NAME

nbdkit-nozero-filter - nbdkit nozero filter

SYNOPSIS

 nbdkit --filter=nozero plugin [plugin-args...] \
   [zeromode=MODE] [fastzeromode=MODE]

DESCRIPTION

"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

zeromode=none
zeromode=emulate
zeromode=notrim
zeromode=plugin: Optional, controls which mode the filter will use. Mode none (default) means that zero support is not advertised to the client. Mode emulate means that zero support is emulated by the filter using the plugin's "pwrite" callback, regardless of whether the plugin itself implemented the "zero" callback with a more efficient way to write zeros. Since nbdkit ≥ 1.13.4, mode notrim means that zero requests are forwarded on to the plugin, except that the plugin will never see the NBDKIT_MAY_TRIM flag, to determine if the client permitting trimming during zero operations makes a difference. Since nbdkit ≥ 1.15.0, mode plugin leaves normal zero requests up to the plugin, useful when combined with "fastzeromode" for experimenting with the effects of fast zero requests. It is an error to request notrim or plugin if the plugin does not support the "zero" callback.
fastzeromode=none
fastzeromode=slow
fastzeromode=ignore
fastzeromode=default: Optional since nbdkit ≥ 1.15.0, controls whether fast zeroes are advertised to the client, and if so, how the filter will react to a client fast zero request. Mode none avoids advertising fast zero support. Mode slow advertises fast zero support unconditionally, but treats all fast zero requests as an immediate "ENOTSUP" failure rather than performing a fallback. Mode ignore advertises fast zero support, but treats all client fast zero requests as if the flag had not been used (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). Mode default is selected by 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 1.2.

AUTHORS

Eric Blake

COPYRIGHT

LICENSE

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
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Neither the name of Red Hat nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

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