Server log flooded with "WARNING: Failed to write to file... waiting..."

Server
1

One of my clients has stopped backing up successfully. When I looked at the server log to investigate, I saw a flood of the same messages.

2026-03-24 14:37:36: WARNING: Failed to write to file… waiting…
2026-03-24 14:37:37: Write failed. errno=28
2026-03-24 14:37:37: WARNING: Failed to write to file… waiting…
2026-03-24 14:37:46: Write failed. errno=28
2026-03-24 14:37:46: WARNING: Failed to write to file… waiting…
2026-03-24 14:37:46: Write failed. errno=28
2026-03-24 14:37:46: WARNING: Failed to write to file… waiting…
2026-03-24 14:37:47: Write failed. errno=28
2026-03-24 14:37:47: WARNING: Failed to write to file… waiting…
2026-03-24 14:37:56: Write failed. errno=28
2026-03-24 14:37:56: WARNING: Failed to write to file… waiting…

My log is in debug mode but I can’t see where it’s actually failing. The error starts right in the middle of server initialization.

2026-03-24 14:33:36: Connecting to target service…
2026-03-24 14:33:36: Connecting to target service…
2026-03-24 14:33:36: Established internet connection. Service=0
2026-03-24 14:33:36: Established internet connection. Service=0
2026-03-24 14:33:36: Authed+capa for client ‘********’ (encrypted-v2, compressed-zstd, token auth) - 1 spare connections
2026-03-24 14:33:36: Getting client settings…
2026-03-24 14:33:36: Encrypting with key /[redacted redacted redacted]/ (server)
2026-03-24 14:33:36: Flushing FileClient…
2026-03-24 14:33:36: Encrypting with key /[redacted redacted redacted]/ (server)
2026-03-24 14:33:36: Write failed. errno=28
2026-03-24 14:33:36: WARNING: Failed to write to file… waiting…
2026-03-24 14:33:36: Authed+capa for client ‘********’ (encrypted-v2, compressed-zstd, token auth) - 1 spare connections
2026-03-24 14:33:36: Channel message: STARTUP timestamp=1774042197
2026-03-24 14:33:36: Connecting to target service…
2026-03-24 14:33:36: Connecting to target service…
2026-03-24 14:33:36: Established internet connection. Service=0
2026-03-24 14:33:36: Established internet connection. Service=0

My storage seems fine, other clients can backup and verify-hashes and remove-unknown all work without error.

Any ideas as to what this error might be?

2

Sry, but here is the ChatGPT answer… which is good enough for such a common problem.

That error is pretty straightforward once you decode the errno:

errno=28 = “No space left on device.”

Your system (or the filesystem the program is writing to) has run out of available space, so every write attempt fails and the app keeps retrying.

What’s happening

  • The program tries to write to a file
  • The OS returns error 28
  • The program logs a warning and retries
  • It keeps failing because the underlying issue (disk full) isn’t resolved

Common causes

  • Disk/partition is completely full
  • Temporary directory (e.g. /tmp) is full
  • A mounted volume (Docker volume, network drive, etc.) is full
  • Disk quotas exceeded (even if overall disk has space)
  • Too many inodes used (rare, but possible with tons of small files)

How to fix it

Try these checks:

1. Check disk space

df -h

2. Check inode usage

df -i

3. Find large files

du -sh /* 2>/dev/null

4. Clean up

  • Delete unnecessary files/logs

  • Clear temp files:

    rm -rf /tmp/*

  • Rotate or truncate logs

  • Clean Docker if relevant:

    docker system prune

5. Check quotas (if on shared system)

quota -v

If you tell me what program produced this log (database, backup tool, etc.), I can point you to the exact place it’s likely filling up.