nbd_get_block_size(3)

✨ nbd_get_block_size — bestie's guide to server block size constraints ✨

okay so this is a page from LIBNBD (manual section 3), and it's about nbd_get_block_size — the little function that tells you what block size constraints the server is out here advertising. it's giving "reading the room" but for I/O. 🎀

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🌸 NAME 🌸

nbd_get_block_size — return a specific server block size constraint

💖 SYNOPSIS 💖

#include <libnbd.h>

int64_t nbd_get_block_size (
          struct nbd_handle *h, int size_type
        );

🦋 DESCRIPTION 🦋

okay so what this does: it returns a specific block size constraint that the server advertised. and if you get a 0 back? that means the server straight-up did NOT advertise a constraint. no cap. 💀

and here's the thing — constraints are hints. servers literally differ in their behaviour as to whether they enforce constraints or not. so like, don't @ me if a server plays fast and loose with its own rules.

the size_type parameter selects which constraint to read. it can be one of these icons 👑:

• 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. also, the image size will generally also be a multiple of this value (and if it's not, the final few bytes are inaccessible while obeying alignment constraints — awkward 😭). if it's 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. and if the server does provide a constraint, then libnbd defaults to honoring that constraint client-side unless LIBNBD_STRICT_ALIGN is cleared in nbd_set_strict_mode(3). slay, defaults. ✨
• 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. and lowkey this value may be larger than the size of the export. if it's zero (meaning no information was returned by the server), using 4k as a preferred block size tends to give decent performance. cute. 🍓
• 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, so don't assume. and note that this function returns what the server advertised, but libnbd itself imposes a maximum of 64M. important distinction, bestie. 💅 if it's 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. like, just hang up on you. rude but real. 🔥
• 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_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. iconic consistency. 🌈

future NBD extensions may result in additional size_type values, so stay tuned i guess. also note that by default, libnbd requests all available block sizes, but a server may differ in what sizes it chooses to report if nbd_set_request_block_size(3) alters whether the client requests sizes. the drama of negotiation. 🙈

⭐ RETURN VALUE ⭐

this call returns a 64 bit signed integer ≥ 0. that's it, that's the vibe.

💀 ERRORS 💀

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). so like, actually pass a handle, bestie. 🫶

🧚 NOTES 🧚

Non-blocking call

this call does not block, because it returns data that is saved in the handle from the NBD protocol handshake. it already knows the answer. main character energy. 👑

👑 HANDLE STATE 👑

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

so basically: mid-handshake, connected, or shut down? you're good. before connecting, mid-connect, or straight-up dead? nope. vibe check failed. 💀

🎀 VERSION 🎀

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

🔥 SEE ALSO 🔥

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).

💜 AUTHORS 💜

Eric Blake

Richard W.M. Jones

🌸 COPYRIGHT 🌸

Copyright Red Hat

🌈 LICENSE 🌈

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

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and that's the whole page, bestie! 🫶 this one's from libnbd-1.25.8, dated 2026-10-13 — so like, extremely fresh. go forth and align your offsets. periodt. 💅✨