#include <libnbd.h> int64_t nbd_get_block_size ( struct nbd_handle *h, int size_type );
Returns a specific size constraint advertised by the server, if any. If the return is zero, the server did not advertise a constraint.
size_type must be one of the following constraints:
- 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
EINVAL. 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
LIBNBD_STRICT_ALIGNis cleared in
- 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.
- LIBNBD_SIZE_MAXIMUM = 2
If non-zero, this represents the maximum length that the server is willing to handle during nbd_pread(3) or nbd_pwrite(3). Other functions like nbd_zero(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 NBD servers will abruptly disconnect if a transaction involves more than 32M.
- LIBNBD_SIZE_PAYLOAD = 3
This value is not advertised by the server, but rather represents the maximum outgoing payload size for a given connection that libnbd will enforce unless
LIBNBD_STRICT_PAYLOADis cleared in
nbd_set_strict_mode(3). It is always non-zero: never smaller than 1M, never larger than 64M, and matches
Future NBD extensions may result in additional
size_type 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 nbd_set_request_block_size(3) alters whether the client requests sizes.
This call does not block, because it returns data that is saved in the handle from the NBD protocol handshake.
This call returns a 64 bit signed integer ≥
-1 is returned.
Refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error.
The following parameters must not be NULL:
h. For more information see "Non-NULL parameters" in libnbd(3).
nbd_get_block_size can be called when the handle is in the following states:
┌─────────────────────────────────────┬─────────────────────────┐ │ Handle created, before connecting │ ❌ error │ │ Connecting │ ❌ error │ │ Connecting & handshaking (opt_mode) │ ✅ allowed │ │ Connected to the server │ ✅ allowed │ │ Connection shut down │ ✅ allowed │ │ Handle dead │ ❌ error │ └─────────────────────────────────────┴─────────────────────────┘
This function first appeared in libnbd 1.4.
If you need to test if this function is available at compile time check if the following macro is defined:
#define LIBNBD_HAVE_NBD_GET_BLOCK_SIZE 1
nbd_create(3), nbd_get_protocol(3), nbd_get_size(3), nbd_opt_info(3), nbd_pread(3), nbd_pwrite(3), nbd_set_request_block_size(3), nbd_zero(3), libnbd(3).
Richard W.M. Jones
Copyright Red Hat
This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License along with this library; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
libnbd(3), libnbd-release-notes-1.4(1), NBD(3), nbd_block_status(3), nbd_block_status_64(3), nbd_cache(3), nbd_get_extended_headers_negotiated(3), nbd_get_protocol(3), nbd_pread(3), nbd_pread_structured(3), nbd_pwrite(3), nbd_set_request_block_size(3), nbd_set_strict_mode(3), nbd_trim(3), nbd_zero(3).