=head1 NAME

nbd_cache - send cache (prefetch) command to the NBD server

=head1 SYNOPSIS

 #include <libnbd.h>

 int nbd_cache (
       struct nbd_handle *h, uint64_t count,
       uint64_t offset, uint32_t flags
     );

=head1 DESCRIPTION

Issue the cache (prefetch) command to the NBD server, which
if supported by the server causes data to be prefetched
into faster storage by the server, speeding up a subsequent
L<nbd_pread(3)> call.  The server can also silently ignore
this command.  Note this will generally return an error if
L<nbd_can_cache(3)> is false.

Note that not all servers can support a C<count> of 4GiB or larger;
L<nbd_get_extended_headers_negotiated(3)> indicates which servers
will parse a request larger than 32 bits.
The NBD protocol does not yet have a way for a client to learn if
the server will enforce an even smaller maximum cache size, although
a future extension may add a constraint visible in
L<nbd_get_block_size(3)>.

The C<flags> parameter must be C<0> for now (it exists for future NBD
protocol extensions).

By default, libnbd will reject attempts to use this function with
parameters that are likely to result in server failure, such as
requesting an unknown command flag.  The L<nbd_set_strict_mode(3)>
function can be used to alter which scenarios should await a server
reply rather than failing fast.

=head1 RETURN VALUE

If the call is successful the function returns C<0>.

=head1 ERRORS

On error C<-1> is returned.

Refer to L<libnbd(3)/ERROR HANDLING>
for how to get further details of the error.

The following parameters must not be NULL: C<h>.
For more information see L<libnbd(3)/Non-NULL parameters>.

=head1 HANDLE STATE

nbd_cache
can be called when the handle is in the following state:

 ┌─────────────────────────────────────┬─────────────────────────┐
 │ Handle created, before connecting   │ ❌ error                │
 │ Connecting                          │ ❌ error                │
 │ Connecting & handshaking (opt_mode) │ ❌ error                │
 │ Connected to the server             │ ✅ allowed              │
 │ Connection shut down                │ ❌ error                │
 │ Handle dead                         │ ❌ error                │
 └─────────────────────────────────────┴─────────────────────────┘

=head1 VERSION

This function first appeared in libnbd 1.0.

If you need to test if this function is available at compile time
check if the following macro is defined:

 #define LIBNBD_HAVE_NBD_CACHE 1

=head1 SEE ALSO

L<nbd_aio_cache(3)>,
L<nbd_can_cache(3)>,
L<nbd_create(3)>,
L<nbd_get_block_size(3)>,
L<nbd_get_extended_headers_negotiated(3)>,
L<nbd_pread(3)>,
L<nbd_set_strict_mode(3)>,
L<libnbd(3)>.

=head1 AUTHORS

Eric Blake

Richard W.M. Jones

=head1 COPYRIGHT

Copyright Red Hat
