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NAME

nbd_zero - send write zeroes command to the NBD server

SYNOPSIS

#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 "offset" + "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.

The "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.

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

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_ZERO 1

AUTHORS

Eric Blake

Richard W.M. Jones

COPYRIGHT

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

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