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Customer telling us that they see high %iowait in top and iostat output on a busy RHEL8 NFS client.

iostat man page clearly says that iowait is related to % of time kernel is waiting on "disk" I/O

Question - does NFS I/O count as "disk" I/O in %iowait calculations?

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Yes, on Linux, NFS is included in iowait time, and contributes to the load average.

Red Hat KB Does NFS cient I/O count iowait% on linux? shows packet loss to an NFS server increasing iowait. Eventually your NFS server may go away unexpectedly, try looking at iostat in that bad state.

Tasks that sleep on I/O contribute to these iowait metrics in process accounting. NFS does this, it calls io_schedule().

"Disk" is a shorthand for processes waiting on I/O, which typically but not always are block devices.

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