About VMware Asynchronous read operations during backup

Description
 

Overview

For vSphere 6.7 and higher versions, asynchronous I/O for NBD/NBDSSL transport mode is available. Performance of NBD/NBDSSL mode data transfer can be improved with this option.
 

Backup Exec 21.4 onwards, the asynchronous read APIs can be used for Backup. Below prerequisites should be met.

• When the source ESXi host is 6.7 or later version.
• NBD/NBDSSL transport mode is being used for the backup.
• The option to enable asynchronous read is enabled in the backup job.

When the backup job has completed, the job log will record information whether the backup was done using the asynchronous read mode.
 

New job options for asynchronous read operations:
 

New job options are provided in ‘Virtual Machines’ tab in the backup job settings.

By default, asynchronous read option is enabled for new backup jobs.

Default values:

Number of read requests: 8   Buffer size: 1MB data size.

In an upgrade scenario the existing jobs settings are not changed automatically. Enable the asynchronous read option and modify ‘Number of asynchronous read requests’ and ‘buffer size’ values after update is installed.
 

Registry Overrides

Registry values that can override the settings made in the Backup Exec User Interface

Registry values:

·       DWORD – “HKLM\SOFTWARE\Veritas\Backup Exec For Windows\Backup Exec\Engine\VMware Agent\Disable Async Read Operation”

·       DWORD – “HKLM\SOFTWARE\Veritas\Backup Exec For Windows\Backup Exec\Engine\VMware Agent\Number of Async Read Calls”

·       DWORD – “HKLM\SOFTWARE\Veritas\Backup Exec For Windows\Backup Exec\Engine\VMware Agent\Size of Async Read Buffers” to be specified in KB

Note:

1.     The number of asynchronous read calls can be entered between 1 to 256 (any even/odd number).
2.     The size of read buffer can be entered between 1MB to 16MB

Hardware Requirements

It is important that the ESX server meets the recommended hardware and memory configuration to get the optimum performance while running the VMware agent-based backups. Refer to VMware documentation for more information.

Typically, the memory requirements per backup job = Number of asynchronous read requests * buffer size.

Based on the Default settings the memory requirement will be 8 * 1MB = 8MB per backup job.

Run few backups with different values of the Number of asynchronous read requests and buffer size to conclude what is the optimum settings that work best in every environment.

Note:

1.     The backup performance is dependent on multiple factors like network, disk IO rate, etc. and will vary from one setup to another. 

2.     The Backup Exec server and the source ESXi host must be capable to run multiple concurrent operations and the performance will depend on the network between Backup Exec server and ESXi host. While setting the number of asynchronous read requests and the buffer size such aspects should be considered. These parameters should not be set to very high values as it may have an impact on other virtual machines hosted on the ESXi host or other backup jobs running.