Learning Ceph - Second Edition

Resizing RADOS Block Devices

Ceph RBD block devices are thin-provisioned volumes, which means that the underlying physical storage on Ceph clusters (provided by OSDs) is not fully written or allocated until the user actually writes data to the device. For example, say we provision a 5 TB volume— something that the author's users do daily on local disks, all 5 terabytes are allocated immediately and the available capacity is immediately reduced by 5 TB. This holds true whether the user writes 5 TB of data, just 1 GB, or none at all. With thin provisioning, almost no space is immediately allocated on the underlying storage, and the available capacity is not significantly reduced. Thus there is an important distinction between provisioned and ...