# Patent

## Title
Coordinated space reclamation in space-efficient flashcopy volumes

## Abstract

A method for releasing storage space in a space-efficient volume is disclosed. In one embodiment, such a method includes maintaining, for a space-efficient volume, a bitmap comprising a bit for each track in the space-efficient volume. A host system indicates whether a track is one of used and unused by marking the corresponding bit in the bitmap. A storage controller reads the bitmap and frees tracks from the space-efficient volume that the bitmap indicates are unused. In certain embodiments, the bitmap is stored on the space-efficient volume. For example, the bitmap may be appended to a volume table of contents (VTOC) stored on the space-efficient volume. A corresponding system and computer program product are also disclosed.

## Background
BACKGROUND
Field of the Invention
This invention relates to systems and methods for more effectively reclaiming space in space-efficient FlashCopy volumes.
Background of the Invention
On storage systems such as the IBM DS8000TM enterprise storage system, FlashCopy is a function used to create nearly instantaneous point-in-time copies of volumes or data sets. These point-in-time copies may be used for backups and other purposes. Once created, the point-in-time copies are immediately available for both read and write access. Space Efficient FlashCopy is a function similar to conventional FlashCopy except that space-efficient volumes created with this function are not allocated all their defined storage space at the time of creation. Rather, storage space is allocated when data is actually written to a space-efficient volume. This allows the space-efficient volumes to be thinly provisioned, which reduces the amount of physical storage space needed to back the space-efficient volumes. Space Efficient FlashCopy achieves the aforementioned space efficiency by pooling physical storage requirements of many space-efficient volumes in a common repository. A mapping structure is used to keep track of where the space-efficient volumes' data is physically located within the repository.
While physical storage space may be allocated to space-efficient volumes when needed, the same physical storage space may be reclaimed from the space-efficient volumes when it is no longer needed. Currently, when tracks are deleted by host-system-based software, the DS8000TM storage controller relies on updates to its metadata to release the tracks from a space-efficient volume. More specifically, the DS8000TM storage controller relies on certain metadata associated with the tracks to be overwritten with zeros or marked to indicate that the tracks need to be removed from the space-efficient volume. This metadata is later scanned by the storage controller to identify which tracks should be removed. Once identified, the storage controller may add the tracks to a free storage pool to be used for future allocations. Over time, events such as errors or miscommunications may occur which may result in the required metadata not being zeroed out correctly. For example, if software on a host system deletes a data set or particular tracks of a data set, the software on the host system may notify the storage controller so that the storage controller can release the tracks back into the free storage pool. If the notifications from the software are lost or not properly registered or handled by the storage controller, the tracks may not be released back into the free storage pool. Over time this may result in storage space in the storage system that is not utilized, but nevertheless tied up and unavailable for use.
In view of the foregoing, what are needed are systems and methods to more effectively reclaim space in space-efficient point-in-time-copy volumes. Ideally, such systems and methods will prevent situations where tracks, even though deleted or marked as unused by a host system, are not released to a free storage pool by the storage controller.
SUMMARY
The invention has been developed in response to the present state of the art and, in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available systems and methods. Accordingly, the invention has been developed to provide systems and methods to more effectively reclaim space in space-efficient FlashCopy volumes. The features and advantages of the invention will become more fully apparent from the following description and appended claims, or may be learned by practice of the invention as set forth hereinafter.
Consistent with the foregoing, a method for releasing storage space in a space-efficient volume is disclosed. In one embodiment, such a method includes maintaining, for a space-efficient volume, a bitmap comprising a bit for each track in the space-efficient volume. A host system indicates whether a track is one of used and unused by marking the corresponding bit in the bitmap. A storage controller reads the bitmap and frees tracks from the space-efficient volume that the bitmap indicates are unused. In certain embodiments, the bitmap is stored on the space-efficient volume. For example, the bitmap may be appended to a volume table of contents (VTOC) stored on the space-efficient volume. A corresponding system and computer program product are also disclosed and claimed herein.



BRIEF DESCRIPTION OF THE DRAWINGS
In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the embodiments of the invention will be described and explained with additional specificity and detail through use of the accompanying drawings, in which:
FIG. 1 is a high-level block diagram showing one example of a network architecture in which a system and method in accordance with the invention may be implemented;
FIG. 2 is a high-level block diagram showing one example of a storage system hosting one or more space efficient volumes;
FIG. 3 is a high-level block diagram showing logical volumes exposed by the storage system, and particularly showing a volume table of contents (VTOC) and data sets stored on a logical volume;
FIG. 4 is a high-level block diagram showing the use of space-efficient logical volumes for storing point-in-time copies; and
FIG. 5 is a high-level block diagram showing use of a free space bitmap by both a host system and storage controller.



DETAILED DESCRIPTION
It will be readily understood that the components of the present invention, as generally described and illustrated in the Figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the invention, as represented in the Figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of certain examples of presently contemplated embodiments in accordance with the invention. The presently described embodiments will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout.
The present invention may be embodied as a system, method, and/or computer program product. The computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present invention.
The computer readable storage medium may be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanically encoded device such as punch-cards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire.
Computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and/or edge servers. A network adapter card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing/processing device.
Computer readable program instructions for carrying out operations of the present invention may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Smalltalk, C++ or the like, and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The computer readable program instructions may execute entirely on a user's computer, partly on a user's computer, as a stand-alone software package, partly on a user's computer and partly on a remote computer, or entirely on a remote computer or server. In the latter scenario, a remote computer may be connected to a user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present invention.
Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, may be implemented by computer readable program instructions.
These computer readable program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and/or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein comprises an article of manufacture including instructions which implement aspects of the function/act specified in the flowchart and/or block diagram block or blocks.
The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus, or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions/acts specified in the flowchart and/or block diagram block or blocks.
Referring to FIG. 1, one example of a network architecture 100 is illustrated. The network architecture 100 is presented to show one example of an environment where an apparatus and method in accordance with the invention may be implemented. The network architecture 100 is presented only by way of example and not limitation. Indeed, the apparatus and methods disclosed herein may be applicable to a wide variety of network architectures, in addition to the network architecture 100 shown.
As shown, the network architecture 100 includes one or more computers 102, 106 interconnected by a network 104. The network 104 may include, for example, a local-area-network (LAN) 104, a wide-area-network (WAN) 104, the Internet 104, an intranet 104, or the like. In certain embodiments, the computers 102, 106 may include both client computers 102 and server computers 106 (also referred to herein as “host systems” 106). In general, the client computers 102 initiate communication sessions, whereas the server computers 106 wait for requests from the client computers 102. In certain embodiments, the computers 102 and/or servers 106 may connect to one or more internal or external direct-attached storage systems 112 (e.g., arrays of hard-disk drives, solid-state drives, tape drives, etc.). These computers 102, 106 and direct-attached storage systems 112 may communicate using protocols such as ATA, SATA, SCSI, SAS, Fibre Channel, or the like.
The network architecture 100 may, in certain embodiments, include a storage network 108 behind the servers 106, such as a storage-area-network (SAN) 108 or a LAN 108 (e.g., when using network-attached storage). This network 108 may connect the servers 106 to one or more storage systems 110, such as arrays 110a of hard-disk drives or solid-state drives, tape libraries 110b, individual hard-disk drives 110c or solid-state drives 110c, tape drives 110d, CD-ROM libraries, or the like. To access a storage system 110, a host system 106 may communicate over physical connections from one or more ports on the host 106 to one or more ports on the storage system 110. A connection may be through a switch, fabric, direct connection, or the like. In certain embodiments, the servers 106 and storage systems 110 may communicate using a networking standard such as Fibre Channel (FC).
Referring to FIG. 2, one embodiment of a storage system 110a containing an array of hard-disk drives 204 and/or solid-state drives 204 is illustrated. As shown, the storage system 110a includes a storage controller 200, one or more switches 202, and one or more storage devices 204, such as hard disk drives 204 or solid-state drives 204 (such as flash-memory-based drives 204). The storage controller 200 may enable one or more hosts 106 (e.g., open system and/or mainframe servers 106 running operating systems such as MVS, z/OS, or the like) to access data in the one or more storage devices 204.
In selected embodiments, the storage controller 200 includes one or more servers 206. The storage controller 200 may also include host adapters 208 and device adapters 210 to connect the storage controller 200 to host devices 106 and storage devices 204, respectively. Multiple servers 206a, 206b may provide redundancy to ensure that data is always available to connected hosts 106. Thus, when one server 206a fails, the other server 206b may pick up the I/O load of the failed server 206a to ensure that I/O is able to continue between the hosts 106 and the storage devices 204. This process may be referred to as a “failover.”
One example of a storage system 110a having an architecture similar to that illustrated in FIG. 2 is the IBM DS8000TM enterprise storage system. The DS8000TM is a high-performance, high-capacity storage controller providing disk storage that is designed to support continuous operations. Nevertheless, the apparatus and methods disclosed herein are not limited to operation with the IBM DS8000TM enterprise storage system 110a, but may operate with any comparable or analogous storage system 110, regardless of the manufacturer, product name, or components or component names associated with the system 110. Furthermore, any storage system that could benefit from one or more embodiments of the invention is deemed to fall within the scope of the invention. Thus, the IBM DS8000TM is presented only by way of example and is not intended to be limiting.
In selected embodiments, each server 206 may include one or more processors 212 and memory 214. The memory 214 may include volatile memory (e.g., RAM) as well as non-volatile memory (e.g., ROM, EPROM, EEPROM, hard disks, flash memory, etc.). The volatile and non-volatile memory may, in certain embodiments, store software modules that run on the processor(s) 212 and are used to access data in the storage devices 204. The servers 206 may host at least one instance of these software modules. These software modules may manage all read and write requests to logical volumes in the storage devices 204.
Referring to FIG. 3, in certain embodiments, a storage system 110 such as that illustrated in FIG. 2 may be configured to present or expose one or more volumes 300a-j to a host system 106. The volumes 300 may be logical volumes 300, meaning that the volumes 300 may appear to be physical drives 204 (e.g., hard drives, solid state drives, etc.) to a host system 106 but do not necessarily directly correlate to physical drives 204 on the storage system 110. For example, in certain embodiments, a physical drive 204 may be used by more than one logical volume 300 or a logical volume 300 may span all or part of multiple physical drives 204. A storage virtualization layer 302 within the storage system 110 or may expose the logical volumes 300 and handle the mapping between the logical volumes 300 and the physical drives 204.
As further shown in FIG. 3, in certain embodiments, each logical volume 300 may store a volume table of contents (VTOC) 304 and one or more data sets 306. A VTOC 304 may contain information for locating data sets 306 on the associated logical volume 300. In certain embodiments, the VTOC 304 is located at the beginning of the logical volume 300 and may list the names of each data set 306 on the logical volume 300 as well as the data set's size, location, and permissions. The VTOC 304 may also store information describing each area of contiguous free space in the logical volume 300. The VTOC 304 is typically created at the time the logical volume 300 is initialized.
To access a particular data set 306 on a storage system 110, a host 106 may query a host-based catalog to determine the logical volume 300 on which the data set 306 resides. Once the correct logical volume 300 is determined, the host 106 locates the VTOC 304 on the logical volume 300 and searches the VTOC 304 to determine where the data set 306 is stored. The host 106 may then access the data set 306 at the determined location.
In general, a host 106 and host-system-based software is able to recognize, understand, and utilize the VTOC 304 to access data sets 306 on the logical volume 300. However, the storage controller 200 hosting the logical volume 300 typically will not recognize or understand the VTOC 304. Instead, the storage controller 200 may store and maintain internal metadata 216 (See FIG. 2) in its memory 214 to understand the configuration of a logical volume 300.
Referring to FIG. 4, as previously mentioned, on storage systems such as the IBM DS8000TM enterprise storage system, FlashCopy is a function used to create nearly instantaneous point-in-time copies of volumes 300 or data sets 306 on a storage system 110. These point-in-time copies may be used for backups or other purposes. Once created, the point-in-time copies are immediately available for both read and write access to host systems 106. Space Efficient FlashCopy is a function similar to conventional FlashCopy except that space-efficient volumes 300b created with this function are not allocated all their defined storage space at the time of creation. Rather, storage space is allocated when data is actually written to a space-efficient volume 300b. 
FIG. 4 shows a scenario where space-efficient logical volumes 300b are created to store point-in-time copies of data in corresponding source logical volumes 300a. When the space-efficient logical volumes 300b are created, no physical space may be allocated to the space-efficient logical volumes 300b. Rather, space may be allocated to the space-efficient logical volumes 300b on an as-need basis. For example, when new data is written to a track 400a of a source logical volume 300a, the old data in the track 400a may be copied over to the associated space-efficient logical volume 300b in order to preserve the point-in-time copy of the data.
When the data is copied over, a track 400b may be allocated to the space-efficient logical volume 300b from a repository 402 of free tracks in order to store the old data. In this way, tracks may be allocated to space-efficient logical volumes 300b if and when they are needed, providing more efficient utilization of storage space. This also allows the space-efficient logical volumes 300b to be over-provisioned, meaning that the space-efficient logical volumes 300b may be logically larger than the amount of physical storage space backing them.
When a track is no longer being used in a space-efficient logical volume 300b, such as when a track of data or an entire data set 306 has been deleted from the space-efficient logical volume 300b, the track may be released to the repository 402 so that it can be used again either in the same or different space-efficient logical volume 300b. In conventional implementations, if a host system 106 or host-system-based software deletes data, a volume space manager 508 (See FIG. 5) on the host system 106 updates the VTOC 304 to indicate that the storage space used to store the data is no longer in use. The volume space manager 508 then notifies the storage controller 200 that the storage space is no longer being used. A space reclamation module 510 (See FIG. 5) on the storage controller 200 may, in turn, update the track or tracks formerly used to store the data (such as by writing all zeros to the track or tracks) or update the internal metadata 216 of the storage controller 200 to indicate that the tracks are no longer in use. The space reclamation module 510 may then periodically scan the tracks and/or the metadata 216 to determine which tracks have been zeroed out or marked, and then release the tracks to the repository 402 so that they can be used again.
Unfortunately, scenarios may occur where storage space that is freed by a volume space manager 508 on the host system 106 is not released to the repository 402 by the storage controller 200. Because the space reclamation module 510 on the storage controller 200 relies on notifications from the volume space manager 508 on the host system 106 to identify tracks to release to the repository 402, these tracks may not be released if the notifications are not received or logged correctly by the storage controller 200. Lost or incorrectly logged notifications may be the result of errors, miscommunications, code glitches, or the like. Over time, tracks that are not properly released may result in storage space in the storage system 110 that is not utilized, but nevertheless tied up and unavailable for use.
Referring to FIG. 5, in certain embodiments in accordance with the invention, a new data structure may be created that allows a host system 106 and storage controller 200 to coordinate the release of storage space back to the repository 402. In certain embodiments, this new data structure may be stored in the VTOC 304 previously discussed. Since a storage controller 200 may not normally recognize and/or understand a VTOC 304, various mechanisms may be put in place to enable a space reclamation module 510 on the storage controller 200 to identify and utilize the new data structure in the VTOC 304. In other embodiments, the new data structure is located on a space efficient logical volume 300 outside of a VTOC 304, such as in a data set 306 or other location external to the VTOC 304.
In certain embodiments, the new data structure is a bitmap 506, where each bit in the bitmap 506 represents a storage area in a space-efficient logical volume 300b. For example, each bit in the bitmap 506 may represent a track in the space-efficient logical volume 300b. When a host system 106 or host-system-based software uses or releases a track, the volume space manager 508 on the host system 106 may mark the corresponding bit in the bitmap 506 to indicate that the track is either used or unused. The space reclamation module 510 on the storage controller 200 may, in turn, periodically scan the bitmap 506 to determine which tracks to release to the repository 402. If a bit for a track is marked as unused, the space reclamation module 510 may release the corresponding track to the repository 402. In certain embodiments, the bitmap 506 is embodied as a new type of data set control block (DSCB) 500 in the VTOC 304. However, unlike other data set control blocks 500 (e.g., data set descriptor DSCBs 502, free space descriptor DSCBs 504, etc.), the new bitmap 506 may be recognized and utilized not only by a host system 106, but also by a storage controller 200.
When a volume space manager 508 on a host system 106 changes the status of a track in a logical volume 300, such as by changing the track status from used to unused, or unused to used, the volume space manager 508 may update the bitmap 506 to reflect the new status. To accomplish this, the volume space manager 508 may acquire a lock on the track storing the bitmap 506 to ensure that the storage controller 200 or other processes can't access the bitmap 506 while updates are being made. The volume space manager 508 may then update the bitmap 506 to reflect the new status and release the lock when the operation is complete. The space reclamation module 510 in the storage controller 200 may likewise acquire a lock on the bitmap 506 when the bitmap 506 is analyzed or updated. This will ensure that neither the host system 106 nor storage controller 200 overwrites a pending operation. If the storage controller 200 attempts to access the bitmap 506 while the host system 106 has a lock, the storage controller 200 may need to wait until the lock is released to access the bitmap 506, and vice versa.
As mentioned above, various mechanisms may be put in place to enable a storage controller 200 to find and utilize the bitmap 506 on a space-efficient logical volume 300b. In one embodiment, the bitmap 506 is created and registered with the storage controller 200 at the time a space-efficient logical volume 300b is initialized. This will allow the storage controller 200 to store the location of the bitmap 506 in its internal metadata 216 so that unused tracks can be determined and released. If a logical volume 300 is reinitialized in a way that changes the location of the bitmap 506, the bitmap 506 may be reregistered with the storage controller 200.
In another embodiment, a unique identifier is stored on the space-efficient logical volume 300b with the bitmap. This may enable a storage controller 200 to locate the bitmap by scanning the logical volume 300 for the unique identifier. In yet another embodiment, the storage controller 200 may be updated to have knowledge of the VTOC 304, there enabling the storage controller 200 to scan the VTOC 304 and locate the bitmap 506. Other techniques for enabling the storage controller 200 to locate and identify the bitmap 506 are possible and within the scope of the invention.
The flowcharts and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowcharts or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the Figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. Other implementations may not require all of the disclosed steps to achieve the desired functionality. It will also be noted that each block of the block diagrams and/or flowchart illustrations, and combinations of blocks in the block diagrams and/or flowchart illustrations, may be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.

## Claims

1. A method for releasing storage space in a space-efficient volume, the method comprising:
maintaining, for a space-efficient volume, a bitmap comprising a bit for each track in the space-efficient volume;
storing the bitmap in a volume table of contents (VTOC) associated with the space-efficient volume;
enabling a storage controller to identify and utilize the bitmap within the VTOC by performing at least one of the following: (1) registering the bitmap with the storage controller at the time the space-efficient volume is initialized; and (2) storing a unique identifier with the bitmap to enable the storage controller to locate the bitmap on the space-efficient volume;
indicating, by a host system, whether a track is one of used and unused by marking the corresponding bit in the bitmap;
reading, by the storage controller, the bitmap; and
freeing, by the storage controller, tracks from the space-efficient volume that the bitmap indicates are unused.

2. The method of claim 1, further comprising enabling the host system and storage controller to lock the bitmap when accessing data therein.

3. The method of claim 2, further comprising enabling the host system and storage controller to update the bitmap.

4. The method of claim 1, wherein the bitmap is created when the space-efficient volume is initialized.

5. The method of claim 1, wherein freeing tracks further comprises adding the tracks to a free storage pool.

6. A computer program product to release storage space in a space-efficient volume, the computer program product comprising a non-transitory computer-readable storage medium having computer-usable program code embodied therein, the computer-usable program code comprising:
computer-usable program code to maintain, for a space-efficient volume, a bitmap comprising a bit for each track in the space-efficient volume;
computer-usable program code to store the bitmap in a volume table of contents (VTOC) associated with the space-efficient volume;
computer-usable program code to enable a storage controller to identify and utilize the bitmap within the VTOC by performing at least one of the following: (1) registering the bitmap with the storage controller at the time the space-efficient volume is initialized; and (2) storing a unique identifier with the bitmap to enable the storage controller to locate the bitmap on the space-efficient volume;
computer-usable program code to enable a host system to indicate, in the bitmap, whether a track is one of used and unused by marking the corresponding bit;
computer-usable program code to enable the storage controller to read the bitmap; and
computer-usable program code to enable the storage controller to free, from the space-efficient volume, tracks that the bitmap indicates are unused.

7. The computer program product of claim 6, further comprising computer-usable program code to enable the host system and storage controller to lock the bitmap when accessing data therein.

8. The computer program product of claim 7, further comprising computer-usable program code to enable the host system and storage controller to update the bitmap.

9. The computer program product of claim 6, wherein the bitmap is readable by: (1) a storage controller hosting the space-efficient volume; and (2) host-system-based software for managing space in the space-efficient volume.

10. The computer program product of claim 6, wherein freeing tracks further comprises adding the tracks to a free storage pool.

11. A system to release storage space in a space-efficient volume, the system comprising:
at least one processor;
at least one memory device operably coupled to the at least one processor and storing instructions for execution on the at least one processor, the instructions causing the at least one processor to:
maintain, for a space-efficient volume, a bitmap comprising a bit for each track in the space-efficient volume;
store the bitmap in a volume table of contents (VTOC) associated with the space-efficient volume;
enable a storage controller to identify and utilize the bitmap within the VTOC by performing at least one of the following: (1) registering the bitmap with the storage controller at the time the space-efficient volume is initialized; and (2) storing a unique identifier with the bitmap to enable the storage controller to locate the bitmap on the space-efficient volume;
enable a host system to indicate, in the bitmap, whether a track is one of used and unused by marking the corresponding bit;
enable the storage controller to read the bitmap; and
enable the storage controller to free, from the space-efficient volume, tracks that the bitmap indicates are unused.

12. The system of claim 11, wherein the instructions further enable the host system and storage controller to lock the bitmap when accessing data therein.

13. The system of claim 12, wherein the instructions further enable the host system and storage controller to update the bitmap.

14. The system of claim 11, wherein the bitmap is readable by: (1) a storage controller hosting the space-efficient volume; and (2) host-system-based software for managing space in the space-efficient volume.