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.\" ========================================================================
.\"
.IX Title "nbd_get_block_size 3"
.TH nbd_get_block_size 3 "2023-01-04" "libnbd-1.14.2" "LIBNBD"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
nbd_get_block_size \- return a specific server block size constraint
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <libnbd.h>
\&
\& int64_t nbd_get_block_size (struct nbd_handle *h,
\&                             int size_type);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Returns a specific size constraint advertised by the server, if any.  If
the return is zero, the server did not advertise a constraint.  \f(CW\*(C`size_type\*(C'\fR
must be one of the following constraints:
.ie n .IP """LIBNBD_SIZE_MINIMUM"" = 0" 4
.el .IP "\f(CWLIBNBD_SIZE_MINIMUM\fR = 0" 4
.IX Item "LIBNBD_SIZE_MINIMUM = 0"
If non-zero, this will be a power of 2 between 1 and 64k; any client
request that is not aligned in length or offset to this size is likely
to fail with \f(CW\*(C`EINVAL\*(C'\fR.  The image size will generally also be a
multiple of this value (if not, the final few bytes are inaccessible
while obeying alignment constraints).  If zero, it is safest to
assume a minimum block size of 512, although many servers support
a minimum block size of 1.  If the server provides a constraint,
then libnbd defaults to honoring that constraint client-side unless
\&\f(CW\*(C`LIBNBD_STRICT_ALIGN\*(C'\fR is cleared in \f(CWnbd_set_strict_mode(3)\fR.
.ie n .IP """LIBNBD_SIZE_PREFERRED"" = 1" 4
.el .IP "\f(CWLIBNBD_SIZE_PREFERRED\fR = 1" 4
.IX Item "LIBNBD_SIZE_PREFERRED = 1"
If non-zero, this is a power of 2 representing the preferred size for
efficient I/O.  Smaller requests may incur overhead such as
read-modify-write cycles that will not be present when using I/O that
is a multiple of this value.  This value may be larger than the size
of the export.  If zero, using 4k as a preferred block size tends to
give decent performance.
.ie n .IP """LIBNBD_SIZE_MAXIMUM"" = 2" 4
.el .IP "\f(CWLIBNBD_SIZE_MAXIMUM\fR = 2" 4
.IX Item "LIBNBD_SIZE_MAXIMUM = 2"
If non-zero, this represents the maximum length that the server is
willing to handle during \fBnbd_pread\fR\|(3) or \fBnbd_pwrite\fR\|(3).  Other
functions like \fBnbd_zero\fR\|(3) may still be able to use larger sizes.
Note that this function returns what the server advertised, but libnbd
itself imposes a maximum of 64M.  If zero, some \s-1NBD\s0 servers will
abruptly disconnect if a transaction involves more than 32M.
.PP
Future \s-1NBD\s0 extensions may result in additional \f(CW\*(C`size_type\*(C'\fR values.
Note that by default, libnbd requests all available block sizes,
but that a server may differ in what sizes it chooses to report
if \fBnbd_set_request_block_size\fR\|(3) alters whether the client
requests sizes.
.PP
This call does not block, because it returns data that is saved in
the handle from the \s-1NBD\s0 protocol handshake.
.SH "RETURN VALUE"
.IX Header "RETURN VALUE"
This call returns a 64 bit signed integer ≥ \f(CW0\fR.
.SH "ERRORS"
.IX Header "ERRORS"
On error \f(CW\*(C`\-1\*(C'\fR is returned.
.PP
Refer to \*(L"\s-1ERROR HANDLING\*(R"\s0 in \fBlibnbd\fR\|(3)
for how to get further details of the error.
.PP
The following parameters must not be \s-1NULL:\s0 \f(CW\*(C`h\*(C'\fR.
For more information see \*(L"Non-NULL parameters\*(R" in \fBlibnbd\fR\|(3).
.SH "HANDLE STATE"
.IX Header "HANDLE STATE"
The handle must be
negotiating, or
connected with the server, or
shut down,
otherwise this call will return an error.
.SH "VERSION"
.IX Header "VERSION"
This function first appeared in libnbd 1.4.
.PP
If you need to test if this function is available at compile time
check if the following macro is defined:
.PP
.Vb 1
\& #define LIBNBD_HAVE_NBD_GET_BLOCK_SIZE 1
.Ve
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbd_create\fR\|(3),
\&\fBnbd_get_protocol\fR\|(3),
\&\fBnbd_get_size\fR\|(3),
\&\fBnbd_opt_info\fR\|(3),
\&\fBnbd_pread\fR\|(3),
\&\fBnbd_pwrite\fR\|(3),
\&\fBnbd_set_request_block_size\fR\|(3),
\&\fBnbd_zero\fR\|(3),
\&\fBlibnbd\fR\|(3).
.SH "AUTHORS"
.IX Header "AUTHORS"
Eric Blake
.PP
Richard W.M. Jones
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright (C) 2019\-2021 Red Hat Inc.
.SH "LICENSE"
.IX Header "LICENSE"
This library is free software; you can redistribute it and/or
modify it under the terms of the \s-1GNU\s0 Lesser General Public
License as published by the Free Software Foundation; either
version 2 of the License, or (at your option) any later version.
.PP
This library is distributed in the hope that it will be useful,
but \s-1WITHOUT ANY WARRANTY\s0; without even the implied warranty of
\&\s-1MERCHANTABILITY\s0 or \s-1FITNESS FOR A PARTICULAR PURPOSE.\s0  See the \s-1GNU\s0
Lesser General Public License for more details.
.PP
You should have received a copy of the \s-1GNU\s0 Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, \s-1MA 02110\-1301 USA\s0