folders and ignore the rest, and that rest comprises a lot of folders and files, then I think it will be more efficient to just add those five folders to Syncthing separately and not deal with ignore patterns at all. This is because ignored files are still included in the database (see [https://forum.syncthing.net/t/solved-large-index-size-with-nothing-synced-but-a-lot-of-ignores/14284](https://forum.syncthing.net/t/solved-large-index-size-with-nothing-synced-but-a-lot-of-ignores/14284)), and the larger the database, the slower accessing it will be, which may have some impact here, especially considering that the hardware is weak.
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### Author:  [@SyncOrSwim](https://forum.syncthing.net/u/SyncOrSwim)
#### Post date: [November 15, 2025, 5:38pm UTC](https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676/26 "2025-11-15T17:38:23Z")
I considered that approach when I first set up Syncthing, but to be frank it feels really messy in terms of organization for my liking.
Also, that still wouldn’t explain the root issue here. Worked fine for years, suddenly switched TrueNAS from a host OS to a VM on Proxmox and suddenly this started occurring. Not saying it has anything to do with that, but it is interesting on timing.
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### Author:  [@system](https://forum.syncthing.net/u/system)
#### Post date: [December 15, 2025, 5:39pm UTC](https://forum.syncthing.net/t/syncthing-stuck-at-100-cpu-while-scanning-preparing-to-sync/25676/27 "2025-12-15T17:39:00Z")
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