IO_Completion wait type:5 deep analysis

IO_COMPLETION delay types are crucial in the world of tweaking SQL Server performance. For database administrators and developers aiming for the highest level of system efficiency, understanding their causes, effects, and remedies is essential. This article goes deeply into the complexities of IO_COMPLETION waits, offering thorough explanations and practical advice.


When a SQL Server query is waiting for I/O operations to finish before continuing, IO_COMPLETION wait type delays happen. These delays can have a major negative influence on database performance, resulting in slow user response times. Understanding the background, root causes, benefits, and drawbacks of IO_COMPLETION waits is crucial for anybody managing SQL Server databases.

A Glimpse into History

Since the beginning of SQL Server, IO_COMPLETION wait type delays have been a problem. I/O operations turned into a bottleneck as databases expanded in size and complexity. Over time, SQL Server has added several optimizations, but handling IO_COMPLETION waits successfully requires an awareness of how they have changed over time.

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Advantages & Disadvantages of IO_COMPLETION wait type

The time that the session is spent waiting for I/O (Input/Output) activities to complete is indicated by the value IO_COMPLETION. The IO_COMPLETION wait type has the following advantages and disadvantages:


Efficient Resource Utilization

The CPU is not actively being used when a session is in the IO_COMPLETION wait type state, freeing up resources for other processes to run simultaneously.

Asynchronous I/O

The SQL Server’s ability to execute asynchronous I/O operations allows the Database engine to do other tasks while awaiting the completion of I/O operations.

Avoids Blocking

Sessions in the IO_COMPLETION state do not interfere with other sessions’ ability to perform their duties. This non-blocking behavior allows subsequent requests and transactions to proceed without being halted.

Improved Performance

Asynchronous I/O operations can boost SQL Server performance because they reduce the amount of time sessions must wait for I/O to complete. This can result in a quicker pace of query and transaction execution.

IO_COMPLETION wait types in sql server


Resource Contention

Instances of SQL Server may perform worse overall if there are too many connections awaiting the completion of I/O operations. Other sessions’ execution can take longer than expected.

Complex Troubleshooting

It could be challenging to pinpoint the main reason for IO_COMPLETION wait type delays. Examining the I/O subsystem, disc latency, and storage settings may be essential to locate and address issues causing I/O delays.

Potential Bottlenecks

An under-optimized or bottlenecked I/O subsystem, such as storage arrays or disc drives, can significantly slow down database operations. This might cause significant IO_COMPLETION delays.

Higher Latency

A protracted IO_COMPLETION Queries and transactions may perform more slowly as a result of waits, which may slow down database-dependent applications. Users can encounter delays when obtaining data or carrying out transactions.

Timeout Risk

Applications may time out if I/O operations take too long to complete, which can result in errors and interruptions to services that rely on the SQL Server database.

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Why Do IO_COMPLETION Wait Types Matter

The IO_COMPLETION In SQL Server, wait types are important for a variety of reasons, most of which are connected to system responsiveness, performance, and user experience.

Efficient Resource Utilization

The IO_COMPLETION wait type delays can be understood and resolved to ensure the effective use of system resources. The ability of other processes to use CPU resources while sessions wait for I/O operations to finish can improve system performance.

Reduced Latency

Reducing the IO_COMPLETION waits through management and monitoring reduces I/O latency. Because database operations are done more quickly with lower latency, applications, and services that rely on the SQL Server database respond to user input more quickly.

Identifying Resource Bottlenecks

The IO_COMPLETION Waits may be a sign of I/O subsystem problems, such as sluggish disc drives or storage arrays. By addressing these problems, I/O bottlenecks are avoided, guaranteeing that database operations may be carried out quickly.

Faster Query Execution

Database queries can run more quickly by reducing the IO_COMPLETION wait type delays. As a result, apps and websites that use SQL Server databases benefit from quicker data retrieval and processing times, which improve user experience.

Avoiding Application Timeouts

The IO_COMPLETION wait type with long lifetime Applications can time out due to waiting, which can result in mistakes and service interruptions. I/O operations may be made more efficient to lower the chance of timeouts and make sure that programs run uninterruptedly.

Effective Troubleshooting

For debugging, it’s essential to keep track of the IO_COMPLETION delays. Database administrators can optimize queries, indexes, and I/O subsystem configurations to address performance concerns and improve overall system efficiency by identifying queries and processes that are generating these pauses.

Optimizing for Scalability

Planning for capacity benefits from being aware of the IO_COMPLETION wait type delays. Based on current use patterns, database managers may predict future I/O requirements and optimize the infrastructure to properly handle growing workloads.

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Why Do IO_COMPLETION Wait Types Occur

IO_COMPLETION When a session is waiting for I/O (Input/Output) activities to finish, wait types might appear in SQL Server. There are various causes for this kind of wait:

Disk Operations

The session transitions to the IO_COMPLETION state while waiting for the disk read operation to complete when a query or transaction asks for data that is not already in memory (RAM) and must be read from the disk.

Data Retrieval

Queries that need a lot of data but don’t have it in the buffer cache have to wait for the I/O subsystem to read it from the disk. The session is now in the IO_COMPLETION state.

Index Operations

Operations like index generation or rebuilding frequently entail substantial I/O operations. While for the I/O operations to finish, sessions executing these actions can enter the IO_COMPLETION state.

Backup and Restore

Data is read from database files and written to backup files during backup processes. Similar to backup operations, restoration operations read and write data from backup files to database files. IO_COMPLETION delays may result from either of these operations.

Data Modification

Large volumes of data may need to be written to the data files during transactions that include adding, updating, or removing a lot of data. Sessions making these changes can encounter IO_COMPLETION wait type delays as they wait for the I/O to finish.

Resource Contention

Sessions may experience IO_COMPLETION waits while they wait their turn to access the I/O subsystem if there is competition for I/O resources, such as when numerous sessions are attempting to access the same disk concurrently.

Slow Disk Drives or Storage Systems

The time it takes to finish I/O operations can be greatly increased by slow or overburdened disk drives or storage systems. On certain slow systems, sessions waiting for I/O may face extended IO_COMPLETION wait type delays.

Concurrency Issues

I/O requests and subsequent IO_COMPLETION delays, while sessions wait for their turn to access the required data, might result from contention issues at the database or table level that allow several sessions to access the same data concurrently.

How to Avoid IO_COMPLETION Wait Types

It’s important to optimize your system’s I/O operations, queries, and configurations to prevent IO_COMPLETION wait types in SQL Server. Here are a few methods to reduce or avoid IO_COMPLETION waits:

Use Solid State Drives (SSDs)

Comparing SSDs to conventional hard disk drives, I/O operations are performed much quicker on SSDs. Utilizing SSDs will help you minimize IO_COMPLETION wait type delays by lowering the latency involved with reading and writing data.

Solid State Drive
Hard DiskTape DriveNetwork Attached Storage (NAS)Redundant Array of Independent Disks (RAID)Solid State Drive (SSD)
Read SpeedFrom 80 MB/s to 160 MB/s.From as low as 30 MB/s to 300 MB/s.From 100 MB/s to 125 MB/s.In case of two drives with 160 MB/s read speeds, RAID 0 could potentially offer around 320 MB/s read speeds.From 200 MB/s to 550 MB/s or even higher.
Write SpeedFrom 80 MB/s to 160 MB/s.From 20 MB/s to 250 MB/s.From 100 MB/s to 125 MB/s.Write speeds in RAID 0 can be similar to the speeds of individual drives, so in the example above, around 160 MB/s.From 200 MB/s to 550 MB/s or even higher.
Type Of Storage devices with Read & Write Speed

Optimize Disk Configuration

To disperse I/O load across numerous disks, use RAID setups. Performance for I/O can be enhanced by properly set RAID levels. To lessen conflict, divide data, logs, and tempdb onto different physical disks.

Proper Indexing

Create and keep up with the right indexes for your queries. The quantity of information that has to be read from the disk can be significantly decreased via indexes. To make sure your indexes are compatible with your query patterns, regularly examine and optimize them.

Partition Large Tables

Consider dividing huge tables if you have to. Partitioning reduces the amount of data SQL Server must scan during I/O operations by breaking huge tables and indexes into smaller, more manageable chunks.

Use Filtered Indexes

You can index a portion of a table’s rows using filtered indexes. You can lower the number of I/O operations necessary to read the index by only indexing the data that is essential.

Update Statistics

Ineffective query strategies might be the result of outdated facts, which add extra I/O operations. Update statistics often to aid the query optimizer in creating effective execution plans.

Update stats of a specific object:

USE AdventureWorks2022;
UPDATE STATISTICS [Sales].[SalesOrderDetail] [AK_SalesOrderDetail_rowguid];   

Update all stats of a table:

USE AdventureWorks2022;
UPDATE STATISTICS [Sales].[SalesOrderDetail];   

Update all stats of all tables in a database:

USE AdventureWorks2022;   
EXEC sp_updatestats; 

Batch and Cache Operations

I/O activities can be reduced by grouping smaller transactions. Additionally, memory caching of frequently requested material might lessen the frequency of disc reads.

Use Proper Hardware

Invest in high-performance storage systems and confirm that the server’s CPU, RAM, and disc controllers can handle the anticipated I/O demand.

Monitor and Analyze

To determine which actions and queries are producing IO_COMPLETION delays, use the SQL Server monitoring tools. You can identify troublesome queries and get insight into I/O bottlenecks with the use of performance monitoring tools.

Consider Asynchronous I/O

Asynchronous I/O operations are supported by SQL Server, allowing the database engine to handle additional tasks while waiting for I/O operations to finish. When appropriate, set up the system to use asynchronous I/O.

Review and Optimize Queries

Excessive I/O operations can be caused by poorly crafted queries. Review and improve SQL queries to make sure they only obtain the information that is required and make effective use of indexes.

Regular Maintenance

To ensure peak performance and reduce pointless I/O operations, undertake routine maintenance procedures like index rebuilding and reorganization.


Maintaining a high-performing SQL Server system requires mastering the knowledge and control of IO_COMPLETION waits. Database managers may lessen the effects of these waits and guarantee a flawless and responsive user experience by using best practices, optimizing queries, and spending money on the right hardware.


Q: What does the wait type IO_COMPLETION mean?
It happens whenever a query waits for I/O operations to finish before continuing.

Q: How do IO_COMPLETION delays affect the speed of SQL Server?
They may cause resource conflicts and impede query response times, which would reduce server efficiency as a whole.

Q: How come IO_COMPLETION delays happen?
Slow disc I/O, insufficient indexing, poorly optimized queries, and excessive cursor usage are among the causes.

Q: How can queries be made more efficient to decrease IO_COMPLETION wait times?
Reduce cursor usage, minimize I/O operations, and ensure adequate indexing to optimize searches.

Q: How can SSDs contribute to shorter IO_COMPLETION wait times?
Because SSDs provide quicker I/O operations, queries don’t have to wait as long for disc reads and writes to finish.

Q: How frequently should statistics be updated to prevent IO_COMPLETION waits?
Update statistics often, especially after significant data changes, to aid the query optimizer in making informed judgments.

Q: Are IO_COMPLETION delays a thing of the past?
Although total eradication may be difficult, effective query optimization and machine upgrades can greatly reduce these pauses.

Q: What effect do too many cursors have on IO_COMPLETION wait times?
High I/O overhead is involved with cursors. Their usage can be minimized to lower IO_COMPLETION wait times.

Q: How can appropriate indexing reduce IO_COMPLETION delay times?
Correctly indexed tables minimize I/O operations and wait times by reducing the number of rows read during queries.

Q: Is addressing IO_COMPLETION delays dependent on monitoring disc performance?
Monitoring disk performance measurements does assist in pinpointing bottlenecks and enables prompt action to improve I/O operations.

Q: What does SQL Server’s IO_COMPLETION wait type mean?
When a job is waiting for I/O (Input/Output) operations to finish, the wait type IO_COMPLETION is used. When data needs to be recovered from or written to disc, this might occur during read or write operations.

Q: What are the common causes of IO_COMPLETION waits?
 IO_COMPLETION Waits frequently result from sluggish disc I/O subsystems, excessive disc latency, or competition for available disc resources. Insufficient memory may result in excessive paging or I/O operations, or poorly optimized queries may do the same.

Q: How can I identify the specific queries causing IO_COMPLETION waits?
To find the queries that are presently awaiting IO_COMPLETION, utilize dynamic management views (DMVs) like sys.dm_exec_requests and sys.dm_os_waiting_tasks. Additionally, programs like SQL Server Profiler and Extended Events can assist in identifying queries that generate a lot of I/O.

Q: How can the IO_COMPLETION wait times be reduced?
To reduce IO_COMPLETION latency, you can wish to optimize your queries to eliminate needless I/O operations. Ensure that indexes are properly constructed and defragmented. Your disc subsystem’s I/O performance might be significantly improved by switching to faster storage devices, such as SSDs.

Q: Can IO_COMPLETION delays impact SQL Server’s overall performance?
The impact of IO_COMPLETION delays on SQL Server performance might be significant since delayed I/O operations may cause longer query response times. It is crucial to deal with the underlying causes of these delays, such as optimizing queries and improving disc speed, for the SQL Server system to stay responsive.

Q: How can I monitor disk I/O performance to prevent IO_COMPLETION waits?
You may monitor disc I/O performance parameters including disc queue length, disc response time, and throughput with monitoring tools like Performance Monitor (PerfMon), SQL Server Profiler, and Extended Events. You can find possible problems before they result in IO_COMPLETION delays by routinely monitoring these metrics.

Q: Can IO_COMPLETION wait to be caused by network-related issues?
No, IO_COMPLETION delays only apply to I/O operations on disc, not to problems with the network. Other wait types, such as ASYNC_NETWORK_IO, which happens while a job is waiting for network activities to finish, might be caused by network-related issues.

Q: Are there specific scenarios where IO_COMPLETION waits are normal?
Any database system that includes reading from or writing to disc will inevitably have IO_COMPLETION delays. However, excessive or lengthy IO_COMPLETION waits are abnormal and point to a performance problem that has to be fixed.

Q: Is it possible for hardware issues, such as a failed disc, to result in IO_COMPLETION waits?
A malfunctioning disc or RAID controller, for example, might result in IO_COMPLETION delays if the I/O operations are affected. Monitoring the system event logs and the SQL Server error logs can assist in locating hardware-related problems that are creating I/O difficulties.

Q: How should I prioritize IO_COMPLETION delay troubleshooting in my SQL Server environment?
Prioritise troubleshooting by first determining which DMV queries are to blame for the delays. Concentrate on improving these queries, and make sure the disc subsystem is operating at peak efficiency. To quickly fix any hardware issues, frequently check the system for performance concerns.


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