[SQL SERVER] I/O requests taking longer than 15 seconds to complete on file

What does the “I/O request” error below represent?

 

2008-04-21 13:26:42.480 spid364      Microsoft SQL Server 2005 - 9.00.3177.00 (Intel X86)

2008-04-22 16:30:02.140 spid6s       SQL Server has encountered 2 occurrence(s) of I/O requests taking longer than 15 seconds to complete on file [F:\sql data files\xxx.MDF] in database [xxx] (5). 

2008-04-22 16:32:08.780 spid6s       SQL Server has encountered 2 occurrence(s) of I/O requests taking longer than 15 seconds to complete on file [H:\sql data files\xxx_data_4.NDF] in database [xxx] (5). 

 

Research and Findings

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These errors may occur as result of “CPU Drift” and can be ignored and disabled, howerver, first verify both SQL DMV sys.dm_io_pending_io_requests and Windows Performance counters don’t indicate any IO delays.

 

On computers with multiple CPUs, the CPUs are designed to “sleep” during periods of low workload.  When CPUs sleep, SQL may not accurately determine CPUs overall workload and incorrectly report this as IO WARNING shown above, however, this does not represent an actual CPU performance problem.

 

To confirm if the CPUs entered low-power state, SQL Server Escalation Services created RDTSC (Read Time Stamp Counter) utility to report total CPU sleep time.  The report confirmed the CPUs were sleeping up to 24 seconds.  This would be enough for SQL Server to incorrectly report this a slow IO.  Both AMD’s and INTEL’s web sites describe the ability for CPUs to sleep.

 

RDTSCTest.exe [-md|-mt]

      -md   Detailed output (default)

      -mt   CPU speeds in table format

 

-- Current CPU Speeds --

Runtime              CPU  ExpectedMHz ActualMHz RDTSCTicks           DriftMS         

-------------------- ---- ----------- --------- -------------------- ----------------

2008-04-22 17:53:36     0        3502      3503 0x0001564772F87FA72    16123.4

2008-04-22 17:53:36     8        3502      3506 0x00015647D8B7AE21D    23922.5

2008-04-22 17:53:36    16        3502      3507 0x00015647B5FEB4A39    21260.9

 

For more information on  RDTSC can be found at

http://blogs.msdn.com/psssql/archive/2007/08/19/sql-server-2005-rdtsc-truths-and-myths-discussed.aspx

 

Starting with SQL 2005 SP2 we’ve included two trace flags to disable the reporting of CPU Drift errors in the SQL Server errorlog. 

 

Disable this error using Trace Flag 8033

The time stamp counter of CPU on scheduler id 1 is not synchronized with other CPUs.

 

Disable this error using Trace Flag 830

SQL Server has encountered 2 occurrence(s) of I/O requests taking longer than 15 seconds to complete

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In the majority of cases this warning indicates that an I/O operation has taken too long. This article looks at the possible reasons for this and describes what can be done to reduce the likelihood of it recurring.

There is another possible reason that is not I/O related and this is briefly referred to at the end of the article.

Let's start by examining the reason behind it. I/O requests from SQL Server are handled asynchronously by the operating system. This means that when a read or write request is made, the thread making it waits for the I/O to complete.
This frees CPU for use by another thread. Optimally an I/O should take a few ms, but can take considerably more than this. This warning is reported when the time between requesting an I/O and its completion is greater than 15 seconds.

The possible reasons for this are: I/O subsystem problems or misconfiguration, excessive I/O being requested by SQL Server, data files not optimally placed on the disk, and fragmentation.

I/O Subsystem

The first thing to do is to examine the I/O subsystem. Hardware errors are a common cause and you should run diagnostics if you are at all suspicious that this might be the case.

Two common reasons for poor I/O throughput on a SAN are out of date firmware, and insufficient queue length on the HBA. Be aware that if you upgrade SAN firmware you often need to upgrade the HBA drivers at the same time, or the server may fail to access the SAN altogether. Contact the SAN vendor for confirmation and details.

Another common cause of poor I/O performance is if a file system filter driver has been installed. A filter driver intercepts requests before they reach the file system, and performs additional processing such as anti-virus checking and encryption. It goes without saying that this can only have a negative impact on I/O performance.

If you have to have anti-virus software installed on your SQL Server, ensure that mdf, ndf and ldf files are added to the exclusion list. Even better is for realtime virus checking to be disabled completely; schedule a regular scan during quiet times instead.

I/O Load

It is often the case that there are I/O issues at night, when batch jobs are running, but it performs well during the day with no warnings in the logs.

You may find that there are several jobs running at the same time, either through poor scheduling or because one or more jobs have overrun. Provided there is sufficient free time you could reschedule one or more jobs and monitor for a couple of days to see if it has made a difference.

This is more complicated in environments where a SAN is used by several servers, as it may be jobs running on non-SQL servers overloading the SAN and causing your problems. In this case you need to discuss the problem with the system administrators responsible for these servers and agree a schedule that is mutually acceptable.

Fragmentation

Fragmentation may be internal (within tables/indexes) or external (file fragmentation on the disk).

Internal fragmentation is only a problem where in-order scans are being performed, and only on large tables. The warning message that is the subject of this article is unlikely to occur in this situation. Best practise is to monitor logical fragmentation and regularly rebuild or reorganize affected indexes.

Disk fragmentation can also be reduced through best practise, such as presizing data files (capacity planning), and not using autoshrink , or manually shrinking database files.

If you suspect disk fragmentation may be causing performance problems, I can recommend Diskeeper . This runs continuously in the background using idle CPU time only, and is able to defragment SQL Server files while they are in use. Your mileage may vary, but I have seen staggering improvements from running this tool.

I/O Configuration

Other best practise includes splitting tempdb into several files, putting data and log files on separate disks, placing clustered and non-clustered indexes in separate filegroups (on different disks), keeping statistics up-to-date, etc. All these will improve performance and some are described in SQL Server Configuration

.

One Last Possibility

In a minority of cases there may be not be an issue with I/O at all, but a problem caused by CPU drift. This is described in this article .

I stress that this is rare, and you should always examine the possible causes of slow I/O described above.

Summary

This article has described some of the most common reasons for "SQL Server has encountered n occurrence(s) of I/O requests taking longer than 15 seconds to complete on file <filename> in database <dbname>" .

In most cases this message is an accurate description of the problem, and reasons for slow I/O should be investigated.

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