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doc/rbd/rbd-config-ref: add more detail on QoS settings

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Signed-off-by: Josh Durgin <jdurgin@redhat.com>
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Josh Durgin 2022-01-11 22:17:15 -05:00
parent 52995842a9
commit e1548ef36a
2 changed files with 33 additions and 3 deletions
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32
doc/rbd/rbd-config-ref.rst
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@ -212,11 +212,36 @@ creating images or the default features can be configured via
:KRBD support: no
QOS Settings
QoS Settings
============
librbd supports limiting per-image IO, controlled by the following
settings.
librbd supports limiting per-image IO in several ways. These all apply
to a given image within a given process - the same image used in
multiple places, e.g. two separate VMs, would have independent limits.
* **IOPS:** number of I/Os per second (any type of I/O)
* **read IOPS:** number of read I/Os per second
* **write IOPS:** number of write I/Os per second
* **bps:** bytes per second (any type of I/O)
* **read bps:** bytes per second read
* **write bps:** bytes per second written
Each of these limits operates independently of each other. They are
all off by default. Every type of limit throttles I/O using a token
bucket algorithm, with the ability to configure the limit (average
speed over time) and potential for a higher rate (a burst) for a short
period of time (burst_seconds). When any of these limits is reached,
and there is no burst capacity left, librbd reduces the rate of that
type of I/O to the limit.
For example, if a read bps limit of 100MB was configured, but writes
were not limited, writes could proceed as quickly as possible, while
reads would be throttled to 100MB/s on average. If a read bps burst of
150MB was set, and read burst seconds was set to five seconds, reads
could proceed at 150MB/s for up to five seconds before dropping back
to the 100MB/s limit.
The following options configure these throttles:
.. confval:: rbd_qos_iops_limit
.. confval:: rbd_qos_iops_burst
@ -237,3 +262,4 @@ settings.
.. confval:: rbd_qos_write_bps_burst
.. confval:: rbd_qos_write_bps_burst_seconds
.. confval:: rbd_qos_schedule_tick_min
.. confval:: rbd_qos_exclude_ops

4
src/common/options/rbd.yaml.in
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@ -742,6 +742,10 @@ options:
type: uint
level: advanced
desc: minimum schedule tick (in milliseconds) for QoS
long_desc: This determines the minimum time (in milliseconds) at which I/Os
can become unblocked if the limit of a throttle is hit. In terms of the
token bucket algorithm, this is the minimum interval at which tokens are
added to the bucket.
default: 50
services:
- rbd