You must first create the regional persistent disk and then attach it to an existing instance.
You can't create regional persistent disks as boot disks because they can't be force-attached during a failover.
Create a regional persistent disk.
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Go to the Disks page.
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Select your project.
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Click Create Disk.
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Specify a Name for your disk.
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Choose a disk Type.
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Check the Replicate this disk within region box.
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Select the region.
Make a note of the region because you must select the same region when you create your instance.
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Select two zones in that region, and click OK.
You must select two zones. Note the zones that you select because you must attach the disk to your instance in one of those zones.
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Choose the Source Type.
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Optionally, choose your Physical block size (KB). The disk default size is 4 KB. However, you can select 16 KB from the drop-down menu to increase the physical block size of the disk. This feature is in beta.
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Select the Encryption.
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Click Create to finish creating your disk.
After you create your regional persistent disk, attach it to your instance.