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
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.
-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
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