Using scratch extents to facilitate copying operations in an append-only storage system

A non-transitory computer-readable storage medium storing instructions that when executed by a computer cause the computer to perform a method, the method comprising:

• operating a data storage system that is append-only and stores sets of data blocks in extents that are located in storage devices, wherein operating the data storage system includes performing the following operations:
• receiving a request to copy an extent from a source storage device to a destination storage device;
• and
• in response to the request,
• creating a scratch extent on the destination storage device,
• associating the scratch extent with a private identifier,
• whereby the scratch extent can only be accessed through the private identifier,
• using the private identifier to perform a copying operation that copies the extent from the source storage device to the scratch extent on the destination storage device, and
• after the copying operation is complete and the scratch extent is closed, associating the scratch extent with a public identifier, whereby the copy of the extent on the destination storage device becomes publically accessible to other entities in the data storage system.

A data storage system, comprising:

• one or more storage devices containing extents, wherein each extent stores a set of data blocks;
• wherein upon receiving a request to copy an extent from a source storage device to a destination storage device, the data storage system is configured to,
• create a scratch extent on the destination storage device,
• associate the scratch extent with a private identifier, whereby the scratch extent can only be accessed through the private identifier,
• use the private identifier to perform a copying operation that copies the extent from the source storage device to the scratch extent on the destination storage device, and
• after the copying operation is complete and the scratch extent is closed, associate the scratch extent with a public identifier, whereby the copy of the extent on the destination storage device becomes publically accessible to other entities in the data storage system.