1. Excellent – You Disk Subsystem is responding perfectly for respective counter.
2. Good – You Disk Subsystem is responding fairly for respective counter and you can consider it good considering your workload and other resource utilization.
3. Poor – You Disk Subsystem is not responding well for respective counter and it requires your intervention to drill down more to understand the reason behind slowness.

If you want to read about CPU and Memory related performance counters then i would suggest you to go through with below attached articles.

• Top Perfmon Counters to Identify SQL Server CPU Bottleneck
• Top 10 Perfmon Counters to Identify SQL Server Memory Pressure

Below are the list of important perfmon disk counters that we will be discussing in this article.

• Average Disk Sec/Read
• Average Disk Sec/Write
• Average Disk Sec/Transfer
• Average Disk Queue Length
• Current Disk Queue Length
• Disk Read/Sec and Disk Write/Sec
• %Disk Time
• %Idle Time

Average Disk Sec/Read

Average Disk Sec/Read is important perfmon disk counter that shows the average time in seconds needed to read data from disk and better explain the disk latency. We can find these counters at the logical and physical disk object level. The value for this counter is generally the number of seconds it takes to do each read. The shorter the time needed to read or write data, the faster the system. If you constantly get this counter value more than 20ms then it means your disk is not performing well.

Your disk performance is:

• Excellent, if the value of this counter is less than 6 milliseconds (0.006 sec)
• Good, if the value of this counter is up to 15 milliseconds (0.015 sec)
• Poor, if the value is constantly more than 15 milliseconds (0.015 sec)

Average Disk Sec/Write

Average Disk Sec/Write is also an important counter that shows the average time in seconds needed to write data to disk. that can better explain the disk latency. This counter can be tracked at the logical and physical disk level. The value for this counter is generally the number of seconds it takes to do each read. The shorter the time needed to read or write data, the faster the system. If you constantly get this counter value more than 20ms then it means your disk is not performing well.

Your disk performance is:

• Excellent, if the value of this counter is less than 6 milliseconds
• Good, if the value of this counter is up to 15 milliseconds
• Poor, if the value is constantly more than 15 milliseconds.

Average Disk Sec/Transfer

Average Disk Sec/Transfer is a time that measures average latency for read or write operations. If the Average Disk Sec/Transfer value increases, this indicates that the I/O subsystem is not optimally keeping up with the I/O demand. This counter is available under Logical Disk and Physical Disk object. If you are keeping a watch on this counter make sure to have a look at above perfmon disk counters Average Disk Sec/ Read and Average Disk Sec/Write as well because Average Disk Sec/Transfer is the average of both read and write counters so you need to find out high value is due to read operation or write operation. Below are the values that will tell you whether your disk is performing excellent, good or it is performing slow and requires your intervention to improve its performance.

Your disk performance is:

• Excellent, if the value of this counter is less than 6 milliseconds
• Good, if the value of this counter is up to 15 milliseconds
• Poor, if the value is constantly more than 15 milliseconds.

Average Disk Queue Length

Average Disk Queue Length counter shows you the average number of read and write requests that were queued on the selected physical disk. If this value is exceeding its threshold frequently then we should also look at other perfmon disk counters Average Disk Read Queue Length and Average Disk Write Queue Length. This way you can diagnose whether Average Disk Queue Length is high due to high read or high write operation. The recommended value for this counter is less than 2 per individual disk so if you have multiple disks then your recommended value for Average disk queue length would be twice the no of disks.

Be careful when using thresholds of 2 with SQL Server and SANs.  This value can be much higher than 2 if you have multiple disks.  Let’s say you have 6 drives and Average disk queue length counter is showing around 12, then you should consider your Average disk queue length value 9/6 = 1.5 not 9.

Your disk performance is:

• Excellent, if the value of this counter is less than 1.5 per drive.
• Good, if the value of this counter is up to 2 per drive.
• Poor, if the value is constantly more than 2 per drive,

Current Disk Queue Length

Current Disk Queue Length is the number of disk requests that are currently waiting as well as requests currently being serviced. The difference between average disk queue length and current queue length is CDQL tells current load whereas ADQL tells load during a given time frame. The Current Disk Queue Length metric in Windows Performance Monitor is available for both physical and logical disk. The Current Disk Queue Length value should be less than 2 per disk.

Your disk performance is:

• Excellent, if the value of this counter is less than 1.5 per drive.
• Good, if the value of this counter is up to 2 per drive.
• Poor, if the value is constantly more than 2 per drive.

Disk Read/Sec and Disk Write/Sec

Disk Reads/Sec and Disk Writes/Sec show you the number of read and write operations happening per second on the disk. If you want to set the benchmark for disk capacity then you can determine it by gradually increasing the load on the system.

Your disk performance is:

• Excellent, if the value of this counter is less than 75% of total disk capacity.
• Good, if the value of this counter is up to 85% of total disk capacity.
• Poor, if the value is constantly more than 85% of total disk capacity.

%Disk Time

%Disk Time counter tells how busy the disk is in performing read and write operations. You can see its values normal even your disk has serious disk performance issue. You should compare its value with current/average disk queue length before reaching on any conclusion about disk issue.

Your disk performance is:

• Excellent, if the value of this counter is less than 75% of total disk capacity.
• Good, if the value of this counter is up to 85% of total disk capacity.
• Poor, if the value is constantly more than 85% of total disk capacity.

%Idle Time

%Idle Time measures the percentage of time the disk was idle during the sample interval. The disk is idle when it’s not processing read and write requests. If you are constantly getting its value less than 40% then you can either move some applications from your machine to another machine if you are running other applications as well apart from SQL Server or You can change the current disk system with a faster disk system.

Your disk performance is:

• Excellent, if the value of this counter is greater than 60% of total disk capacity.
• Good, if the value of this counter is greater than 50% of total disk capacity.
• Poor, if the value is constantly less than 40% of total disk capacity.

Related Articles:

• Disk and Performance Impact of running Online Index Rebuild Operation
• How to fix IO error: SQL Server has encountered N occurrence(s) of I/O requests taking longer than 15 seconds to complete.
• Improving Bulk Data Load Performance in SQL Server
• Understanding ASYNC_IO_COMPLETION Wait Type

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SQL Server has encountered 32 occurrence(s) of I/O requests taking longer than 15 seconds to complete on file [E:\MSSQL\Data\database_file.mdf] in database [Database_Name] (%d). The OS file handle is 0x000000F543. The offset of the latest long I/O is: 0x000000004h583.

This message indicates that SQL Server has issued a read or write request from disk, and the I/O requests taking longer than 15 seconds to return. Microsoft SQL Server reports Event Id 833 to the event log, and above text in the ERRORLOG file along with other symptoms like high wait times for PAGEIOLATCH waits or other disk IO related wait types in SQL Server.

Why we get this issue/warning?

SQL Server database is designed to perform intensive disk I/O operations. An I/O operation may be either a read or a write operation and the standard disk speed to perform these operations is around 10 milliseconds or less. If any of the read or write operation takes more time to perform their activity and generally disk speed reaches or exceeds 15 seconds of time, then database engine reports event id 833 along with above log information in error log file.

If you receive such messages, it does not mean problems with SQL Server. They are reported to let DBAs and system administrator know about poor storage performance and help them to investigate why the I/O request took so long. Long I/Os often indicate a SQL Server workload that is too intense for the disk subsystem. Disk Subsystems perform poorly/slowly because of either stuck or stalled IO operations. Before going ahead let’s first understand about stuck and stalled IO operation.

Stuck I/O

Stuck I/O is defined as an I/O request that does not finish.  To resolve a stuck I/O condition, you must usually restart the computer or perform a similar action. A stuck I/O condition typically indicates one of the following:

• Faulty hardware
• A bug in an I/O path component

Stalled I/O

Stalled I/O is defined as an I/O request that does finish or that takes excessive time to finish. Stalled I/O behavior typically occurs because of one of the following reasons:

• The hardware configuration
• The firmware settings

Below is the list of items that can cause stuck or stalled I/O operations on disk subsystem that can adversely affect SQL Server performance.

• Faulty hardware
• Incorrectly configured hardware
• Firmware settings
• Filter drivers
• Compression
• Bugs
• All transactions are in one drive like (backup, live database updates)
• Blocking
• Latch contentions
• Missing Indexes in databases

Resolution

Disk I/O problems are one of the most difficult problems to diagnose and to find the root cause. There might be multiple reasons due to which your disk subsystems perform slow. First step towards fixing such issue is to examining the event logs for any hardware related error messages and then proceed towards fix accordingly. Follow below steps to reduce the IO overheads on your database server if I/O requests taking longer than 15 seconds or perfoming slow.

1. Check & update all missing device drivers and firmware with the latest updates.

2. Analyze memory pressure and CPU usages trends as well because if you will run with CPU or memory pressure, there might be possibility slow disk performance.

3. Monitor disk I/O performance counters either using Performance Monitor or using DMVs to examine your disk subsystem workload. Read attached article to get the optimum values for all given counters. It’s highly recommended to monitor these counters for a while, and then compare it to your disk baselines value that you had captured during stress testing or load testing exercise.

• Average Disk Sec/Transfer
• Average Disk Sec/Read
• Average Disk Sec/Write
• Average Disk Queue Length
• Current Disk Queue Length
• Disk Read/Sec
• Disk Write/Sec
• %Disk Time

4. Microsoft suggests being careful when you use the performance counters because SQL Server takes full advantage of asynchronous I/O capabilities that push the disk queue lengths heavily. Therefore, longer disk queue lengths alone do not indicate a problem. So, compare every counter with their baseline and then analyze the trend.

5. Monitor SQL Server Wait Types, If the disk is slowing all the times then you will get wait types PAGEIOLATCH, WRITELOG or LOGBUFFER with high wait time.

6. Exclude SQL Server Data and Log files from antivirus software scans. If your database files are configured to be scanned by antivirus software then this will have slowed your disk performance.

7. If you are getting this issue on a specific time then I would suggest checking the conflict between your SQL Server Agent Jobs? There might be some data load/Disk IO intensive query or database maintenance activities will be running parallelly at the same time. Change the Agent jobs schedule time carefully so that they should not conflict to each other.

8. Don’t place database files and backups on single drive. Always place data file, log files and tempdb files on separate drives to reduce disk IO overhead. Also, it is highly recommended to run backups on separate drive because if you run backups on data drives then this can reduce disk performance.

9. Check your database files property for which you are getting this error. The size of database files should not be limited to fixed size with auto growth disabled. Enabling the auto growth of all the data files with appropriate value can fix this issue if you have put limits of database files growth. Read the attached article to understand the best value for database file autogrowth.

10. Disk I/O can perform poorly if there is no proper indexing on your tables. You can use Index Turning Wizard to resolve I/O pressure on the system. You can find missing indexes from most used or accessed tables and create appropriate indexes to reduce query time and resources overhead.

Disable this Error to Log using Trace flag 830

Microsoft has given us an option to disable stalled or stuck I/O detection by using trace flag 830, although we do not recommend that you do this.

To disable detection when SQL Server starts, use the -T830 startup parameter to disable detection every time that SQL Server is started. To disable detection for an instance of SQL Server that is currently running, use the following statement:

dbcc traceoff(830, -1)

This setting is effective only for the life of the SQL Server process.

Related Articles:

• Identify Disk Bottleneck in SQL Server using Perfmon Disk Counters
• How to Improve Bulk Data Load Performance?
• What is ASYNC_IO_COMPLETION wait type?
• Understanding BACKUPBUFFER wait type

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