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mjallday
Asked: 2009-07-09 19:04:02 +0800 CST

Understanding Disk Queue Length

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I understand that the disk queue should ideally be under the total number of disks within the system. How does this apply to RAID arrays?

Raid 0 I assume would be ideal queue = # of disks, RAID 10 / 1 = # of disks / 2, what about RAID 5, is it # of disks - 1?

What is happening exactly when the disk queue length exceeds this value, this is the number of writes waiting to be applied to the disks so why is any value above 0 OK?

hard-drive raid

3 Answers

  1. Best Answer

    According to this page I/O wait is more important than the queue length.

    http://blogs.msdn.com/psssql/archive/2007/02/21/sql-server-urban-legends-discussed.aspx

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  2. Everything that I've seen (don't have any links handy) says no more than 2 IOs in queue per disk which is servicing the data.

    When IOs are in queue the command to read or write that IO is waiting for the platter to be moved into the correct position so that the block of data can be read or written.

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  3. Just be aware that in this context the term disks means disk spindles in use, not logical disks. If you have a 3 drive RAID array that's 3 spindles, no matter what the RAID level.

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