Provided by: nbdkit_1.40.4-5ubuntu1_amd64 
NAME
nbdkit-spinning-filter - add seek delays to simulate a spinning hard disk
SYNOPSIS
nbdkit --filter=spinning PLUGIN [heads=N] [separate-heads=true|false]
[max-seek-time=N] [half-seek-time=N]
[min-seek-time=N]
DESCRIPTION
"nbdkit-spinning-filter" is a filter for nbdkit(1) which adds seek delays to simulate a
spinning hard disk with a mechanical arm and head. You can use this to test the NBD
rotational property.
In the default configuration this filter simulates a disk with one head. Accesses which
are close to the current position of this head have a very small delay. Accesses which
are far away suffer increasingly larger delays as the simulated head seeks to that
position on the disk. Each access moves the simulated position of the head.
For example to turn nbdkit-memory-plugin(1) (a RAM disk) into something which behaves more
like a spinning disk:
nbdkit --filter=spinning memory 1G
Multiple heads
You can control the number of heads and whether they can move independently or not.
Setting "heads=4" and "separate-heads=false" simulates a disk with a single mechanical
arm, two platters and four heads. From the point of view of logical blocks this looks
like the following, where the heads move in tandem over 4 equally spaced sections of the
disk:
arm
│ head
┌────────────┬──────┴─────┬────────────┐ ↙
⇓ ⇓ ⇓ ⇓
┌─────────────────────────────────────────────────┐
└─────────────────────────────────────────────────┘
In this mode, requests are serialized waiting for the arm to move even if the requests are
for different heads (as happens on a real disk).
Using "separate-heads=true" simulates heads which move on independent arms (but each head
is still confined to its quarter of the disk):
range of movement for 4ᵗʰ arm/head
┌──────┴─────┐
⇓ ⇓ ⇓ ⇓
┌─────────────────────────────────────────────────┐
└─────────────────────────────────────────────────┘
In this mode, requests may operate in parallel if they are handled by different heads.
The rotational property
The rotational property is a boolean flag in the NBD protocol which hints to the client
whether the plugin behaves like a spinning disk (if rotational is true) or like a solid
state disk or RAM (if rotational is false).
This filter always advertises the rotational property, because it behaves like a spinning
disk.
Well-behaved NBD clients should observe this property and adjust their behaviour, for
instance by scheduling disk accesses to reduce seeking.
You can place nbdkit-rotational-filter(1) in front of this filter if you want to simulate
what happens with a spinning disk which does not advertise the property correctly:
nbdkit --filter=rotational --filter=spinning PLUGIN rotational=false
Use nbdinfo(1) to examine the rotational property of an NBD server.
Seek times
By default, seeks to adjacent tracks take 10 milliseconds, seeking from the first sector
to the last sector of the disk takes ½ second, and seeking half way across the disk takes
0.2 seconds (so the curve of seek times is mildly quadratic). Note these seek times are
at least an order of magnitude larger than a typical hard disk, in order to exaggerate the
effect for testing purposes.
You can adjust the time taken to seek across the whole disk ("max-seek-time"), across half
of the disk ("half-seek-time"), and from one track to an adjacent track ("min-seek-time")
in order to change this curve. For example, setting "half-seek-time=500ms
max-seek-time=1000ms" will make seek times twice as slow as the default and approximately
linear with distance.
PARAMETERS
heads=N
Set the number of simulated heads to "N" (default is 1).
separate-heads=true
separate-heads=false
Simulate separate, independently moving heads (true), or heads connected together on a
single arm (false). See the description above. The default is "false".
min-seek-time=N
half-seek-time=N
max-seek-time=N
Adjust the time taken to seek to an adjacent track, seek across half of the disk, and
seek across the whole disk. All numbers are in seconds. The defaults are
"min-seek-time=10ms", "half-seek-time=200ms" and "max-seek-time=500ms".
See nbdkit_parse_delay(3) for the precise format of these parameters.
FILES
$filterdir/nbdkit-spinning-filter.so
The filter.
Use "nbdkit --dump-config" to find the location of $filterdir.
VERSION
"nbdkit-spinning-filter" first appeared in nbdkit 1.40.
SEE ALSO
nbdkit(1), nbdkit-delay-filter(1), nbdkit-rate-filter(1), nbdkit-rotational-filter(1),
nbdkit-memory-plugin(1), nbdinfo(1), nbdkit-filter(3), nbdkit_parse_delay(3),
https://en.wikipedia.org/wiki/Hard_disk_drive,
https://en.wikipedia.org/wiki/Hard_disk_drive_performance_characteristics.
AUTHORS
Richard W.M. Jones
COPYRIGHT
Copyright Red Hat
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