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nbd_cache - Man Page

send cache (prefetch) command to the NBD server


 #include <libnbd.h>

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


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; 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 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.


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

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                │


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:


See Also

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


Eric Blake

Richard W.M. Jones


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

Referenced By

libnbd(3), NBD(3), nbd_aio_cache(3), nbd_can_cache(3), nbd_get_extended_headers_negotiated(3), nbd_set_request_extended_headers(3).

2024-06-07 libnbd-1.20.1