NAME

nbdkit-fua-filter - 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. It is mainly useful for testing 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=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, 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 1.2.

AUTHORS

Eric Blake

COPYRIGHT

LICENSE

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