OpenStack Temporary storage back end

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OpenStack now offers RAW,QCOW2,LVM,RBD four types of image back end.
The so-called back end, that is, image/temporary volume root Disk Management storage mode.

You can see that the image is cached as base on compute, the disk that creates the virtual machine as base, and the disks of multiple virtual machines may share the same base.

nova/virt/libvirt/imagebackend.py:
There are four RAW,QCOW2,LVM,RBD in four classes. All inherit the image class. Create_image methods and Snapshot_extract methods are mainly provided.
The image parent class provides the cache method. The Create_image method is called.
The cache method provides the image folder creation, calls the Create_image method to create an image, and completes the preallocate (implemented through Fallocate).
The Create_image method provides an image download with the ability to create backing_image, depending on the backend type.

The backend class mainly provides Raw,qcow2, LVM, RBD object generation capabilities.

Call flow for the cache method:

Cache method "------_create_image Method"------------------(Rescue/finish_migration/spawn)
Cache method "------_create_images_and_backing Method"-----(hard_reboot/pre_livemigration)

Configuration items:

The images_type is used to configure the default image storage management mode. Can be set to four kinds of random one.
The volume_clear is used to define how the temporary volume is deleted (various security removal policies).
Volume_clear_size the size of the shred corrupted data will be deleted.

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OpenStack Temporary storage back end