openzfs/zfs

Allow configuration of zvol logical blocksize

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#4203 opened on Jan 11, 2016

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Component: ZVOLType: Featuregood first issue

Description

The logical blocksize of a zvol seems to always be 512 byte, despite the fact that volblocksize is set to 4 KB:

# zfs get volblocksize data/iscsi 
NAME        PROPERTY      VALUE     SOURCE
data/vol1   volblocksize  4K        -

# lsblk -o name,min-io,opt-io,phy-sec,log-sec,rq-size
NAME    MIN-IO OPT-IO PHY-SEC LOG-SEC RQ-SIZE
zd0       4096   4096    4096     512     128

Is there any way to set logical blocksize = physical blocksize?

Contributor guide

Tech stack
c
Domain
backend
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
ZFS internalsLinux kernel module developmentC programming
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
Investigate the zvol block device implementation in module/zfs/zvol.c to understand how logical blocksize is currently forced to 512 bytes. Look for any existing discussions or patches in the issue comments. The goal is to add a zvol property or module parameter to allow overriding the logical blocksize, similar to volblocksize but for logical reporting. Ensure compatibility with existing zvol behavior and handle potential alignment constraints.