nbd_zero - send write zeroes command to the NBD server
#include <libnbd.h> int nbd_zero ( struct nbd_handle *h, uint64_t count, uint64_t offset, uint32_t flags );
Issue a write zeroes command to the NBD server, which if supported by the server causes a zeroes to be written efficiently starting at
offset and ending at
count - 1. The call returns when the command has been acknowledged by the server, or there is an error. Note this will generally return an error if nbd_can_zero(3) is false or nbd_is_read_only(3) is true.
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 zero size, although a future extension may add a constraint visible in nbd_get_block_size(3). Also, some servers may permit a larger zero request only when the
LIBNBD_CMD_FLAG_FAST_ZERO is in use.
flags parameter may be
0 for no flags, or may contain
LIBNBD_CMD_FLAG_FUA meaning that the server should not return until the data has been committed to permanent storage (if that is supported - some servers cannot do this, see nbd_can_fua(3)),
LIBNBD_CMD_FLAG_NO_HOLE meaning that the server should favor writing actual allocated zeroes over punching a hole, and/or
LIBNBD_CMD_FLAG_FAST_ZERO meaning that the server must fail quickly if writing zeroes is no faster than a normal write (if that is supported - some servers cannot do this, see nbd_can_fast_zero(3)).
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.
If the call is successful the function returns
-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).
nbd_zero 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:
#define LIBNBD_HAVE_NBD_ZERO 1
nbd_aio_zero(3), nbd_can_fast_zero(3), nbd_can_fua(3), nbd_can_zero(3), nbd_create(3), nbd_get_block_size(3), nbd_get_extended_headers_negotiated(3), nbd_is_read_only(3), nbd_set_strict_mode(3), libnbd(3).
Richard W.M. Jones
Copyright Red Hat
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.
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