1nbd_get_block_size(3) LIBNBD nbd_get_block_size(3)
2
6 nbd_get_block_size - return a specific server block size constraint
7
9 #include <libnbd.h>
10 int64_t nbd_get_block_size (struct nbd_handle *h,
12 int size_type);
13
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:
18 "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).
30 "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.
38 "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.
46 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.
51 This call does not block, because it returns data that is saved in the
53 handle from the NBD protocol handshake.
54
56 This call returns a 64 bit signed integer ≥ 0.
57
59 On error "-1" is returned.
60 Refer to "ERROR HANDLING" in libnbd(3) for how to get further details
62 of the error.
63
65 The handle must be negotiating, or connected with the server, or shut
66 down, otherwise this call will return an error.
67
69 This function first appeared in libnbd 1.4.
70 If you need to test if this function is available at compile time check
72 if the following macro is defined:
73 #define LIBNBD_HAVE_NBD_GET_BLOCK_SIZE 1
75
77 nbd_create(3), nbd_get_protocol(3), nbd_get_size(3), nbd_opt_info(3),
78 nbd_pread(3), nbd_pwrite(3), nbd_set_request_block_size(3),
79 nbd_zero(3), libnbd(3).
80
82 Eric Blake
83 Richard W.M. Jones
85
87 Copyright (C) 2019-2021 Red Hat Inc.
88
90 This library is free software; you can redistribute it and/or modify it
91 under the terms of the GNU Lesser General Public License as published
92 by the Free Software Foundation; either version 2 of the License, or
93 (at your option) any later version.
94 This library is distributed in the hope that it will be useful, but
96 WITHOUT ANY WARRANTY; without even the implied warranty of
97 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
98 Lesser General Public License for more details.
99 You should have received a copy of the GNU Lesser General Public
101 License along with this library; if not, write to the Free Software
102 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
103 02110-1301 USA
104libnbd-1.12.5 2022-07-10 nbd_get_block_size(3)