Disk queue length vs sec/read or write

sakuraime 2,321 Reputation points
2020-12-15T10:05:28.69+00:00

If sec/read or write is up to hundreds ms , while Disk queue length is around 5.
then is it means that the disk has some latency issue ?

SQL Server
SQL Server
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  1. AmeliaGu-MSFT 13,961 Reputation points Microsoft Vendor
    2020-12-16T06:12:50.39+00:00

    Hi @sakuraime ,

    A high Avg. Disk sec/Write and Avg. Disk sec/Read performance counter value may occur due to a burst of disk write and read requests by either an operating system or application. In general, for the performance counter value of Avg. Disk sec/Write and Avg. Disk sec/Read, under 10ms is good. 10-20ms is average. Over 50ms is bad.

    For both Current and Avg. Disk Queue Length, the higher the number the more disk operations are waiting. It requires attention if this value frequently exceeds a value of 2 during peak usage of SQL Server. If you have multiple drives you should take this number and divide by the number of drives in the array to see if the number is above 2. For example, you have 4 drives and a disk queue length of 10, this would be 2.5(10/4 ). 5 or more requests per disk could suggest that the disk subsystem is bottlenecked.
    Please refer to How to Identify IO Bottlenecks in MS SQL Server and Current & Average Disk Queue Length Counter for more details.

    Best Regards,
    Amelia

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  2. AmeliaGu-MSFT 13,961 Reputation points Microsoft Vendor
    2020-12-17T09:48:17.763+00:00

    Hi @sakuraime ,

    sec/write is hundred and avg queue length is just 2-5 < is it normal ?

    No, it seems it is an indication of poor performance.

    sec/write is hundred and avg queue length is around 50 more <<< is believe this is really a issue .

    Yes, you are right.

    You can use Performance Monitor to monitor the SAN which used to store SQL Server database files. Please refer to Investigating I/O bottlenecks which might help.

    Best Regards,
    Amelia

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