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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
the RAM. It is supported since kernel 5.0. Use ``swapon(8)`` to activate it as
a swap area, until then it's just a normal file (with NODATACOW set), for which
the special restrictions for active swapfile areas don't apply.
A swapfile, when active, is a file-backed swap area. It is supported since kernel 5.0.
Use ``swapon(8)`` to activate it, until then (respectively again after deactivating it
with ``swapoff(8)``) it's just a normal file (with NODATACOW set), for which the special
restrictions for active swapfiles don't apply.
There are some limitations of the implementation in BTRFS and Linux swap
subsystem:
* filesystem - must be only single device
* filesystem - must have only *single* data profile
* swapfile - the containing subvolume cannot be snapshotted if there's an active swapfile
* swapfile - the file must be preallocated (i.e. no holes)
* swapfile - the file must be NODATACOW (i.e. also NODATASUM, no compression)
* subvolume - cannot be snapshotted if it contains any active swapfiles
* swapfile - must be preallocated (i.e. no holes)
* swapfile - must be NODATACOW (i.e. also NODATASUM, no compression)
The limitations come namely from the COW-based design and mapping layer of
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
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
* 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
* device add - a new device does not interfere with existing active swapfile
and this operation will work, though no new swapfile can be activated
* device add - if the new devices do not interfere with any already active swapfiles
this operation will work, though no new swapfile can be activated
afterwards
* device delete - if the device has been added as above, it can be also deleted
* device replace - ditto