NAME

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

SYNOPSIS

        #include <libnbd.h>

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

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
       nbd_pread(3) call.  The server can also silently ignore this command.  Note this will
       generally return an error if nbd_can_cache(3) is false.

       Note that not all servers can support a "count" of 4GiB or larger.  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
       nbd_get_block_size(3).

       The "flags" parameter must be 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
       nbd_set_strict_mode(3) function can be used to alter which scenarios should await a server
       reply rather than failing fast.

RETURN VALUE

       If the call is successful the function returns 0.

ERRORS

       On error "-1" is returned.

       Refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error.

       The following parameters must not be NULL: "h".  For more information see "Non-NULL
       parameters" in libnbd(3).

HANDLE STATE

       The handle must be connected with the server, otherwise this call will return an error.

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

SEE ALSO

       nbd_aio_cache(3), nbd_can_cache(3), nbd_create(3), nbd_get_block_size(3), nbd_pread(3),
       nbd_set_strict_mode(3), libnbd(3).

AUTHORS

       Eric Blake

       Richard W.M. Jones

COPYRIGHT

       Copyright (C) 2019-2021 Red Hat Inc.

LICENSE

       This library is free software; you can redistribute it and/or modify it under the terms of
       the GNU Lesser General Public License as published by the Free Software Foundation; either
       version 2 of the License, or (at your option) any later version.

       This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
       without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
       See the GNU Lesser General Public License for more details.

       You should have received a copy of the GNU Lesser General Public License along with this
       library; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth
       Floor, Boston, MA 02110-1301 USA