nbd_get_block_size - return a specific server block size constraint


 #include <libnbd.h>

 int64_t nbd_get_block_size (
           struct nbd_handle *h, int size_type


Returns a specific block size constraint advertised by the server. If zero is returned it means the server did not advertise a constraint.

Constraints are hints. Servers differ in their behaviour as to whether they enforce constraints or not.

The size_type parameter selects which constraint to read. It can be one of:


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 (meaning no information was returned by the server), 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_ALIGN is cleared in nbd_set_strict_mode(3).


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 (meaning no information was returned by the server), using 4k as a preferred block size tends to give decent performance.


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 (meaning no information was returned by the server), some NBD servers will abruptly disconnect if a transaction sends or receives more than 32M of data.


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_PAYLOAD is cleared in nbd_set_strict_mode(3). It is always non-zero: never smaller than 1M, never larger than 64M, and matches LIBNBD_SIZE_MAXIMUM when possible.

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 ≥ 0.


On error -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:



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).


Eric Blake

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