1nbd_get_block_size(3)               LIBNBD               nbd_get_block_size(3)
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NAME

6       nbd_get_block_size - return a specific server block size constraint
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SYNOPSIS

9        #include <libnbd.h>
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11        int64_t nbd_get_block_size (struct nbd_handle *h,
12                                    int size_type);
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DESCRIPTION

15       Returns a specific size constraint advertised by the server, if any.
16       If the return is zero, the server did not advertise a constraint.
17       "size_type" must be one of the following constraints:
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19       "LIBNBD_SIZE_MINIMUM" = 0
20           If non-zero, this will be a power of 2 between 1 and 64k; any
21           client request that is not aligned in length or offset to this size
22           is likely to fail with "EINVAL".  The image size will generally
23           also be a multiple of this value (if not, the final few bytes are
24           inaccessible while obeying alignment constraints).  If zero, it is
25           safest to assume a minimum block size of 512, although many servers
26           support a minimum block size of 1.  If the server provides a
27           constraint, then libnbd defaults to honoring that constraint
28           client-side unless "LIBNBD_STRICT_ALIGN" is cleared in
29           nbd_set_strict_mode(3).
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31       "LIBNBD_SIZE_PREFERRED" = 1
32           If non-zero, this is a power of 2 representing the preferred size
33           for efficient I/O.  Smaller requests may incur overhead such as
34           read-modify-write cycles that will not be present when using I/O
35           that is a multiple of this value.  This value may be larger than
36           the size of the export.  If zero, using 4k as a preferred block
37           size tends to give decent performance.
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39       "LIBNBD_SIZE_MAXIMUM" = 2
40           If non-zero, this represents the maximum length that the server is
41           willing to handle during nbd_pread(3) or nbd_pwrite(3).  Other
42           functions like nbd_zero(3) may still be able to use larger sizes.
43           Note that this function returns what the server advertised, but
44           libnbd itself imposes a maximum of 64M.  If zero, some NBD servers
45           will abruptly disconnect if a transaction involves more than 32M.
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47       Future NBD extensions may result in additional "size_type" values.
48       Note that by default, libnbd requests all available block sizes, but
49       that a server may differ in what sizes it chooses to report if
50       nbd_set_request_block_size(3) alters whether the client requests sizes.
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52       This call does not block, because it returns data that is saved in the
53       handle from the NBD protocol handshake.
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RETURN VALUE

56       This call returns a 64 bit signed integer ≥ 0.
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ERRORS

59       On error "-1" is returned.
60
61       Refer to "ERROR HANDLING" in libnbd(3) for how to get further details
62       of the error.
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64       The following parameters must not be NULL: "h".  For more information
65       see "Non-NULL parameters" in libnbd(3).
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HANDLE STATE

68       The handle must be negotiating, or connected with the server, or shut
69       down, otherwise this call will return an error.
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VERSION

72       This function first appeared in libnbd 1.4.
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74       If you need to test if this function is available at compile time check
75       if the following macro is defined:
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77        #define LIBNBD_HAVE_NBD_GET_BLOCK_SIZE 1
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SEE ALSO

80       nbd_create(3), nbd_get_protocol(3), nbd_get_size(3), nbd_opt_info(3),
81       nbd_pread(3), nbd_pwrite(3), nbd_set_request_block_size(3),
82       nbd_zero(3), libnbd(3).
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AUTHORS

85       Eric Blake
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87       Richard W.M. Jones
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90       Copyright (C) 2019-2021 Red Hat Inc.
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LICENSE

93       This library is free software; you can redistribute it and/or modify it
94       under the terms of the GNU Lesser General Public License as published
95       by the Free Software Foundation; either version 2 of the License, or
96       (at your option) any later version.
97
98       This library is distributed in the hope that it will be useful, but
99       WITHOUT ANY WARRANTY; without even the implied warranty of
100       MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
101       Lesser General Public License for more details.
102
103       You should have received a copy of the GNU Lesser General Public
104       License along with this library; if not, write to the Free Software
105       Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
106       02110-1301 USA
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110libnbd-1.14.2                     2023-01-03             nbd_get_block_size(3)
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