SQL Server 2008 : Monitoring Your Server - Monitoring Disk IO

Monitoring disk IO can also be a little tricky. Depending on the counters that you use, you have to evaluate the results based on the number of disk spindles available on the monitored physical disk. Table 1 lists some of the counters and thresholds used when monitoring disk IO; it also lists if the number of spindles needs to be considered when using the counter.

Figure 1 shows PerfMon with the counters listed in Table 1.

Figure 2. Disk IO counters listed in Table 1

1. Disk IO with Windows Counters

The following list explains the Physical Disk object Windows counters listed in Table 1. As you use those counters, remember that the results from some counters must be interpreted in light of the number of spindles within the disk array. For example, if disk D consists of 6 spindles and you are monitoring the Avg Disk Queue length, the counter would have to be greater than 12 before you have reason for concern.

• Avg Disk Queue Length: Represents the number of physical reads and writes queued on the selected disk. When your disk IO subsystem is overwhelmed, physical disk reads or writes have to wait before performing their actions. Keep in mind, sustained values greater than two for each spindle requires attention.

• Avg Disk Reads/Sec or Writes/Sec: Captures the average amount of time it takes to read/ write data to disk. One benefit of this counter is that it measures disk latency. The measurement of disk latency does not care about the number of spindles within the array. The measurement of Avg Disk Reads/Sec or Writes/Sec counter represents the amount of time in milliseconds it takes for the actions to happen. When the counter exceeds 20 ms for a given disk, research the cause of the increased read/write from/to the disk.

• Reads/Sec or Writes/Sec: Measures the number of reads or writes per second from/to disk. These numbers need to be less than 85% of the disk capacity. The specifications of your disk drives should identify the number of reads and writes per second the drives support. Make sure the Reads/Sec and Writes/Sec counters do not exceed 85% because the access time increases exponentially beyond 85% capacity.

• % Disk Time: Represents the busyness of your disk drives. Drives that sustain more than 55% over time indicate that your disks are overworked and that you likely have a disk IO problem. Figuring out why your disks are so busy will go a long way toward reducing IO problems.

• Current Disk Queue: Measures the current processes waiting for IO. Using this counter along with % Disk Time may help you understand what is going on with your disks. If your current disk queue is high and % Disk Time is above your baseline, then that probably is an indicator that application processes are delayed and your disks are bottlenecked.

2. Disk IO with SQL Server Counters

SQL Server also provides some counters to monitor factors that negatively affect the disk IO on your server. Table 1 mentioned the two SQL Server counters that we use the most to determine how factors of SQL Server are impacting disk IO. These counters are located in the SQL Server Access Methods object and are further described in the following list:

• Full Scans/Sec: Identifies the number of table and index scans occurring on your system per second. Scans are IO-intensive and should run within your databases minimally. If your scan counters are high, then identify the tables that have a large number of scans against them.
  

• Page Splits/Sec: Represents the number of page splits per second that occur on your system. Page splits occur when there is not enough space on a page to add the data on an existing page. When the page splits, that causes fragmentation and results in IO-intensive processes. Try to keep page splits under 20 per 100 batches/sec. If the number of page splits is greater than that, then review the fill factor you have set up on your indexes and minimize it where appropriate.