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btrfs-progs: docs: swapfile clarifications

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Clarify active vs. inactive swapfile regarding snapshotting and further minor
improvements.

Issue: #608
Pull-request: #618
Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Christoph Anton Mitterer 2023-04-26 13:05:32 +02:00 committed by David Sterba
parent 93d51b286c
commit 321b2f61fe
1 changed files with 11 additions and 11 deletions
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22
Documentation/ch-swapfile.rst
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@ -1,16 +1,16 @@
A swapfile is file-backed memory that the system uses to temporarily offload A swapfile, when active, is a file-backed swap area. It is supported since kernel 5.0.
the RAM. It is supported since kernel 5.0. Use ``swapon(8)`` to activate it as Use ``swapon(8)`` to activate it, until then (respectively again after deactivating it
a swap area, until then it's just a normal file (with NODATACOW set), for which with ``swapoff(8)``) it's just a normal file (with NODATACOW set), for which the special
the special restrictions for active swapfile areas don't apply. restrictions for active swapfiles don't apply.
There are some limitations of the implementation in BTRFS and Linux swap There are some limitations of the implementation in BTRFS and Linux swap
subsystem: subsystem:
* filesystem - must be only single device * filesystem - must be only single device
* filesystem - must have only *single* data profile * filesystem - must have only *single* data profile
* swapfile - the containing subvolume cannot be snapshotted if there's an active swapfile * subvolume - cannot be snapshotted if it contains any active swapfiles
* swapfile - the file must be preallocated (i.e. no holes) * swapfile - must be preallocated (i.e. no holes)
* swapfile - the file must be NODATACOW (i.e. also NODATASUM, no compression) * swapfile - must be NODATACOW (i.e. also NODATASUM, no compression)
The limitations come namely from the COW-based design and mapping layer of The limitations come namely from the COW-based design and mapping layer of
blocks that allows the advanced features like relocation and multi-device blocks that allows the advanced features like relocation and multi-device
@ -21,13 +21,13 @@ swap.
With active swapfiles, the following whole-filesystem operations will skip With active swapfiles, the following whole-filesystem operations will skip
swapfile extents or may fail: swapfile extents or may fail:
* balance - block groups with an active swapfile extents are skipped and * balance - block groups with extents of any active swapfiles are skipped and
reported, the rest will be processed normally reported, the rest will be processed normally
* resize grow - unaffected * resize grow - unaffected
* resize shrink - works as long as the extents of an active swapfile are * resize shrink - works as long as the extents of any active swapfiles are
outside of the shrunk range outside of the shrunk range
* device add - a new device does not interfere with existing active swapfile * device add - if the new devices do not interfere with any already active swapfiles
and this operation will work, though no new swapfile can be activated this operation will work, though no new swapfile can be activated
afterwards afterwards
* device delete - if the device has been added as above, it can be also deleted * device delete - if the device has been added as above, it can be also deleted
* device replace - ditto * device replace - ditto