1 files changed, 48 insertions, 0 deletions
diff --git a/Notes/iotop_notes.txt b/Notes/iotop_notes.txt
new file mode 100644
index 000000000000..9663ec7b5498
--- /dev/null
+++ b/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.
+
+