69 lines
2.8 KiB
ReStructuredText
69 lines
2.8 KiB
ReStructuredText
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A swapfile is file-backed memory that the system uses to temporarily offload
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the RAM. It is supported since kernel 5.0. Use ``swapon(8)`` to activate the
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swapfile. There are some limitations of the implementation in BTRFS and linux
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swap subsystem:
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* filesystem - must be only single device
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* filesystem - must have only *single* data profile
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* swapfile - the containing subvolume cannot be snapshotted
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* swapfile - must be preallocated
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* swapfile - must be nodatacow (ie. also nodatasum)
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* swapfile - must not be compressed
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The limitations come namely from the COW-based design and mapping layer of
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blocks that allows the advanced features like relocation and multi-device
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filesystems. However, the swap subsystem expects simpler mapping and no
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background changes of the file blocks once they've been attached to swap.
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With active swapfiles, the following whole-filesystem operations will skip
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swapfile extents or may fail:
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* balance - block groups with swapfile extents are skipped and reported, the
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rest will be processed normally
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* resize grow - unaffected
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* resize shrink - works as long as the extents are outside of the shrunk range
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* device add - a new device does not interfere with existing swapfile and this
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operation will work, though no new swapfile can be activated afterwards
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* device delete - if the device has been added as above, it can be also deleted
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* device replace - ditto
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When there are no active swapfiles and a whole-filesystem exclusive operation
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is running (eg. balance, device delete, shrink), the swapfiles cannot be
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temporarily activated. The operation must finish first.
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To create and activate a swapfile run the following commands:
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.. code-block:: bash
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# truncate -s 0 swapfile
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# chattr +C swapfile
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# fallocate -l 2G swapfile
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# chmod 0600 swapfile
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# mkswap swapfile
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# swapon swapfile
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Please note that the UUID returned by the *mkswap* utility identifies the swap
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"filesystem" and because it's stored in a file, it's not generally visible and
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usable as an identifier unlike if it was on a block device.
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The file will appear in */proc/swaps*:
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.. code-block:: none
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# cat /proc/swaps
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Filename Type Size Used Priority
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/path/swapfile file 2097152 0 -2
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--------------------
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The swapfile can be created as one-time operation or, once properly created,
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activated on each boot by the **swapon -a** command (usually started by the
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service manager). Add the following entry to */etc/fstab*, assuming the
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filesystem that provides the */path* has been already mounted at this point.
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Additional mount options relevant for the swapfile can be set too (like
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priority, not the BTRFS mount options).
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.. code-block:: none
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/path/swapfile none swap defaults 0 0
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