nbd_cache3

Usage

 #include <libnbd.h>
 int nbd_cache (
       struct nbd_handle *h, uint64_t count,
       uint64_t offset, uint32_t flags
     );

Overview

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.

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

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                │
 └─────────────────────────────────────┴─────────────────────────┘

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