Transport Write Failed

This article describes the troubleshooting for the following backup error for a protected Windows machine:

Backup error occurred during backup run: The backup could not complete due to a problem writing data to the remote target. (TRANSPORT_WRITE_FAILED)

BAK3001 : Agent Export error occurred mid transfer (TRANSPORT_WRITE_FAILED - The backup could not complete due to a problem writing data to the remote target.)

Environment

  • Datto SIRIS
  • Datto ALTO

Description

This error can be caused by several issues on the protected system during a backup. The most common causes include when the protected machine does not meet the minimum requirements, as outlined Getting Started with the Datto Windows Agent.

The amount of space required by the agent will depend on the agent version.

  • For versions 3.0 (current production release) and above of the Datto Windows Agent, 1 GB of free space each protected volume is the minimum required.

  • For version 2.1 of the Datto Windows Agent, if 10% of the volume is larger than 10 GB, the file's size will be 10 GB, regardless of the total size of the protected volume. The minimum size of the file will be 1GB.

  • For versions 2.0 and below of the Datto Windows Agent, if the volume is large enough that 10 GB is less than 10% of the total disk space, the agent will use the percentage of the volume's size that is closest to and is at least 10 GB. For example, for a 560 GB volume, the COW file size will be 2% of the volume and be about 11.2 GB in size

In all sizing scenarios, the agent requires an additional 2 GB of free space for operational overhead. There may be exceptions to the above guidelines where more space may be required.

The machine must be able to communicate with the Datto device as described:

Outbound connectivity on the protected machine:

  • Port 443 (for Datto Cloud communication)

  • Port 3260 (for iSCSI)

  • Port 3262 (for MercuryFTP)

Inbound connectivity on the protected machine:

  • TCP port 25568 (for Datto Windows Agent service)

Full networking requirements can be found here, Unified Backup Networking and Bandwidth Requirements.

If the VSS writers are unavailable or in a bad state at the start of the backup, it may fail. On the target machine, open an elevated command prompt with administrator privileges.

Run the following command, and make a note of any writers not in a Stable state.

vssadmin list writers

If there are writers in a failed state, refer to Resolving VSS Writer Errors in Windows 8, Server 2012, and Up.

If there are writers in a waiting for completion state, review the protected machine for any VSS aware applications that may be running at the same time as the backup.

The following article explains how the Datto agent uses shadow storage, how to check current storage, and resize it on the protected machine: How the Datto Windows Agent uses Shadow Storage.

If you are experiencing this error you may need to expand the shadow storage on the volume that the backup is failing on.

Similar to the local storage requirement, older versions of Datto Windows Agent may use a COW file, which needs to be able to be written to. If the protected machine OS supports the newer versions of the Windows Agent, then it should be updated as the first troubleshooting step. To check the supported Windows operating systems, refer to the Getting Started with the Datto Windows Agent.

If the operating system is supported, but only with the legacy windows agent, please reach out to Datto Technical Support to confirm and potentially adjust the COW file size.