Tuning the DRBD Knockout Count Setting for DRBD Proxy

This article will explain how you should tune the DRBD® knockout count setting, ko-count, for use with DRBD Proxy.

One way that you can tune DRBD Proxy performance is to tune the ko-count or timeout settings so that the value of (timeout / 10) * ko-count is greater than the amount of time that it takes to flush a full DRBD Proxy buffer. If you divide the value of timeout by 10 (because it is measured in tenths of a second), then ko-count is set to 7 by default, because timeout is set to 60 by default.

The following inequality comparison should be true:

(timeout/10)s * (ko-count) > (memlimit MB) / (Bandwidth MB/s)

For example, if you had a 10MBps link, and a Proxy buffer 500MB in size, you would start with the following inequality comparison:

(60/10)s * 7 > 500 MB / 10 MB/s

This reduces to:

42s > 50s

…which is not true.

To make the inequality true, you could tune ko-count up to 9, or timeout up to 80. It is usually preferred to tune ko-count over timeout because the ko-count option was designed for this use case. Adjusting timeout could affect other DRBD behaviors. Adjusting ko-count up to 9 results in the following:

(60/10)s * 9 > 500MB / 10MB/s

This reduces to:

54s > 50s

…which is true.

Tuning your ko-count appropriately will allow DRBD to more gracefully disconnect from a peer that is having difficulty flushing replicated writes to disk, which could otherwise cripple performance on the Primary node.

Reviewed 2020/12/01 – DGT