MSSQL Server Error 833: A Synthesis of Real-World Case Studies
Technical / Infrastructure-Focused
- Analyzing SQL Server Error 833: Infrastructure-Related Case Studies and Root Causes
- SQL Server Error 833 Explained: Common Infrastructure Scenarios and Resolutions
Practical / Troubleshooting-Oriented
- Troubleshooting SQL Server Error 833: Lessons Learned from Production Environments
- SQL Server Error 833 in the Field: Causes, Impacts, and Remediation Strategies
The SQL Server error 833 – “I/O requests taking longer than 15 seconds to complete” is a well-known but often misunderstood symptom of underlying infrastructure issues rather than a defect in SQL Server itself. This error typically indicates excessive I/O latency at the storage, virtualization, or network layer and can have a significant impact on database performance, availability, and stability.
Over the years, multiple real-world incidents involving this error have been collected across different customer environments. These cases span heterogeneous infrastructures and reveal how a wide range of factors—such as storage configuration, virtualization settings, backup operations, network anomalies, and firmware or code defects—can all converge to produce the same SQL Server error condition.
This guide presents a synthesis of the most representative scenarios encountered in production environments. Each case highlights a distinct root cause and the corresponding remediation, with the objective of providing practical insights for system engineers, database administrators, and infrastructure architects. By correlating SQL Server symptoms with underlying platform behaviors, this article aims to support faster root cause analysis and more effective troubleshooting when faced with SQL Server error 833 in complex, virtualized, and storage-intensive environments.
Error that occurs on SQL Server with this format: ‘…I/O requests taking longer than 15 seconds to complete on file…’
Over the years, real cases involving the above error have been collected from different customers
CASE 1: problem involving two virtual nodes in a cluster, pointing to raw device disks. A multipath balancing policy different from that indicated in IBM best practices for SVC was applied, namely ‘Fixed’ instead of ‘Round robin’.
CASE 2: problem involving many hosts, caused by emptying the SVC cache, resolved with a code upgrade of all SVCs pointing to IBM storage.
CASE 3: problem due to co-stop time - http://www.davidklee.net/2016/03/03/vmware-cpu-co-stop-and-sql-server-performance/, i.e. too many vCPUs configured on a single ESX host, working simultaneously. Co-stop time is the time it takes to align all vCPUs for simultaneous execution. If co-stop time increases, it can impact all VMs allocated to a single ESX host. The co-stop time anomaly was caused by a weekly schedule that ran antivirus scans on all VMs simultaneously.
CASE 4: There was a significant mitigation of the problem inherent in an MSSQL on a VM as soon as the VM was moved from a datastore where deduplication was active to one where deduplication was disabled. Further analysis suggests that the problem is specific to a particular type of storage: Dell Compellent SC5020F.
CASE 5: A network problem caused massive broadcast traffic, causing the storage controllers to crash. They first froze and then failed to restart when an attempt was made to reboot them. The storage controllers were manually forced to restart; after the restart, the latencies to the storage were abnormal. Dell support was engaged and, from the logs, identified errors, which were fixed by upgrading the firmware.
CASE 6: Problem related to the failure to consolidate a backup snapshot: this causes the machine to operate under sub-optimal file system conditions on the storage (since it works on the snapshot files instead of the consolidated virtual disk files). Problem solved by manual consolidation with the VM turned off.
CASE 7: Storage vMotion performed from a low-performance datastore (7k aggregate) to a higher-performance datastore (15k aggregate).
