nbd_get_block_size(3)

1nbd_get_block_size(3)               LIBNBD               nbd_get_block_size(3)
2
3
4

NAME

6       nbd_get_block_size - return a specific server block size constraint
7

SYNOPSIS

9        #include <libnbd.h>
10
11        int64_t nbd_get_block_size (
12                  struct nbd_handle *h, int size_type
13                );
14

DESCRIPTION

16       Returns a specific size constraint advertised by the server, if any.
17       If the return is zero, the server did not advertise a constraint.
18       "size_type" must be one of the following constraints:
19
20       "LIBNBD_SIZE_MINIMUM" = 0
21           If non-zero, this will be a power of 2 between 1 and 64k; any
22           client request that is not aligned in length or offset to this size
23           is likely to fail with "EINVAL".  The image size will generally
24           also be a multiple of this value (if not, the final few bytes are
25           inaccessible while obeying alignment constraints).  If zero, it is
26           safest to assume a minimum block size of 512, although many servers
27           support a minimum block size of 1.  If the server provides a
28           constraint, then libnbd defaults to honoring that constraint
29           client-side unless "LIBNBD_STRICT_ALIGN" is cleared in
30           nbd_set_strict_mode(3).
31
32       "LIBNBD_SIZE_PREFERRED" = 1
33           If non-zero, this is a power of 2 representing the preferred size
34           for efficient I/O.  Smaller requests may incur overhead such as
35           read-modify-write cycles that will not be present when using I/O
36           that is a multiple of this value.  This value may be larger than
37           the size of the export.  If zero, using 4k as a preferred block
38           size tends to give decent performance.
39
40       "LIBNBD_SIZE_MAXIMUM" = 2
41           If non-zero, this represents the maximum length that the server is
42           willing to handle during nbd_pread(3) or nbd_pwrite(3).  Other
43           functions like nbd_zero(3) may still be able to use larger sizes.
44           Note that this function returns what the server advertised, but
45           libnbd itself imposes a maximum of 64M.  If zero, some NBD servers
46           will abruptly disconnect if a transaction involves more than 32M.
47
48       "LIBNBD_SIZE_PAYLOAD" = 3
49           This value is not advertised by the server, but rather represents
50           the maximum outgoing payload size for a given connection that
51           libnbd will enforce unless "LIBNBD_STRICT_PAYLOAD" is cleared in
52           nbd_set_strict_mode(3).  It is always non-zero: never smaller than
53           1M, never larger than 64M, and matches "LIBNBD_SIZE_MAXIMUM" when
54           possible.
55
56       Future NBD extensions may result in additional "size_type" values.
57       Note that by default, libnbd requests all available block sizes, but
58       that a server may differ in what sizes it chooses to report if
59       nbd_set_request_block_size(3) alters whether the client requests sizes.
60
61       This call does not block, because it returns data that is saved in the
62       handle from the NBD protocol handshake.
63

RETURN VALUE

65       This call returns a 64 bit signed integer ≥ 0.
66

ERRORS

68       On error -1 is returned.
69
70       Refer to "ERROR HANDLING" in libnbd(3) for how to get further details
71       of the error.
72
73       The following parameters must not be NULL: "h".  For more information
74       see "Non-NULL parameters" in libnbd(3).
75

HANDLE STATE

77       The handle must be negotiating, or connected with the server, or shut
78       down, otherwise this call will return an error.
79

VERSION

81       This function first appeared in libnbd 1.4.
82
83       If you need to test if this function is available at compile time check
84       if the following macro is defined:
85
86        #define LIBNBD_HAVE_NBD_GET_BLOCK_SIZE 1
87

AUTHORS

94       Eric Blake
95
96       Richard W.M. Jones
97

COPYRIGHT

99       Copyright Red Hat
100

LICENSE

102       This library is free software; you can redistribute it and/or modify it
103       under the terms of the GNU Lesser General Public License as published
104       by the Free Software Foundation; either version 2 of the License, or
105       (at your option) any later version.
106
107       This library is distributed in the hope that it will be useful, but
108       WITHOUT ANY WARRANTY; without even the implied warranty of
109       MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
110       Lesser General Public License for more details.
111
112       You should have received a copy of the GNU Lesser General Public
113       License along with this library; if not, write to the Free Software
114       Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
115       02110-1301 USA
116
117
118
119libnbd-1.16.5                     2023-09-26             nbd_get_block_size(3)