nbdkit-fua-filter1

Usage

Overview

"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.

Note that by default, the NBD protocol does not guarantee that the use of FUA from one connection will be visible from another connection unless the server advertised NBD_FLAG_MULTI_CONN. You may wish to combine this filter with nbdkit-multi-conn-filter(1) if you plan on making multiple connections to the plugin.

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

•

Serve the file disk.img discarding all FUA and flush requests. This can greatly improve performance, but you will likely lose data if there is a crash, so it is not safe.

 nbdkit --filter=fua file disk.img fuamode=discard

•

Serve the file disk.img, but force the client to submit explicit flush requests instead of using "NBD_CMD_FLAG_FUA":

 nbdkit --filter=fua file disk.img

•

Observe that the blocksize filter optimizes its handling of the FUA flag based on whether it knows nbdkit will be emulating FUA with a flush, by comparing the log filter output on top of different fua filter modes:

 nbdkit --filter=blocksize --filter=log --filter=fua file disk.img \
   maxlen=4k logfile=fua_emulated fuamode=emulate
 nbdkit --filter=blocksize --filter=log --filter=fua file disk.img \
   maxlen=4k logfile=fua_native fuamode=native

•

Serve the file disk.img in write-through mode, where all writes from the client are immediately flushed to disk as if the client had always requested FUA:

 nbdkit --filter=fua file disk.img fuamode=force

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 1.4.