c29d517558
unmodified. For others we'll need to add the missing probes and adjust. This is not attached to the build.
49 lines
2.0 KiB
Plaintext
49 lines
2.0 KiB
Plaintext
**************************************************************************
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* The following are additional notes on the iotop program.
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*
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* $Id: iotop_notes.txt,v 1.1.1.1 2015/09/30 22:01:09 christos Exp $
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*
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* COPYRIGHT: Copyright (c) 2007 Brendan Gregg.
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**************************************************************************
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* When using -P, how can a process exceed 100% I/O?
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These percentages are based on disk time. They are in terms of a single disk.
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200% could mean 2 disks @ 100%, or 4 @ 50%, or some such combination.
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I could have capped it at 100% by dividing by disk count. I didn't. Disk
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utilisation is an asymmetric resource (unlike CPUs, which are (mostly)
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symmetric), so it's unfair to divide by all the disks capacity as an
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application cannot use every disks capacity (eg, writing to a /opt disk only).
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Would it be wise to report utilisation as 10% of overall capacity, if it
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could mean that 1 disk was SATURATED out of ten? A value of 10% could
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understate the problem.
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Instead I add the utilisations and don't divide. 1 disk saturated out of 10
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would be reported as 100% utilisation. This has the danger of overstating
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the problem (consider all ten disks at 10% utilisation, this would also be
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reported as 100%).
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Nothing is perfect when you are summarising to a single value!
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* Beware of overcounting metadevices, such as SVM and Veritas.
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The current version of iotop reports on anything the kernel believes to be
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a block disk device. A problem happens when a metadevice contains physical
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disk devices, and iotop reports on activity to both the metadevice and
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the physical devices, which overcounts activity.
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Consider a metadevice that contains two physical disks which are both
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running at 100% utilised. iotop -P may report 300% utilisation, which is
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200% for the disks + 100% for the metadevice. We'd probably want to see
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a value of 200%, not 300%. Eliminating the counting of metadevices in DTrace
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isn't easy (without inelegant "hardwiring" of device types), however I do
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intend to find a way to fix this in future versions.
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