High disk latency on VMs and hard resets with "Failed to issue sync i/o : Busy" PVSCSI error

Products

• VMware vSphere ESXi

Issue/Introduction

The virtual machine experiences intermittent "hard resets" and performance degradation. Analysis of vmware.log and hostd logs reveals the following indicators:

• Error Message: vmkernel.log: PVSCSI: 2682: Failed to issue sync i/o : Busy

• Error Message: Hostd.log:No VMkernel vSCSI handler for disk scsi0:X (ctrl:1000, path:...........

• Guest Behavior: VM vmware.log: The Guest OS SCSI timeout is reached, leading to a "hard reset" request to the virtual hardware.

Environment

ESXi 8.x

Cause

The virtual machine's performance is bottlenecked because all high-intensity data disks are routed through a single PVSCSI Controller (SCSI 0). This configuration forces the database and Operating System's concurrent I/O requests to share a limited virtual execution path and queue depth, quickly exhausting the controller's capacity. The resulting synchronous I/O failures are misinterpreted by the Guest OS as a hardware hang, which initiates a system reset for recovery.

Resolution

Optimizing I/O Performance by Distributing Load with Multiple PVSCSI Controllers

The recommended strategy to eliminate I/O bottlenecks and significantly improve throughput is to distribute the I/O load across several virtual SCSI controllers. This method achieves better performance by creating independent queues and enabling parallel interrupt processing.

1. Add Virtual Controllers

Power off the Virtual Machine (VM): This is strongly recommended before making configuration changes.
Access the VM Settings and edit them.
Add the necessary number of additional VMware Paravirtual SCSI (PVSCSI) controllers.

2. Redistribute Virtual Disks

Re-assign the existing virtual disks to the new SCSI controllers to ensure the I/O load is evenly balanced.

3. Operating System Verification

Linux: Modern kernels will automatically detect the new device paths. To prevent boot issues or mount failures due to changing device names (e.g., /dev/sdb to /dev/sdd), confirm that the /etc/fstab configuration utilizes persistent identifiers like UUID or LABEL.
Windows: The operating system will automatically detect the new controllers and install the required PVSCSI drivers. A system reboot may be necessary to complete the drive mapping and finalize the configuration.