Synology CloudStation Sync Stops Working — 200,000 File Limit
Section: Network & Storage Tags: synology, cloudstation, cloud-drive, sync, file-limit, nas, dsm Source tickets: #27681 Confidence: High — confirmed by Synology support after escalationSymptom
Files on the Synology NAS are not syncing to one or more client machines via CloudStation Drive (now Synology Drive). Users report:
- Files visible on server but not appearing on client
- Folder moved on client does not reflect on server (or vice versa)
- CloudStation Drive shows "syncing" indefinitely or is stuck
- After a folder was accidentally moved and moved back, sync never recovered
Root Cause
Confirmed by Synology Support: CloudStation Drive (and its successor Synology Drive) supports a maximum of 200,000 files per sync task. If the file count in the synced folder exceeds this limit, the sync database becomes inconsistent. The client application may appear to work (it will attempt to sync) but silently fails to propagate changes.Accidentally moving a large folder tree and moving it back can corrupt the local CloudStation database independently, causing files to be placed in wrong locations after a reinstall.
In the confirmed case: the server-side file count was 2,967,651 — nearly 15× the supported limit.
Prerequisites
- Admin access to Synology DSM
- Ability to check the file count on the sync share
- Client machines with CloudStation Drive or Synology Drive installed
Diagnosis Steps
Check current file count on the NAS
- Log in to DSM
- Open File Station
- Right-click the folder being synced → Properties
- Note the file count shown
Remediation Steps
Step 1 — Stop all sync tasks immediately
- On each client machine: open CloudStation Drive / Synology Drive → pause or stop all sync tasks
- In DSM: CloudStation Server / Synology Drive Server → Sync Tasks — stop all active tasks
- Do not attempt to restart sync until the file count is resolved
Step 2 — Restructure the sync scope to stay under 200,000 files
The sync task must be split so no single task covers more than 200,000 files. Options: Option A — Exclude large subfolders from sync
- In CloudStation Drive / Synology Drive on the client: edit the sync task → Selective Sync → untick the largest subdirectories (archive folders, old project data, media libraries)
- Keep only active working folders in sync
- In DSM: CloudStation Server → Sync Tasks → Create — create one task per major top-level folder
- Each task must cover fewer than 200,000 files total
- Assign each task to the relevant users only
- Move infrequently accessed data to a separate NAS share that is not included in any sync task
- Access via mapped drive rather than sync
Step 3 — Clear the CloudStation database on each client and re-sync
After restructuring scope, the local database must be rebuilt cleanly:
- Stop CloudStation Drive / Synology Drive on the client
- Delete the local database folder:
~/Library/Application Support/CloudStation/
- Windows: C:\Users\<username>\AppData\Roaming\CloudStation\
- Relaunch CloudStation Drive — it will re-index and re-sync
- Allow the initial sync to complete fully before considering the task resolved (may take hours for large datasets)
Verification
After sync completes:
- Create a test file on the client
- Confirm it appears on the NAS within 30–60 seconds
- Create a test file in DSM File Station
- Confirm it appears on the client within 30–60 seconds
Prevention
- When initially scoping a CloudStation / Synology Drive sync task, check the file count first (File Station → right-click → Properties)
- For file shares above 150,000 files: use mapped drive access (SMB) instead of sync, or use Hyper Backup for server-to-server replication
- Monitor file count quarterly as the business grows
If This Fails
If the file count is within limits and sync still fails, check DSM logs: Log Centre → Logs → CloudStation for specific error codes. Common follow-ups:
- Port 6690 blocked by firewall (CloudStation's sync port)
- Clock skew between client and NAS > 5 minutes (NTP issue)
- DSM CloudStation Server package needs updating
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