nbdkit-nocache-filter1

Usage

Overview

"nbdkit-nocache-filter" is a filter that intentionally disables efficient handling of advisory client cache requests across the NBD protocol. It is mainly useful for evaluating timing differences to determine the impact of caching requests.

Note that the effects of this filter (in crippling handling of client cache requests) is somewhat orthogonal from that of the nbdkit-cache-filter(1) (adding local caching of client read/write requests); the two filters can be run together to experiment with timings.

Parameters

cachemode=none

cachemode=emulate

cachemode=nop

Optional, controls which mode the filter will use. Mode none (default) means that cache support is not advertised to the client. Mode emulate means that cache support is emulated by the filter using the plugin's "pread" callback, regardless of whether the plugin itself implemented the "cache" callback. Mode nop means that cache requests are always accepted and immediately ignored, rather than having any actual impact.

Examples

Serve the file disk.img, but prevent "NBD_CMD_CACHE" requests altogether, to get a baseline timing of behavior when the client is unable to make cache requests:

 nbdkit --filter=nocache file disk.img

Serve the file disk.img, but with cache requests silently ignored, rather than being forwarded on to the file plugin (which attempts to use posix_fadvise(3)), to compare against the timings without the filter and determine whether the file plugin caching was worthwhile:

 nbdkit --filter=nocache file disk.img cachemode=nop

Files

$filterdir/nbdkit-nocache-filter.so

The filter.

Use "nbdkit --dump-config" to find the location of $filterdir.

Version

"nbdkit-nocache-filter" first appeared in nbdkit 1.14.