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diff --git a/cddl/contrib/dtracetoolkit/Notes/iotop_notes.txt b/cddl/contrib/dtracetoolkit/Notes/iotop_notes.txt new file mode 100644 index 0000000..9663ec7 --- /dev/null +++ b/cddl/contrib/dtracetoolkit/Notes/iotop_notes.txt @@ -0,0 +1,48 @@ +************************************************************************** +* The following are additional notes on the iotop program. +* +* $Id: iotop_notes.txt 44 2007-09-17 07:47:20Z brendan $ +* +* COPYRIGHT: Copyright (c) 2007 Brendan Gregg. +************************************************************************** + + +* When using -P, how can a process exceed 100% I/O? + +These percentages are based on disk time. They are in terms of a single disk. + +200% could mean 2 disks @ 100%, or 4 @ 50%, or some such combination. + +I could have capped it at 100% by dividing by disk count. I didn't. Disk +utilisation is an asymmetric resource (unlike CPUs, which are (mostly) +symmetric), so it's unfair to divide by all the disks capacity as an +application cannot use every disks capacity (eg, writing to a /opt disk only). + +Would it be wise to report utilisation as 10% of overall capacity, if it +could mean that 1 disk was SATURATED out of ten? A value of 10% could +understate the problem. + +Instead I add the utilisations and don't divide. 1 disk saturated out of 10 +would be reported as 100% utilisation. This has the danger of overstating +the problem (consider all ten disks at 10% utilisation, this would also be +reported as 100%). + +Nothing is perfect when you are summarising to a single value! + + + +* Beware of overcounting metadevices, such as SVM and Veritas. + +The current version of iotop reports on anything the kernel believes to be +a block disk device. A problem happens when a metadevice contains physical +disk devices, and iotop reports on activity to both the metadevice and +the physical devices, which overcounts activity. + +Consider a metadevice that contains two physical disks which are both +running at 100% utilised. iotop -P may report 300% utilisation, which is +200% for the disks + 100% for the metadevice. We'd probably want to see +a value of 200%, not 300%. Eliminating the counting of metadevices in DTrace +isn't easy (without inelegant "hardwiring" of device types), however I do +intend to find a way to fix this in future versions. + + |