Avamar: Backup Stalls Around 4.3 MB Due to Jumbo MTU - Align MTU to 1500 to Resolve

Summary: Avamar backup on a Linux client stalls at ~4.3 MB with no errors. The client’s Maximum Transmission Unit (MTU) is set to 5000 (jumbo packets) while other nodes use the standard MTU 1500 causes the issue. Resetting the client MTU to 1500 resolves the issue. ...

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Symptoms

Observed Backup Behavior.

During a backup operation on a Linux client, the following symptoms are observed:

  • Backup starts and runs as expected for a short period.
  • Progress advances to approximately 4.3 MB and then stops.
  • No error messages are displayed while the backup is stalled.
  • If the backup is canceled, the displayed progress increases slightly to about 5 MB, still without errors.

Cause

Network MTU Mismatch Leads to Backup Stagnation.

The backup process was halted because the client system was configured with a Maximum Transmission Unit (MTU) size intended for jumbo packets (MTU=5000). All other systems in the environment used the standard Ethernet MTU of 1500.

This discrepancy caused packet fragmentation or loss, preventing the Avamar client from transmitting data beyond approximately 4 MB of progress.

  • Client MTU set to 5000 bytes (jumbo frames).
  • Peer systems MTU set to the default 1500 bytes.
  • Incompatible MTU sizes caused communication failure during data transfer.
  • No error messages were generated; the backup stopped progressing.

Resolution

Setting the MTU on the client having the issues to match the other systems resolves the issue.
 

Adjust MTU to match network standards.

Step‑by‑step guide

  1. Identify the network interface used by the Avamar client:
# List network interfaces
$ ip link show
  1. Verify the current MTU value for the interface:
$ ip link show dev eth0 | grep mtu
  1. Change the MTU value to the standard size (1500):
# Set MTU temporarily
$ sudo ip link set dev eth0 mtu 1500

# Verify the change
$ ip link show dev eth0 | grep mtu
  1. Make the MTU change persistent across reboots:
    1. On Red Hat Enterprise Linux/CentOS, edit /etc/sysconfig/network-scripts/ifcfg-eth0 and add or modify the line MTU=1500.
    2. On Ubuntu/Debian, edit the NetPlan YAML file (/etc/netplan/01-netcfg.yaml) and add mtu: 1500 under the appropriate interface definition, or edit /etc/network/interfaces and add mtu 1500.
    3. Restart the network service or reboot the host to apply the persistent setting.
# Restart network (RHEL/CentOS)
$ sudo systemctl restart network

# Apply Netplan changes (Ubuntu)
$ sudo netplan apply
 

Verification

  1. Run a new Avamar backup. The job should now progress beyond the previous 4.3 MB stall.
# Check Avamar client status
$ avtar -status
  1. Confirm that the backup completes as expected.
 
CAUTION: Incorrect MTU settings can cause packet loss or connectivity issues. If you are unsure, contact Dell Support before making changes.

CAUTION: Reducing MTU may affect applications that rely on jumbo frames. Test network performance after the change.

NOTE: If your environment requires jumbo frames, ensure all switches, routers, and hosts use the same MTU value.

Affected Products

Avamar

Products

Avamar, Avamar Plug-in for NDMP
Article Properties
Article Number: 000061962
Article Type: Solution
Last Modified: 22 Oct 2025
Version:  4
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