nbdkit-fua-filter - Man Page

modify nbdkit flush and Forced Unit Access (FUA)

Synopsis

 nbdkit --filter=fua plugin [fuamode=MODE] [plugin-args...]

Description

nbdkit-fua-filter is a filter that intentionally modifies handling of the “Forced Unit Access” (FUA) flag across the NBD protocol.

This filter can be used to disable FUA and flush requests for speed (although this is unsafe).  Also it can be used to test client or server fallbacks, and for evaluating timing differences between proper use of FUA compared to a full flush.

Parameters

The fuamode parameter is optional and controls which mode the filter will use.

fuamode=discard

(nbdkit ≥ 1.22)

The filter will discard FUA and flush requests.

This mode is unsafe: If the NBD disk contains a filesystem then you will likely lose data in the event of a crash.  It should only be used for ephemeral data which you can easily recreate, such as caches, builds, test data, etc.

fuamode=pass

(nbdkit ≥ 1.22)

Pass through FUA and flush requests unchanged.  Turns the filter into a no-op.

fuamode=none

FUA support is not advertised to the client.  Clients will not be able to issue FUA write requests, but can send flush commands if the plugin supports it.

This is the default if the fuamode parameter is not specified.

fuamode=emulate

The filter will emulate FUA support using the plugin’s .flush callback, regardless of whether the plugin itself supports more efficient FUA.  It refuses to load if the plugin does not support flush.

fuamode=native

The filter will advertise native FUA support to the client and earlier filters in the chain.  This is useful for comparing optimizations of FUA handling when splitting large requests into sub-requests.  It refuses to load if the plugin’s .can_fua callback returns NBDKIT_FUA_NONE.

fuamode=force

The filter will request FUA on all write transactions, even when the client did not request it (“write-through” mode).  In turn client flush requests become no-ops.  It refuses to load if the plugin’s .can_fua callback returns NBDKIT_FUA_NONE.

Examples

Files

$filterdir/nbdkit-fua-filter.so

The filter.

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

Version

nbdkit-fua-filter first appeared in nbdkit 1.2.

See Also

nbdkit(1), nbdkit-file-plugin(1), nbdkit-filter(3), nbdkit-blocksize-filter(1), nbdkit-log-filter(1), nbdkit-nocache-filter(1), nbdkit-noextents-filter(1), nbdkit-noparallel-filter(1), nbdkit-nozero-filter(1).

Authors

Eric Blake

License

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

THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Referenced By

nbdkit(1), nbdkit-file-plugin(1), nbdkit-filter(3), nbdkit-nocache-filter(1), nbdkit-noextents-filter(1), nbdkit-noparallel-filter(1), nbdkit-nozero-filter(1), nbdkit-release-notes-1.4(1).

2020-06-29 nbdkit-1.21.13