Synology agent stops running scheduled backups with error: "too many open files"

Symptoms

• Synology DSM 7 agent stops running scheduled backups. The following error message appear in the Synology system logs:

  CONFIG_TEXT: too many open files

• Scheduled backups are reported as Missed with the message Backup job did not run at scheduled time, although the device is online. Running the backup manually can still work, or the manually started job does not start.
• Opening the Protected Item to review the file selection shows a "too many open files" error while loading the file tree.

• The same behavior can occur on Linux devices. In this case, the journal of the backup-tool service repeatedly shows:

  CONFIG_TEXT: http: Accept error: accept unix @comet-comet.delegate.authentication.v1: accept4: too many open files; retrying in 1s

Cause

Comet has reached the file handle limit of Synology DSM 7. New connections cannot be made to start new scheduled jobs.

The default limit is between 1024 and 8192 on Synology. The backup-tool processes hold an excessive number of open sockets. This is tracked internally as COMET-59478.

Resolution

A bug was present in the Linux and Synology agents causing an excessive number of sockets to be kept open, exhausting the file handle limit of Synology DSM 7. The fix is available in 26.4.5 Phoebe.

Upgrade to 26.4.5 Phoebe or newer to receive the fix.

Until you can upgrade, restart all backup-tool processes via SSH, or reboot the device. Backups run again after the restart, but the issue can reoccur after some time.

Further improvements to lower open file handles were made in 26.10.0.