Common performance problems in C#

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Common performance problems
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Bear in mind that during disk-bound operations such as backups and restores, it s perfectly normal and reasonable to see sustained disk activity, with cause for concern centered on the speed or duration of the operation. For example, backing up a terabyte database to disk will obviously bottleneck on disk; however, the performance can be maximized by using dedicated backup disks, multiple backup files, and so forth. In closing our brief look at disk I/O, let s examine a couple of DMV queries. DMV QUERIES The two DMV queries in this section identify queries with the largest I/O usage and database files with the highest stall rates. Listing 17.9 lists the top 50 queries ordered by I/O usage.
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Listing 17.9 Top 50 queries by I/O usage
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-- Top 50 Queries by I/O Consumption SELECT TOP 50 (total_logical_reads + total_logical_writes) / execution_count AS [Avg IO] , substring (qt.text,qs.statement_start_offset/2, ( CASE WHEN qs.statement_end_offset = -1 THEN len(convert(nvarchar(max), qt.text)) * 2 ELSE qs.statement_end_offset end - qs.statement_start_offset)/2 ) AS query_text , qt.dbid , qt.objectid FROM sys.dm_exec_query_stats qs CROSS APPLY sys.dm_exec_sql_text (qs.sql_handle) AS qt ORDER BY [Avg IO] DESC
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Listing 17.10 uses the sys.dm_io_virtual_file_stats function to inspect the stall rate of disk I/O per file. A stall occurs when a process waits for I/O to complete. By determining which files are stalling the most, opportunities arise for rebalancing I/O. A good example of this is multiple high-transaction-rate databases using the same physical disk(s). By segregating their database files on separate disks, you should be able to improve the throughput (and reduce the stall rate).
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Listing 17.10 Database file I/O stalls
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-- Identify database files with the highest stall rate SELECT db_name(database_id) , file_id , io_stall_read_ms , num_of_reads , cast(io_stall_read_ms/(1.0+num_of_reads) as numeric(10,1)) as 'avg_read_stall_ms' , io_stall_write_ms , num_of_writes , cast(io_stall_write_ms/(1.0+num_of_writes) as numeric(10,1)) as 'avg_write_stall_ms' , io_stall_read_ms + io_stall_write_ms as io_stalls
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Waits and queues: a performance-tuning methodology
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, num_of_reads + num_of_writes as total_io , cast((io_stall_read_ms+io_stall_write_ms)/(1.0+num_of_reads + num_of_writes) as numeric(10,1)) as 'avg_io_stall_ms' FROM sys.dm_io_virtual_file_stats(null,null) WHERE database_id > 4 ORDER BY database_id, avg_io_stall_ms DESC
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As we covered in chapter 14, the Data File I/O pane in the new Activity Monitor presents some of this information in graphical form. In closing this section on common performance problems, let s turn our attention to blocking.
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17.3.5 Blocking
A block occurs when one query wants to access data that is locked by another. Despite blocks and locks being normal, fundamental components of any relational database management system, they present a significant problem in poorly designed databases and transactions. In chapter 14, we covered the use of SQL Server Profiler in determining the presence of deadlocks and blocks exceeding a particular threshold. In this section, we ll cover blocking from a waits perspective.
LCK_* WAITS
SQL Server Books Online lists all of the wait types that may be encountered, including an impressive collection beginning with LCK_. All of these represent a wait on a partic-
ular lock being released. For example, a transaction with a shared lock on a row will block a separate transaction that requires an exclusive lock on the same row. In this case, the registered wait type will be LCK_M_X. When the volume of locks and subsequent blocks increases, overall transaction throughput decreases, often accompanied by a reduction in Performance Monitor counters such as SQL Server SQL Statistics:Batch Requests/Sec. To the untrained eye, a severe blocking problem is often met with confusion; users complain of poor performance, but a quick check of the classic Performance Monitor counters (CPU, disk, and so forth) reveals little in the way of server load. When the blocking transaction completes, activity returns to normal. A fundamental component of a good database design is short transaction length with the appropriate isolation level (and usually with an optimistic locking mode). Such topics are beyond the scope of this book: however, should you identify blocking as a top wait type, the sys.dm_db_index_operational_stats function can assist you in investigating the problem further.
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