208aed2ed4
All main pages have some content and many typos have been fixed. Signed-off-by: David Sterba <dsterba@suse.com>
77 lines
3.3 KiB
ReStructuredText
77 lines
3.3 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 single
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device filesystem 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, at the same time. The
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UUID that is normally attached to a device is automatically changed to a random
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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 a 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
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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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