globaliolimits.cfg man page

globaliolimits.cfg — global I/O limiting configuration

Description

The file globaliolimits.cfg contains configuration of the global I/O limiter.

Syntax

Syntax is:

option value

Lines starting with # character are ignored.

Options

Configuration options:

subsystem <subsystem>

The cgroups subsystem by which clients are classified. If left unspecified, all clients are considered unclassified (see below).

limit unclassified <throughput in KiB/s>

This is a special entry for clients that don’t match any group specified in configuration file or for all clients if subsystem is unspecified. If this entry is unspecified and subsystem is unspecified as well, I/O limiting is disabled entirely. If this entry is unspecified but subsystem is specified, unclassified clients are not allowed to perform I/O.

limit <group> <throughput in KiB/s>

Set limit for clients belonging to the cgroups group <group>. In LizardFS, subgroups of <group> constitute independent groups; they are not allowed to use <group>'s bandwidth reservation and they don’t count against <group>'s usage.

Examples

# empty file

I/O limiting is disabled, no limits are enforced.

limit unclassified 1024

All clients are unclassified and share 1MiB/s of bandwidth.

subsystem blkio
limit /a 1024

Clients in the blkio /a group are limited to 1MiB/s, no other clients can perform any I/O.

subsystem blkio
limit unclassified 256
limit /a   1024
limit /b/a 2048

The blkio group /a is allowed to transfer 1MiB/s, while /b/a gets 2MiB/s. Clients from other groups (e.g. /b, /z, /a/a, /b/z) are considered unclassified and share 256KiB/s of bandwidth.

Tuning Notes

Global I/O limiting is managed by the master server. Mount instances reserve bandwidth allocations from master when they want to perform I/O to chunkservers.

To avoid overloading the master under heavy traffic, mounts try to predict their future usage and reserve at once all the bandwidth they will for the next renegotiation period (see mfsmaster.cfg(5)).

Such reservation are wasted if the traffic at given mount instance suddenly drops.

The ratio of bandwidth being wasted due to this phenomenon shouldn’t exceed fsp/b, where:

f is the frequency of sudden traffic drops in the whole installation (in 1/s)
s is the average size of such drop (in KiB/s)
p is the renegotiation period (in s)
b is the bandwidth limit (in KiB/s)

This applies to each group separately, because groups reserve their bandwidth independently from each other.

See Also

mfsmaster.cfg(5)

Referenced By

iolimits.cfg(5).

08/27/2017