btrfs-progs/Documentation/Subpage.rst

Subpage support
===============

Subpage block size support, or just *subpage* for short, is a feature to allow
using a filesystem that has different size of data block size (*sectorsize*)
and the host CPU page size. For easier implementation the support was limited
to the exactly same size of the block and page. On x86_64 this is typically
4KiB, but there are other architectures commonly used that make use of larger
pages, like 64KiB on 64bit ARM or PowerPC. This means filesystems created
with 64KiB sector size cannot be mounted on a system with 4KiB page size.

Since v6.7, filesystems are created with a 4KiB sectorsize by default,
though it remains possible to create filesystems with other page sizes
(such as 64KiB with the "-s 64k" option for :command:`mkfs.btrfs`). This
ensures that new filesystems are compatible across other architecture
variants using larger page sizes.

Requirements, limitations
-------------------------

The initial subpage support has been added in v5.15. Most features are
already working without problems. On a 64KiB page system, a filesystem with
4KiB sectorsize can be mounted and used as long as the initial mount succeeds.
Subpage support is used by default for systems with a non-4KiB page size since v6.7.

Please refer to status page of :ref:`status-subpage-block-size` for
compatibility.