Knowledge Base

VMware vCenter Server Heartbeat uses queues to buffer the flow of replication data from the active server to the passive server. This configuration provides resilience in the event of user activity spikes, channel bandwidth restrictions, or channel drops (which may be encountered when operating in a WAN deployment). Some types of file write activity may also require buffering as they may cause a sharp increase in the amount of channel traffic. The queues used by VMware vCenter Server Heartbeat are referred to as either the active server (unsafe) queue or the passive server (safe) queue.

Active server (unsafe) queue

VMware considers the active server queue as unsafe because the data in this queue has not yet been replicated across the channel to the passive server and may therefore be lost in the event of a failover. As a result of failover, some data loss is inevitable, with the exact amount depending upon the relationship between current channel bandwidth and the required data transmission rate. If the required data transmission rate exceeds current channel bandwidth, the active server queue fills. If the current channel bandwidth exceeds the required data transmission rate, the active server queue empties. This situation is most commonly seen in a WAN environment, where channel bandwidth may be restricted. In a LAN, which normally has high bandwidth on a dedicated channel, the size of the active server queue is zero or near zero most of the time. Note that on a server which is not protected with VMware vCenter Server Heartbeat, all data is technically unsafe, and it is therefore possible to lose all data if the server fails.

Passive server (safe) queue

VMware considers the passive server queue safe because the data in this queue has already been transmitted across the channel from the active server, and is not therefore lost in the event of a failover, since all updates to the passive server are applied as part of the failover process.

The queues (both active and passive server) are stored on-disk, by default in the <VMware vCenter Server Heartbeat Install Directory>\R2\log, with a quota configured for the maximum permitted queue size (by default, 1 GB on each server). Both the queue location and the quota are configurable.

There are two ways to set the queue size:

• With VMware vCenter Server Heartbeat started, open the VMware vCenter Server Heartbeat Console and click Communication > Configuration. Set the value of Max Disk Usage and click Apply. It is necessary to shut down and restart VMware vCenter Server Heartbeat for the change to take effect. This can be done without stopping the protected applications.
• With VMware vCenter Server Heartbeat shut down, open the Configure Server wizard and click the Logs tab. Set the value of Maximum Disk Usage and click Finish.

Note: VMware vCenter Server Heartbeat is a symmetrical system, and can operate with either server in the active role. For this reason, the queue size is always set to the same value for both servers.

MaxDiskUsage Errors

If VMware vCenter Server Heartbeat exceeds its pre-configured queue size, it reports an error message. There are several possible reasons for this, with the most common ones shown below.

When VMware vCenter Server Heartbeat reports [L9]Exceeded the maximum disk usage (NFChannelExceededMaxDiskUsageException), the following conditions exist:

• On the active server, it indicates that the size of the active server queue has exceeded the disk quota allocated for it.
• On the passive server, it indicates that the size of the passive server queue has exceeded the disk quota allocated for it.

1. Ensure maximum disk usage is not exceeded on the active server. For more information, see Troubleshooting the [L9] Exceeded the maximum disk space usage on the Active server error (1008563).
2. Ensure maximum disk usage is not exceeded on the passive server. For more information, see Troubleshooting the [L9] Exceeded the maximum disk space usage on the Passive server error (1008565).
3. Ensure a queue is not out of disk space. For more information, see Troubleshooting the [L20] Out of disk space (NFSChannelOutofDiskSpaceException) error (1008566).