123 lines
4.9 KiB
ReStructuredText
123 lines
4.9 KiB
ReStructuredText
The COW mechanism and multiple devices under one hood enable an interesting
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concept, called a seeding device: extending a read-only filesystem on a
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device with another device that captures all writes. For example
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imagine an immutable golden image of an operating system enhanced with another
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device that allows to use the data from the golden image and normal operation.
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This idea originated on CD-ROMs with base OS and allowing to use them for live
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systems, but this became obsolete. There are technologies providing similar
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functionality, like *unionmount*, *overlayfs* or *qcow2* image snapshot.
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The seeding device starts as a normal filesystem, once the contents is ready,
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**btrfstune -S 1** is used to flag it as a seeding device. Mounting such device
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will not allow any writes, except adding a new device by **btrfs device add**.
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Then the filesystem can be remounted as read-write.
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Given that the filesystem on the seeding device is always recognized as
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read-only, it can be used to seed multiple filesystems from one device at the
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same time. The UUID that is normally attached to a device is automatically
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changed to a random UUID on each mount.
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Once the seeding device is mounted, it needs the writable device. After adding
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it, something like **remount -o remount,rw /path** makes the filesystem at
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*/path* ready for use. The simplest use case is to throw away all changes by
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unmounting the filesystem when convenient.
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Alternatively, deleting the seeding device from the filesystem can turn it into
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a normal filesystem, provided that the writable device can also contain all the
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data from the seeding device.
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The seeding device flag can be cleared again by **btrfstune -f -S 0**, eg.
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allowing to update with newer data but please note that this will invalidate
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all existing filesystems that use this particular seeding device. This works
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for some use cases, not for others, and the forcing flag to the command is
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mandatory to avoid accidental mistakes.
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Example how to create and use one seeding device:
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.. code-block:: bash
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# mkfs.btrfs /dev/sda
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# mount /dev/sda /mnt/mnt1
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... fill mnt1 with data
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# umount /mnt/mnt1
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# btrfstune -S 1 /dev/sda
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# mount /dev/sda /mnt/mnt1
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# btrfs device add /dev/sdb /mnt/mnt1
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# mount -o remount,rw /mnt/mnt1
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... /mnt/mnt1 is now writable
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Now */mnt/mnt1* can be used normally. The device */dev/sda* can be mounted
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again with a another writable device:
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.. code-block:: bash
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# mount /dev/sda /mnt/mnt2
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# btrfs device add /dev/sdc /mnt/mnt2
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# mount -o remount,rw /mnt/mnt2
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... /mnt/mnt2 is now writable
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The writable device (*/dev/sdb*) can be decoupled from the seeding device and
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used independently:
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.. code-block:: bash
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# btrfs device delete /dev/sda /mnt/mnt1
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As the contents originated in the seeding device, it's possible to turn
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*/dev/sdb* to a seeding device again and repeat the whole process.
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A few things to note:
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* it's recommended to use only single device for the seeding device, it works
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for multiple devices but the *single* profile must be used in order to make
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the seeding device deletion work
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* block group profiles *single* and *dup* support the use cases above
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* the label is copied from the seeding device and can be changed by **btrfs filesystem label**
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* each new mount of the seeding device gets a new random UUID
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Chained seeding devices
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^^^^^^^^^^^^^^^^^^^^^^^
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Though it's not recommended and is rather an obscure and untested use case,
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chaining seeding devices is possible. In the first example, the writable device
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*/dev/sdb* can be turned onto another seeding device again, depending on the
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unchanged seeding device */dev/sda*. Then using */dev/sdb* as the primary
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seeding device it can be extended with another writable device, say */dev/sdd*,
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and it continues as before as a simple tree structure on devices.
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.. code-block:: bash
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# mkfs.btrfs /dev/sda
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# mount /dev/sda /mnt/mnt1
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... fill mnt1 with data
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# umount /mnt/mnt1
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# btrfstune -S 1 /dev/sda
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# mount /dev/sda /mnt/mnt1
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# btrfs device add /dev/sdb /mnt/mnt1
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# mount -o remount,rw /mnt/mnt1
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... /mnt/mnt1 is now writable
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# umount /mnt/mnt1
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# btrfstune -S 1 /dev/sdb
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# mount /dev/sdb /mnt/mnt1
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# btrfs device add /dev/sdc /mnt
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# mount -o remount,rw /mnt/mnt1
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... /mnt/mnt1 is now writable
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# umount /mnt/mnt1
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As a result we have:
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* *sda* is a single seeding device, with its initial contents
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* *sdb* is a seeding device but requires *sda*, the contents are from the time
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when *sdb* is made seeding, ie. contents of *sda* with any later changes
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* *sdc* last writable, can be made a seeding one the same way as was *sdb*,
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preserving its contents and depending on *sda* and *sdb*
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As long as the seeding devices are unmodified and available, they can be used
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to start another branch.
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