nbdkit-nozero-filter1

Usage

Overview

"nbdkit-nozero-filter" is a filter that intentionally disables efficient handling of sparse file holes (ranges of all-zero bytes) across the NBD protocol. It is mainly useful for evaluating timing differences between naive vs. sparse-aware connections, and for testing client or server fallbacks.

Parameters

The parameters "zeromode" and "fastzeromode" are optional and control which mode the filter will use.

zeromode=none

Zero support is not advertised to the client; clients must explicitly write any regions of zero like any other normal write.

This is the default if the "zeromode" parameter is not specified.

zeromode=emulate

Zero support is advertised, but emulated by the filter by using the plugin's "pwrite" callback, regardless of whether the plugin itself has a more efficient "zero" callback.

zeromode=notrim

(nbdkit ≥ 1.14)

Zero requests are forwarded on to the plugin, except that the plugin will never see the "NBDKIT_MAY_TRIM" flag. This can help determine if the client permitting trimming during zero operations makes a difference. It is an error to request this mode if the plugin lacks the "zero" callback.

zeromode=plugin

(nbdkit ≥ 1.16)

Zero requests are forwarded on to the plugin, unchanged by the filter; this mode is helpful when experimenting with the "fastzeromode" parameter. It is an error to request this mode if the plugin lacks the "zero" callback.

fastzeromode=none

(nbdkit ≥ 1.16)

Support for fast zeroing is not advertised to the client.

fastzeromode=slow

(nbdkit ≥ 1.16)

Fast zero support is advertised to the client, but all fast zero requests result in an immediate "ENOTSUP" failure rather than performing any fallback attempts.

fastzeromode=ignore

(nbdkit ≥ 1.16)

This mode is unsafe: Fast zero support is advertised to the client, but all fast zero requests behave as if the fast zero flag had not been included. This behavior is typically contrary to the NBD specification, but can be useful for comparison against the actual fast zero implementation to see if fast zeroes make a difference.

fastzeromode=default

(nbdkit ≥ 1.16)

This mode is the default. When paired with "zeromode=emulate", fast zeroes are advertised but fast zero requests always fail (similar to "slow"); when paired with "zeromode=notrim" or "zeromode=plugin", fast zero support is left to the plugin (although in the latter case, the nozero filter could be omitted for the same behavior).

Examples

Serve the file disk.img, but force the client to write zeroes explicitly rather than with "NBD_CMD_WRITE_ZEROES":

 nbdkit --filter=nozero file disk.img

Serve the file disk.img, allowing the client to take advantage of less network traffic via "NBD_CMD_WRITE_ZEROES", but fail any fast zero requests up front and force all other zero requests to write data explicitly rather than punching any holes:

 nbdkit --filter=nozero file zeromode=emulate disk.img

Serve the file disk.img, but do not advertise fast zero support to the client even if the plugin supports it:

 nbdkit --filter=nozero file zeromode=plugin fastzeromode=none disk.img

Files

$filterdir/nbdkit-nozero-filter.so

The filter.

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

Version

"nbdkit-nozero-filter" first appeared in nbdkit 1.4.