DRBD Sync Rate Throttling

Objective

This document outlines the functionality to conveniently set rate limits for synchronization tasks. A use-case of this is that moving instances might otherwise clog the network for the nodes. If the replication network differs from the network used by the instances, there would be no benefits.

Namely there should be two limits that can be set:

• resync-rate: which should not be exceeded for each device. This exists already.
• total-resync-rate: which should not be exceeded collectively for each node.

Configuration

Suggested command line parameters for controlling throttling are as follows:

gnt-cluster modify -D resync-rate=<bytes-per-second>
gnt-cluster modify -D total-resync-rate=<bytes-per-second>

Where bytes-per-second can be in the format <N>{b,k,M,G} to set the limit to N bytes, kilobytes, megabytes, and gigabytes respectively.

Semantics

The rate limit that is set for the drbdsetup is at least

rate = min(resync-rate,

total-resync-rate/number-of-syncing-devices)

where number-of-syncing-devices is checked on beginning and end of syncs. This is set on each node with

drbdsetup <minor> disk-options –resync-rate <rate> –c-max-rate <rate>

Later versions might free additional bandwidth on the source/target if the target/source is more throttled.

Architecture

The code to adjust the sync rates is collected in a separate tool hrates that

1. is run when a new sync is started or finished.
2. can be run manually if necessary.

Since the rates don’t depend on the job, an unparameterized RPC perspective_node_run_hrates to NodeD will trigger the execution of the tool.

A first version will query ConfD for the other nodes of the group and request the sync state for all of them.