2022-05-13 16:54:14 +00:00
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Subpage support
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===============
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Subpage block size support, or just *subpage* for short, is a feature to allow
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using a filesystem that has different size of data block size (*sectorsize*)
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and the host CPU page size. For easier implementation the support was limited
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to the exactly same size of the block and page. On x86_64 this is typically
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4KiB, but there are other architectures commonly used that make use of larger
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2022-05-18 05:37:50 +00:00
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pages, like 64KiB on 64bit ARM or PowerPC. This means filesystems created
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with 64KiB sector size cannot be mounted on a system with 4KiB page size.
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While with subpage support, systems with 64KiB page size can create (still needs
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"-s 4k" option for :command:`mkfs.btrfs`) and mount filesystems with 4KiB sectorsize.
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Requirements, limitations
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-------------------------
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The initial subpage support has been added in v5.15, although it's still
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considered as experimental, most features are already working without problems.
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End users can mount filesystems with 4KiB sectorsize and do their usual
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workload, while should not notice any obvious change, as long as the initial
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mount succeeded (there are cases a mount will be rejected though).
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The following features has some limitations for subpage:
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- Supported page sizes: 4KiB, 8KiB, 16KiB, 32KiB, 64KiB
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2023-05-31 23:14:47 +00:00
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- Supported filesystem sector sizes on a given host are exported in
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:file:`/sys/fs/btrfs/features/supported_sectorsizes`
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- No inline extents
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This is an artificial limitation, to prevent mixed inline and regular extents.
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2022-12-07 20:00:25 +00:00
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Thus max_inline mount option will be silently ignored for subpage mounts,
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2022-05-18 05:37:50 +00:00
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and it always acts as "max_inline=0".
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2023-05-31 23:14:47 +00:00
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- Compression writes are limited to page aligned ranges
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Compression write for subpage has been introduced in v5.16, with the
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limitation that only page aligned range can be compressed. This limitation
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is due to how btrfs handles delayed allocation.
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- No support for v1 space cache
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2023-05-31 23:14:47 +00:00
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The old v1 cache has quite some hard coded page size usage, and considering
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it already deprecated, we force v2 cache for subpage.
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