a60afdceb9
Capitals were meant for manual pages but in html it looks better with the first letter. Signed-off-by: David Sterba <dsterba@suse.com>
64 lines
2.7 KiB
ReStructuredText
64 lines
2.7 KiB
ReStructuredText
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.
|
|
|
|
While with subpage support, systems with 64KiB page size can create (still needs
|
|
"-s 4k" option for mkfs.btrfs) and mount filesystems with 4KiB sectorsize,
|
|
allowing us to push 4KiB sectorsize as default sectorsize for all platforms in the
|
|
near future.
|
|
|
|
Requirements, limitations
|
|
-------------------------
|
|
|
|
The initial subpage support has been added in v5.15, although it's still
|
|
considered as experimental at the time of writing (v5.18), most features are
|
|
already working without problems.
|
|
|
|
End users can mount filesystems with 4KiB sectorsize and do their usual
|
|
workload, while should not notice any obvious change, as long as the initial
|
|
mount succeeded (there are cases a mount will be rejected though).
|
|
|
|
The following features has some limitations for subpage:
|
|
|
|
- RAID56 support
|
|
This support is already queued for v5.19 cycle.
|
|
Any fs with RAID56 chunks will be rejected at mount time for now.
|
|
|
|
- Support for page size other than 64KiB
|
|
The support for other page sizes (16KiB, 32KiB and more) are already queued
|
|
for v5.19 cycle.
|
|
Initially the subpage support is only for 64KiB support, but the design makes
|
|
it pretty easy to enable support for other page sizes.
|
|
|
|
- No inline extent creation
|
|
This is an artificial limit, to prevent mixed inline and regular extents.
|
|
|
|
It's possible to create mixed inline and regular extents even with
|
|
non-subpage mount for certain corner cases, it's way easier to create such
|
|
mixed extents for subpage.
|
|
|
|
Thus max_inline mount option will be sliently ignored for subpage mounts,
|
|
and it always acts as "max_inline=0".
|
|
|
|
- Compression write is limited to page aligned ranges
|
|
Compression write for subpage is introduced in v5.16, with the limitation
|
|
that only page aligned range can be compressed.
|
|
This limitation is due to how btrfs handles delayed allocation.
|
|
|
|
- No support for v1 space cache
|
|
V1 space cache is considered deprecated, and we're defaulting to v2 cache
|
|
in btrfs-progs already.
|
|
The old v1 cache has quite some hard coded page size usage, and consider it
|
|
is already deprecated, we force v2 cache for subpage.
|
|
|
|
- Slightly higher memory usage for scrub
|
|
This is due to how we allocate pages for scrub, and will be fixed in the coming
|
|
releases soon.
|