Efficient scavenging of data and metadata file system blocks

A data storage apparatus for scavenging free storage space in a thinly provisioned upper deck file system stored in the form of a file in an underlying lower deck file system of the data storage apparatus, the data storage apparatus comprising:

• a set of storage devices;
• and
• a storage processor, the storage processor including:
• memory;
• and
• a set of processors coupled to the memory to form controlling circuitry, the controlling circuitry constructed and arranged to:
• access a set of data structures to identify the location of each of a set of backed free blocks within the upper deck file system,
• wherein the backed free blocks are blocks that have been provisioned from storage devices of the data storage apparatus to the lower deck file system, allocated from the lower deck file system to the upper deck file system, and later freed from the upper deck file system;
• and
• for each of the set of backed free blocks, (i) free the block in the lower deck file system from which the respective backed free block was allocated and (ii) update the set of data structures to indicate that the respective back free block is no longer a backed free block,
• wherein the controlling circuitry, constructed and arranged to access the set of data structures to identify the location of each of a set of backed free blocks within the upper deck file system, is further constructed and arranged to identify, from the set of data structures, a set of slices in the upper deck file system that each include at least one backed free block,
• wherein blocks in the lower deck file system are arranged in lower deck slices, and
• wherein the controlling circuitry is further constructed and arranged to (i) compute a number of freed blocks in at least one lower deck slice, and (ii) compare the number of freed blocks to a threshold number of freed blocks to produce a comparison result indicative of a readiness to return the at least one lower deck slice to a storage pool.

A computer program product having a non-transitory, computer-readable storage medium which stores instructions which, when executed by a computer, cause the computer to perform a method of scavenging free storage space in a thinly provisioned upper deck file system stored in the form of a file in an underlying lower deck file system of a data storage apparatus, the method comprising:

• accessing, by a storage processor of the data storage apparatus, a set of data structures to identify the location of each of a set of backed free blocks within the upper deck file system, wherein the backed free blocks are blocks that have been provisioned from storage devices of the data storage apparatus to the lower deck file system, allocated from the lower deck file system to the upper deck file system, and later freed from the upper deck file system;
• and
• for each of the set of backed free blocks, (i) freeing the block in the lower deck file system from which the respective backed free block was allocated and (ii) updating the set of data structures to indicate that the respective back free block is no longer a backed free block,
• wherein accessing the set of data structures to identify the location of each of a set of backed free blocks within the upper deck file system includes identifying, from the set of data structures, a set of slices in the upper deck file system that each include at least one backed free block,
• wherein blocks in the lower deck file system are arranged in lower deck slices, and
• wherein the method further comprises (i) computing a number of freed blocks in at least one lower deck slice, and (ii) comparing the number of freed blocks to a threshold number of freed blocks to produce a comparison result indicative of a readiness to return the at least one lower deck slice to a storage pool.