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);

DESCRIPTION¶

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. 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¶

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_ZERO 1

SEE ALSO¶

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_is_read_only(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