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SQL_Disciple asked

Is there a way to quantify read and write traffic separately?

I have been asked to give percentages of read vs write traffic on a SQL Cluster and I cannot readily think of a way to accomplish this without running profiler or another like tool.

administration
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Kev Riley avatar image
Kev Riley answered

Glenn Berry has some good scripts in his Diagnostic Information Queries: https://www.sqlskills.com/blogs/glenn/

You can see reads/writes per Drive:

-- Drive level latency information (Query 29) (Drive Level Latency)
-- Based on code from Jimmy May
SELECT tab.[Drive], tab.volume_mount_point AS [Volume Mount Point], 
	CASE 
		WHEN num_of_reads = 0 THEN 0 
		ELSE (io_stall_read_ms/num_of_reads) 
	END AS [Read Latency],
	CASE 
		WHEN num_of_writes = 0 THEN 0 
		ELSE (io_stall_write_ms/num_of_writes) 
	END AS [Write Latency],
	CASE 
		WHEN (num_of_reads = 0 AND num_of_writes = 0) THEN 0 
		ELSE (io_stall/(num_of_reads + num_of_writes)) 
	END AS [Overall Latency],
	CASE 
		WHEN num_of_reads = 0 THEN 0 
		ELSE (num_of_bytes_read/num_of_reads) 
	END AS [Avg Bytes/Read],
	CASE 
		WHEN num_of_writes = 0 THEN 0 
		ELSE (num_of_bytes_written/num_of_writes) 
	END AS [Avg Bytes/Write],
	CASE 
		WHEN (num_of_reads = 0 AND num_of_writes = 0) THEN 0 
		ELSE ((num_of_bytes_read + num_of_bytes_written)/(num_of_reads + num_of_writes)) 
	END AS [Avg Bytes/Transfer]
FROM (SELECT LEFT(UPPER(mf.physical_name), 2) AS Drive, SUM(num_of_reads) AS num_of_reads,
	         SUM(io_stall_read_ms) AS io_stall_read_ms, SUM(num_of_writes) AS num_of_writes,
	         SUM(io_stall_write_ms) AS io_stall_write_ms, SUM(num_of_bytes_read) AS num_of_bytes_read,
	         SUM(num_of_bytes_written) AS num_of_bytes_written, SUM(io_stall) AS io_stall, vs.volume_mount_point 
      FROM sys.dm_io_virtual_file_stats(NULL, NULL) AS vfs
      INNER JOIN sys.master_files AS mf WITH (NOLOCK)
      ON vfs.database_id = mf.database_id AND vfs.file_id = mf.file_id
	  CROSS APPLY sys.dm_os_volume_stats(mf.database_id, mf.[file_id]) AS vs 
      GROUP BY LEFT(UPPER(mf.physical_name), 2), vs.volume_mount_point) AS tab
ORDER BY [Overall Latency] OPTION (RECOMPILE);

or by database file

-- Calculates average stalls per read, per write, and per total input/output for each database file  (Query 30) (IO Stalls by File)
SELECT DB_NAME(fs.database_id) AS [Database Name], CAST(fs.io_stall_read_ms/(1.0 + fs.num_of_reads) AS NUMERIC(10,1)) AS [avg_read_stall_ms],
CAST(fs.io_stall_write_ms/(1.0 + fs.num_of_writes) AS NUMERIC(10,1)) AS [avg_write_stall_ms],
CAST((fs.io_stall_read_ms + fs.io_stall_write_ms)/(1.0 + fs.num_of_reads + fs.num_of_writes) AS NUMERIC(10,1)) AS [avg_io_stall_ms],
CONVERT(DECIMAL(18,2), mf.size/128.0) AS [File Size (MB)], mf.physical_name, mf.type_desc, fs.io_stall_read_ms, fs.num_of_reads, 
fs.io_stall_write_ms, fs.num_of_writes, fs.io_stall_read_ms + fs.io_stall_write_ms AS [io_stalls], fs.num_of_reads + fs.num_of_writes AS [total_io],
io_stall_queued_read_ms AS [Resource Governor Total Read IO Latency (ms)], io_stall_queued_write_ms AS [Resource Governor Total Write IO Latency (ms)] 
FROM sys.dm_io_virtual_file_stats(null,null) AS fs
INNER JOIN sys.master_files AS mf WITH (NOLOCK)
ON fs.database_id = mf.database_id
AND fs.[file_id] = mf.[file_id]
ORDER BY avg_io_stall_ms DESC OPTION (RECOMPILE);

Obviously the data available in these DMVs is subject to the usual caveats about how long the server has been up etc.

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