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64 lines
2.7 KiB
ReStructuredText
64 lines
2.7 KiB
ReStructuredText
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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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 mkfs.btrfs) and mount filesystems with 4KiB sectorsize,
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allowing us to push 4KiB sectorsize as default sectorsize for all platforms in the
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near future.
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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 at the time of writing (v5.18), most features are
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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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- RAID56 support
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This support is already queued for v5.19 cycle.
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Any fs with RAID56 chunks will be rejected at mount time for now.
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- Support for page size other than 64KiB
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The support for other page sizes (16KiB, 32KiB and more) are already queued
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for v5.19 cycle.
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Initially the subpage support is only for 64KiB support, but the design makes
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it pretty easy to enable support for other page sizes.
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- No inline extent creation
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This is an artificial limit, to prevent mixed inline and regular extents.
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It's possible to create mixed inline and regular extents even with
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non-subpage mount for certain corner cases, it's way easier to create such
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mixed extents for subpage.
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Thus max_inline mount option will be silently ignored for subpage mounts,
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and it always acts as "max_inline=0".
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- Compression write is limited to page aligned ranges
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Compression write for subpage is introduced in v5.16, with the limitation
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that only page aligned range can be compressed.
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This limitation is due to how btrfs handles delayed allocation.
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- No support for v1 space cache
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V1 space cache is considered deprecated, and we're defaulting to v2 cache
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in btrfs-progs already.
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The old v1 cache has quite some hard coded page size usage, and consider it
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is already deprecated, we force v2 cache for subpage.
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- Slightly higher memory usage for scrub
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This is due to how we allocate pages for scrub, and will be fixed in the coming
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releases soon.
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