1nbd_get_block_size(3) LIBNBD nbd_get_block_size(3)
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6 nbd_get_block_size - return a specific server block size constraint
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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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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.
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49 This call does not block, because it returns data that is saved in the
50 handle from the NBD protocol handshake.
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53 This call returns a 64 bit signed integer ≥ 0.
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56 On error "-1" is returned.
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58 Refer to "ERROR HANDLING" in libnbd(3) for how to get further details
59 of the error.
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62 The handle must be negotiating, or connected with the server, or shut
63 down, otherwise this call will return an error.
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66 This function first appeared in libnbd 1.4.
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68 If you need to test if this function is available at compile time check
69 if the following macro is defined:
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71 #define LIBNBD_HAVE_NBD_GET_BLOCK_SIZE 1
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74 nbd_create(3), nbd_get_protocol(3), nbd_get_size(3), nbd_opt_info(3),
75 nbd_pread(3), nbd_pwrite(3), nbd_zero(3), libnbd(3).
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78 Eric Blake
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80 Richard W.M. Jones
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83 Copyright (C) 2019-2021 Red Hat Inc.
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86 This library is free software; you can redistribute it and/or modify it
87 under the terms of the GNU Lesser General Public License as published
88 by the Free Software Foundation; either version 2 of the License, or
89 (at your option) any later version.
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91 This library is distributed in the hope that it will be useful, but
92 WITHOUT ANY WARRANTY; without even the implied warranty of
93 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
94 Lesser General Public License for more details.
95
96 You should have received a copy of the GNU Lesser General Public
97 License along with this library; if not, write to the Free Software
98 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
99 02110-1301 USA
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103libnbd-1.7.12 2021-05-29 nbd_get_block_size(3)