Proposal: enable sparse mode by default

[soredake]

Is your feature request related to a problem? Please describe. Currently, WSL does not enable NTFS sparse file support by default for its virtual disk (ext4.vhdx). This leads to unnecessary disk space usage, especially when working with large development environments, package managers, or Docker images. Users must manually convert or recreate the VHDX in sparse mode to benefit from actual disk savings.

Describe the solution you'd like Enable sparse mode by default when creating new WSL distributions or VHDX disks. This would ensure that unused or zeroed blocks inside the VHDX do not consume physical disk space unnecessarily. Ideally, this should be transparent to the user and require no additional setup.

Describe alternatives you've considered wsl --manage <distro> --set-sparse true --allow-unsafe

Additional context Sparse support can significantly reduce the disk footprint for users working with data-heavy tools like Docker, compilers, and large codebases. As SSDs remain relatively expensive and WSL is used heavily in developer workflows, enabling sparse mode by default would be a smart default.

[github-actions[bot]]

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[soredake]

/feature

[github-actions[bot]]

Diagnostic information

Found '/feature', adding tag 'feature'

[craigloewen-msft]

Agreed, we would like to enable sparse mode by default. Right now we are still seeing quality reports and issues with sparse VHDs and so have made it an opt in experimental feature. We're investigating how to improve our storage story in the future!

[dporf]

@craigloewen-msft as far as I understand sparse mode is incomplete. It does not call ioctl FSCTL_SET_ZERO_DATA for vhd regions being deallocated

[dporf]

@soredake

Sparse support can significantly reduce the disk footprint

It could if it worked but it does not (see issue #12103 for example). May be you know how it may be configured to actually work?