Patent Publication Number: US-11023333-B2

Title: Online recovery approach to space accounting

Description:
BACKGROUND 
     Data storage systems include storage processors coupled to arrays of non-volatile storage devices, such as magnetic disk drives, electronic flash drives, and/or optical drives. The storage processors are configured to service storage input/output (IO) requests received from host computers, which send the storage IO requests to the data storage systems over one or more networks. The storage IO requests specify data pages, data files, data blocks, and/or other data elements to be written to, read from, created on, and/or deleted from data volumes, file systems, and/or other storage objects stored on the respective non-volatile storage devices. Computer software application programs running on the storage processors are configured to manage the received storage IO requests, and to perform various data processing tasks to organize and/or secure the data elements and/or storage objects on the non-volatile storage devices. 
     From time to time, data files of a file system stored on a data storage system may become corrupted. Such corruption of data files may be due to metadata associated with the data files being erased or failing to update correctly in response to changes in content of the data files, possibly resulting in an inability to access the data file content. Having detected such data file corruption, the data storage system executes a file system checking (FSCK) utility to address and/or repair any inconsistencies in the metadata caused by the data file corruption, thereby restoring the integrity of the file system stored on the data storage system. 
     SUMMARY 
     Having to execute or run an FSCK utility in a data storage system can be problematic, however, in that such an FSCK utility can require hours or days to address and/or repair metadata inconsistencies in a file system, which can have a size of one or more terabytes (Tb). Further, because data files in the file system cannot be written to while the FSCK utility is running, the data storage system is typically brought offline, preventing users from accessing their data stored on the data storage system while file system checking is taking place. 
     Techniques are disclosed herein for performing recovery (i.e., checking and fixing) of space accounting metadata (including counters) while a data storage system is online for regular user data access. Each volume family can include at least one branch, and each branch can include one or more child volumes (e.g., snapshot volumes) that are sequential copies of a parent volume. The disclosed techniques can be performed while the data storage system is online, allowing users of the data storage system full or at least partial access to their stored data while space accounting metadata recovery activities are in progress. The disclosed techniques can include reestablishing a plurality of counters for tracking amounts of physical storage space that are committed and/or unique to the data volumes and/or volume families, including (i) a first counter that can track a first amount of physical storage space (referred to herein as the “VolumeCommittedCount”) committed to each data volume in each branch of a respective volume family, (ii) a second counter that can track an amount of physical storage space (referred to herein as the “FamilyCommittedCount”) committed to a respective volume family, and (iii) a third counter that can track an amount of physical storage space (referred to herein as the “FamilyUniqueCount”) unique to (or unshared by) a respective volume family. 
     The disclosed techniques can further include, while reestablishing the respective counters, conducting a tree walk through a mapping hierarchy of each data volume and/or volume family, marking a starting point and an ending point for the tree walk through the mapping hierarchy, and monitoring a logical offset from the starting point during the tree walk. Upon receipt of a storage input/output (IO) request at the data storage system, the disclosed techniques can determine whether the storage IO request and the space accounting activities are attempting to access the same region of metadata, based on the logical offset relative to the starting point and/or ending point of the tree walk. Based on the result of the determination, the disclosed techniques can update the VolumeCommittedCount, the FamilyCommittedCount, and/or the FamilyUniqueCount, and/or temporary “bookkeeping” versions of the VolumeCommittedCount, the FamilyCommittedCount, and/or the FamilyUniqueCount, as appropriate. By providing techniques for performing recovery (i.e., checking and fixing) of space accounting metadata (including counters) while a data storage system is online for regular user data access, metadata inconsistencies can be addressed and/or repaired while still allowing users of the data storage system full or at least partial access to their stored data. 
     In certain embodiments, a method of performing recovery of space accounting metadata while a data storage system is online for regular user data access includes, in an online process, performing recovery of space accounting metadata of at least one data volume in a volume family, including accessing a region of metadata pertaining to the at least one data volume in the volume family, and maintaining corresponding metadata pertaining to the at least one data volume in the volume family. The method further includes receiving a storage IO request for servicing at the data storage system, and determining whether the servicing of the storage IO request includes accessing the same region of metadata being accessed in the recovery of space accounting metadata of the at least one data volume in the volume family. The method further includes, having determined that the servicing of the storage IO request includes accessing the same region of metadata as being accessed in the recovery of space accounting metadata of the at least one data volume in the volume family, permitting access to the region of metadata for servicing of the storage IO request, thereby assuring prompt access to data stored on the data storage system. 
     In certain arrangements, the performing of the recovery of the space accounting metadata of the at least one data volume in the volume family includes detecting a discrepancy in an amount of physical storage space committed to a respective data volume in a branch of the volume family. 
     In certain arrangements, the performing of the recovery of the space accounting metadata of the at least one data volume in the volume family further includes setting a starting point and an ending point for a tree walk through a mapping hierarchy to verify space accounting metadata for the respective data volume, in which the space accounting metadata corresponds to a first count representative of the amount of physical storage space committed to the respective data volume. 
     In certain arrangements, the performing of the recovery of the space accounting metadata of the at least one data volume in the volume family further includes maintaining corresponding space accounting metadata for the respective data volume, in which the maintained corresponding space accounting metadata corresponds to a newly calculated count representative of the amount of physical storage space committed to the respective data volume. 
     In certain arrangements, the performing of the recovery of the space accounting metadata of the at least one data volume in the volume family further includes performing the tree walk through the mapping hierarchy for the respective data volume from the starting point to the ending point. 
     In certain arrangements, the performing of the recovery of the space accounting metadata of the at least one data volume in the volume family further includes, upon receipt of the storage IO request, checking a current logical offset from the starting point for the tree walk, and determining whether the servicing of the storage IO request includes accessing the same region of metadata being accessed in the recovery of the space accounting metadata based on the current logical offset. 
     In certain arrangements, the method further includes determining that the recovery of the space accounting metadata of the at least one data volume in the volume family has proceeded past the region of metadata being accessed by the storage IO request, and, having determined that the recovery of the space accounting metadata of the at least one data volume in the volume family has proceeded past the region of metadata being accessed by the storage IO request, updating the first count representative of the amount of physical storage space committed to the respective data volume, and updating the newly calculated count representative of the amount of physical storage space committed to the respective data volume. 
     In certain arrangements, the method further includes determining that the recovery of the space accounting metadata of the at least one data volume in the volume family has not proceeded past the region of metadata being accessed by the storage IO request, and, having determined that the recovery of the space accounting metadata of the at least one data volume in the volume family has not proceeded past the region of metadata being accessed by the storage IO request, updating the first count representative of the amount of physical storage space committed to the respective data volume. 
     In certain arrangements, the performing of the recovery of the space accounting metadata of the at least one data volume in the volume family further includes checking a current logical offset from the starting point for the tree walk, and determining whether the recovery of the space accounting metadata of the at least one data volume in the volume family has completed based on the current logical offset reaching the ending point for the tree walk. 
     In certain arrangements, the performing of the recovery of the space accounting metadata of the at least one data volume in the volume family further includes, having determined that the recovery of the space accounting metadata of the at least one data volume in the volume family has completed, replacing the first count representative of the amount of physical storage space committed to the respective data volume with the newly calculated count representative of the amount of physical storage space committed to the respective data volume. 
     In certain arrangements, the performing of the recovery of the space accounting metadata of the at least one data volume in the volume family includes detecting a discrepancy in an amount of physical storage space committed or unique to the volume family. 
     In certain arrangements, the performing of the recovery of the space accounting metadata of the at least one data volume in the volume family further includes setting a starting point and an ending point for a tree walk through a mapping hierarchy to verify space accounting metadata for the volume family, the space accounting metadata corresponding to a count representative of the amount of physical storage space committed or unique to the volume family. 
     In certain arrangements, the performing of the recovery of the space accounting metadata of the at least one data volume in the volume family further includes maintaining corresponding space accounting metadata for the volume family, the maintained corresponding space accounting metadata corresponding to a newly calculated count representative of the amount of physical storage space committed or unique to the volume family. 
     In certain arrangements, the performing of the recovery of the space accounting metadata of the at least one data volume in the volume family further includes checking a current logical offset from the starting point for the tree walk, and determining whether the recovery of the space accounting metadata of the volume family has completed based on the current logical offset reaching the ending point for the tree walk. 
     In certain arrangements, the performing of the recovery of the space accounting metadata of the at least one data volume in the volume family further includes, having determined that the recovery of the space accounting metadata of the volume family has completed, replacing the count representative of the amount of physical storage space committed or unique to the volume family with the newly calculated count representative of the amount of physical storage space committed or unique to the volume family. 
     In certain embodiments, a data storage appliance configured to perform recovery of space accounting metadata while online for regular user data access includes at least one storage device configured to store a plurality of data volumes of a volume family, a memory configured to store executable instructions, and storage processing circuitry configured to execute the executable instructions out of the memory, in an online process, (i) to perform recovery of space accounting metadata of at least one data volume in the volume family, the recovery of the space accounting metadata including accessing a region of metadata pertaining to the at least one data volume in the volume family, and maintaining corresponding metadata pertaining to the at least one data volume in the volume family; (ii) to receive a storage IO request; (iii) to determine whether servicing of the storage IO request includes accessing the same region of metadata being accessed in the recovery of the space accounting metadata of the at least one data volume in the volume family; and (iv) having determined that the servicing of the storage IO request includes accessing the same region of metadata as being accessed in the recovery of the space accounting metadata of the at least one data volume in the volume family, to permit access to the region of metadata for servicing of the storage IO request, thereby assuring prompt access to data stored on the data storage system. 
     Other functions and aspects of the claimed features of this disclosure will be evident from the Detailed Description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages will be apparent from the following description of particular, embodiments of the disclosure, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. 
         FIG. 1 a    is a block diagram of an exemplary clustered storage environment, in which techniques may be practiced for performing recovery of space accounting metadata while a data storage system is online for regular user data access; 
         FIG. 1 b    is a block diagram of an exemplary data storage appliance included in the clustered storage environment of  FIG. 1   a;    
         FIG. 1 c    is a block diagram of an exemplary data storage node included in the data storage appliance of  FIG. 1   b;    
         FIG. 2  is a block diagram of an exemplary volume family including a plurality of branches, each of which includes a sequence of data volumes; 
         FIG. 3  is a block diagram of an exemplary namespace index node (Mode) configured to include a volume family identifier (ID), a branch ID, and a volume ID for each data volume in a volume family; 
         FIG. 4  is a block diagram of an exemplary mapping hierarchy for a plurality of exemplary data volumes in a volume family; 
         FIG. 5 a    is a block diagram of an exemplary first key-value store for storing a plurality of first key-value pairs, each key in a first key-value pair including a volume family ID, a branch ID, and a volume ID, and each value in the first key-value pair providing an indication of an amount of physical storage space (the “VolumeCommittedCount”) committed to a respective data volume in a respective branch of a respective volume family; 
         FIG. 5 b    is a block diagram of a detailed view of the first key-value store of  FIG. 5   a;    
         FIG. 5 c    is a block diagram of an exemplary second key-value store for storing a plurality of second key-value pairs, each key in a second key-value pair including a volume family ID, and each value in the second key-value pair providing indications of an amount of physical storage space (the “FamilyCommittedCount”) committed to a respective volume family, and an amount of physical storage space (the “FamilyUniqueCount”) unique to (or unshared by) the respective volume family; 
         FIG. 5 d    is a block diagram of a detailed view of the second key-value store of  FIG. 5 c   ; and 
         FIG. 6  is a flow diagram of an exemplary method of performing recovery of space accounting metadata while a data storage system is online for regular user data access. 
     
    
    
     DETAILED DESCRIPTION 
     Techniques are disclosed herein for performing recovery of space accounting metadata while a data storage system is online for regular user data access. The disclosed techniques can include reestablishing a plurality of counters for tracking amounts of physical storage space that are committed and/or unique to the data volumes and/or volume families. The disclosed techniques can further include, while reestablishing the respective counters, conducting a tree walk through a mapping hierarchy of each data volume and/or volume family, marking a starting point and an ending point for the tree walk through the mapping hierarchy, and monitoring a logical offset from the starting point during the tree walk. Upon receipt of a storage input/output (IO) request at the data storage system, the disclosed techniques can determine whether or not the storage IO request and the space accounting metadata recovery activities are attempting to access the same region of metadata based on the logical offset relative to the starting point and/or ending point of the tree walk, and update the respective counters and/or temporary “bookkeeping” versions of the counters, as appropriate. By providing techniques for performing recovery (i.e., checking and fixing) of space accounting metadata (including counters) while a data storage system is online for regular user data access, metadata inconsistencies can be addressed and/or repaired while still allowing users of the data storage system full or at least partial access to their stored data. 
       FIG. 1 a    depicts an illustrative embodiment of a clustered storage environment  100 , in which techniques can be practiced for performing recovery (i.e., checking and fixing) of space accounting metadata (including counters) while a data storage system is online for regular user data access. As shown in  FIG. 1 a   , the clustered storage environment  100  can include a plurality of host computers  102 . 1 ,  102 . 2 , . . . ,  102 . n , at least one storage domain  104 , and a system administrator computer  107 , which are interconnected by a communications medium  103  that can include at least one network  106 . For example, each of the plurality of host computers  102 . 1 , . . . ,  102 . n  may be configured as a web server computer, a file server computer, an email server computer, an enterprise server computer, or any other suitable client or server computer or computerized device. Further, the system administrator computer  107  may be remote from (such as in a data center) or local to the storage domain  104  within the clustered storage environment  100 . 
     As further shown in  FIG. 1 a   , the storage domain  104  can include, as members of the storage domain  104 , a plurality of data storage appliances  108 . 1 ,  108 . 2 , . . . ,  108 . m . In the storage domain  104 , the data storage appliance  108 . 1  can be elected or otherwise designated to perform (at least temporarily) a role of a primary storage appliance, while each of the remaining data storage appliances  108 . 2 , . . . ,  108 . m  perform (at least temporarily) a role of a secondary storage appliance. The storage domain  104  can further include a local area network (LAN)  110  such as an Ethernet network or any other suitable network, which is configured to interconnect the plurality of data storage appliances  108 . 1 , . . . ,  108 . m . A plurality of LANs (like the LAN  110 ) included in a plurality of storage domains (like the storage domain  104 ) can be interconnected by a network  105 , such as a metropolitan area network (MAN), a wide area network (WAN), or any other suitable network. 
     Within the clustered storage environment  100  of  FIG. 1 a   , the system administrator computer  107  can be configured to execute program instructions to enable a system administrator or other user to define and/or configure the storage domain  104 . Further, the plurality of host computers  102 . 1 , . . . ,  102 . n  can be configured to provide, over the network  106 , storage input/output (IO) requests (e.g., small computer system interface (SCSI) commands, network file system (NFS) commands) to the respective storage appliances (primary or secondary)  108 . 1 , . . . ,  108 . m  of the storage domain  104 . For example, such storage IO requests (e.g., read requests, write requests) may direct the respective storage appliances (primary or secondary)  108 . 1 , . . . ,  108 . m  to read and/or write data pages, data files, data blocks, and/or any other suitable data elements from/to data volumes (e.g., virtual volumes (VVOLs), logical units (LUNs)), file systems, and/or any other suitable storage objects stored in association with the respective storage appliances  108 . 1 , . . . ,  108 . m.    
     The communications medium  103  can be configured to interconnect the plurality of host computers  102 . 1 , . . . ,  102 . n  with the respective storage appliances  108 . 1 , . . . ,  108 . m  of the storage domain  104  to enable them to communicate and exchange data/control signals. As shown in  FIG. 1 a   , the communications medium  103  is illustrated as a “cloud” to represent a variety of different communications topologies, including, but not limited to, a backbone topology, a hub and spoke topology, a loop topology, an irregular topology, or any suitable combination thereof. As such, the communications medium  103  can include, but is not limited to, copper-based data communications devices and cabling, fiber optic devices and cabling, wireless devices, or any suitable combination thereof. Further, the communications medium  103  can be configured to support storage area network (SAN)-based communications, network attached storage (NAS)-based communications, LAN-based communications, MAN-based communications, WAN-based communications, wireless communications, distributed infrastructure communications, and/or any other suitable wired, wireless, or fiber communications. 
       FIG. 1 b    depicts an illustrative embodiment of an exemplary data storage appliance  108  included in the storage domain  104  of  FIG. 1 a   . It is noted that each of the data storage appliances (primary or secondary)  108 . 1 , . . . ,  108 . m  included in the storage domain  104  can be configured like the data storage appliance  108  of  FIG. 1 b   . As shown in  FIG. 1 b   , the data storage appliance  108  can include two (2) data storage nodes  112 . 1 ,  112 . 2  for providing high availability within the clustered storage environment  100 . In the data storage appliance  108 , the data storage node  112 . 1  can be elected or otherwise designated to perform (at least temporarily) a role of a primary storage node, while the data storage node  112 . 2  performs (at least temporarily) a role of a secondary storage node. For example, in the data storage appliance  108 , the data storage node (primary)  112 . 1  may (i) receive storage IO requests from one or more of the host computers  102 . 1 , . . . ,  102 . n  over the network  106 , (ii) in response to the storage IO requests, read and/or write data pages, data files, data blocks, and/or any other suitable data elements from/to one or more VVOLs, LUNs, file systems, and/or any other suitable storage objects stored in association with the data storage node (primary)  112 . 1 , and, (iii) at least at intervals, synchronize data stored in association with the data storage node (primary)  112 . 1  with corresponding data stored in association with the data storage node (secondary)  112 . 2 . In the event of a failure of the data storage node (primary)  112 . 1 , the data storage node (secondary)  112 . 2  can assume the role of the primary storage node, providing high availability within the clustered storage environment  100 . 
       FIG. 1 c    depicts an illustrative embodiment of an exemplary data storage node  112  included in the data storage appliance  108  of  FIG. 1 b   . It is noted that each of the data storage nodes (primary and secondary)  112 . 1 ,  112 . 2  of  FIG. 1 b    can be configured like the data storage node  112  of  FIG. 1 c   . As shown in  FIG. 1 c   , the data storage node  112  can include a communications interface  116 , storage processing circuitry  118 , and a memory  120 . The communications interface  108  can include SCSI target adapters, network interface adapters, and/or any other suitable adapters for converting electronic, wireless, and/or optical signals received over the network  106  to a form suitable for use by the storage processing circuitry  118 . The memory  120  can include persistent memory (e.g., flash memory, magnetic memory) and non-persistent cache memory (e.g., dynamic random access memory (DRAM), static random access memory (SRAM)), and can accommodate a variety of specialized software constructs, including, but not limited to, namespace management code and data (also referred to herein as the “namespace manager”; see reference numeral  126 ) and mapping management code and data (also referred to herein as the “mapping manager”; see reference numeral  128 ). 
     The namespace manager  126  can be configured to maintain a namespace of storage objects, such as volumes (e.g., VVOLs, LUNs), file systems, and/or any other suitable storage objects, accessible to the plurality of host computers  102 . 1 , . . . ,  102 . n . In general, a namespace is a point-in-time (PIT) logical collection of such storage objects, each of which can be represented by an index node (also referred to herein as an “inode”). In one embodiment, the namespace maintained by the namespace manager  126  can include a set of storage objects (e.g., VVOLs) organized as a collection of inodes. For example, each such VVOL may be made up of one or more extents, each of which may correspond to a range of storage sizes (e.g., 1 megabyte (Mb), 4 Mbs) in a logical address space. Further, the range of storage sizes may correspond to a range of contiguous or noncontiguous logical addresses spanning some or all of the VVOL. 
     The mapping manager  128  can be configured to map extents of volumes (e.g., VVOLs, LUNs) to corresponding redundant array of independent disk (RAID) addresses, which, in turn, can be mapped to corresponding drive locations in one or more underlying storage units  114 , such as magnetic disk drives, electronic flash drives, and/or any other suitable storage drives. The storage unit(s)  114  can be configured to store storage objects  122  such as volumes (e.g., VVOLs), file systems, and/or any other suitable storage objects, as well as metadata  124  such as a namespace superblock  130 , a mapper superblock  132 , one or more VLB pages  134 , and one or more bookkeeping data structures  136 , each of which can be employed in the techniques disclosed herein. It is noted that the storage unit(s)  114  can be locally attached to an 10 channel of the data storage node  112 , while also being accessible over the network  106 . It is further noted that the storage unit(s)  114  can be implemented as a system of storage drives or devices, such as a collection of drives (e.g., a RAID group). In one embodiment, the storage unit(s)  114  can be implemented as a dual-ported drive, which can be shared between the data storage node (primary)  112 . 1  and the data storage node (secondary)  112 . 2  of the data storage appliance  108 . 
     The storage processing circuitry  118  can include one or more physical storage processors or engines running specialized software, data movers, director boards, blades, IO modules, storage drive controllers, switches, and/or any other suitable computer hardware or combination thereof. In one embodiment, the storage processing circuitry  118  can process storage IO requests provided by the respective host computers  102 . 1 , . . . ,  102 . n  over the communications medium  103 , and store host data in a RAID environment implemented by the storage unit(s)  114 . 
     In the context of the storage processing circuitry  118  being implemented using one or more processors running specialized software, a computer program product can be configured to deliver all or a portion of the software constructs to the respective processor(s). Such a computer program product can include one or more non-transient computer-readable storage media, such as a magnetic disk, a magnetic tape, a compact disk (CD), a digital versatile disk (DVD), an optical disk, a flash drive, a solid state drive (SSD), a secure digital (SD) chip or device, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), and so on. The non-transient computer-readable storage media can be encoded with sets of instructions that, when executed by the respective processor(s), perform the techniques disclosed herein. For example, such media may be considered to be articles of manufacture, and may be transportable from one data storage appliance to another data storage appliance. 
     During operation, the data storage node  112  (see  FIG. 1 c   ) included in the data storage appliance  108  (see  FIG. 1 b   ) can perform recovery (i.e., checking and fixing) of space accounting metadata (including counters) of data volumes and/or volume families stored on the storage unit(s)  114 . Each volume family can include at least one branch, and each branch can include one or more child volumes (e.g., snapshot volumes) that are sequential copies of a parent volume. The data storage node  112  can perform such recovery of space accounting metadata of data volumes and/or volume families while remaining online, allowing users of the data storage appliance  108  full or at least partial access to their stored data while space accounting metadata recovery activities are in progress. The data storage node  112  can reestablish a plurality of counters for tracking amounts of physical storage space that are committed and/or unique to the data volumes and/or volume families, including (i) a first counter that can track a first amount of physical storage space (referred to herein as the “VolumeCommittedCount”) that is committed to each data volume in each branch of a respective volume family, (ii) a second counter that can track an amount of physical storage space (referred to herein as the “FamilyCommittedCount”) that is committed to a respective volume family, and (iii) a third counter that can track an amount of physical storage space (referred to herein as the “FamilyUniqueCount”) that is unique to (or unshared by) a respective volume family. 
     The data storage node  112  can also, while reestablishing the respective counters, conduct a tree walk through a mapping hierarchy of each data volume and/or volume family, marking a starting point and an ending point for the tree walk through the mapping hierarchy, and monitoring a logical offset from the starting point during the tree walk. Upon receipt of a storage input/output (IO) request, the data storage node  112  can determine whether or not the storage IO request and the space accounting metadata recovery activities are attempting to access the same region of metadata, based on the logical offset relative to the starting point and/or ending point of the tree walk. Based on the result of the determination, the data storage node  112  can update the VolumeCommittedCount, the FamilyCommittedCount, and/or the FamilyUniqueCount, and/or temporary “bookkeeping” versions of the VolumeCommittedCount, the FamilyCommittedCount, and/or the FamilyUniqueCount maintained in the bookkeeping data structure(s)  136 , as appropriate. By performing recovery of space accounting metadata of data volumes and/or volume families while online, the data storage node  112  can address and/or repair metadata inconsistencies while still allowing users of the data storage appliance  108  full or at least partial access to their stored data. 
       FIG. 2  depicts an acyclic graph  200  representing an exemplary volume family VF 1 . The volume family VF 1  includes a plurality of branches B 1 , B 2 , B 3 , each of which includes one or more read-only snapshot volumes that are sequential copies of a writable parent volume. As shown in  FIG. 2 , the branch B 1  includes a plurality of snapshot volumes T 1 , T 2 , T 3 , T 4 , which are sequential point-in-time (PIT) copies of a parent volume T 5  (also referred to herein as the “primary volume”). The branch B 2  includes a plurality of snapshot volumes T 6 , T 7 , which are sequential PIT copies of a parent volume T 8  (also referred to herein as a “clone volume”). The branch B 3  includes a single snapshot volume T 9 , which is a PIT copy of a parent volume T 10  (also referred to herein as a “clone volume”). It is noted that the volumes T 1 , T 2 , T 3 , T 4 , T 5  in the branch B 1  may each correspond to a version of a volume V 1 , the volumes T 6 , T 7 , T 8  in the branch B 2  may each correspond to a version of a volume V 2 , and the volumes T 9 , T 10  in the branch B 3  may each correspond to a version of a volume V 3 . 
     In general, an acyclic graph representing a volume family (such as the acyclic graph  200  representing the volume family VF 1 ; see  FIG. 2 ) can be constructed by assigning a volume identifier (ID) to a newly created primary volume, and, over time, assigning monotonically increasing volume IDs to the primary volume and one or more sequential snapshot copies of the primary volume to form a first branch of the volume family (as illustrated in  FIG. 2  by the monotonically increasing tag designations T 1 , T 2 , T 3 , T 4  of the four (4) snapshot volumes and T 5  of the single primary volume in the branch B 1  of the volume family VF 1 ). It is noted that the writable primary volume (e.g., the primary volume T 5 ; see  FIG. 2 ) is assigned the latest volume ID in the first branch (e.g., the branch B 1 ; see  FIG. 2 ) of the volume family (e.g., the volume family VF 1 ; see  FIG. 2 ). In other words, if a snapshot volume is created based on a primary volume of a volume family, then the snapshot volume is assigned the current latest volume ID in a first branch of the volume family, while the primary volume is assigned a new latest volume ID in the first branch of the volume family. 
     Having obtained at least part of the first branch of the volume family, a writable clone volume can be created based on a selected snapshot volume in the first branch. With reference to the acyclic graph  200  representing the volume family VF 1  (see  FIG. 2 ), it is understood that an initial version of the clone volume T 8  in the branch B 2  was created based on the snapshot volume T 4  in the branch B 1 . Similarly, an initial version of the clone volume T 10  in the branch B 3  was created based on the snapshot volume T 2  in the branch B 1 . Like the primary volume (e.g., the primary volume T 5 ; see  FIG. 2 ) in the first branch of the volume family described hereinabove, each clone volume (e.g., the clone volume T 8  or T 10 ; see  FIG. 2 ) is assigned the latest volume ID in a new branch (e.g., the branch B 2  or B 3 ; see  FIG. 2 ) of the volume family. In other words, if a snapshot volume is created based on a clone volume in a new branch of a volume family, then the snapshot volume is assigned the current latest volume ID in the new branch of the volume family, while the clone volume is assigned a new latest volume ID in the new branch of the volume family. It is noted that the first branch and subsequent new branches of a volume family are assigned monotonically increasing branch IDs (as illustrated in  FIG. 2  by the monotonically increasing tag designations B 1 , B 2 , B 3  of the three (3) branches in the volume family VF 1 ). Further, each branch of a volume family has a corresponding branch root volume. 
     To facilitate the space accounting metadata recovery activities performed by the data storage node  112  (see  FIG. 1 c   ), the data storage node  112  can assign, to each data volume in a volume family, (i) a corresponding volume family ID (“Family ID”), (ii) a corresponding branch ID (“Branch ID”), and (iii) a corresponding volume ID (“Volume ID”). In one embodiment, the namespace manager  126  (see  FIG. 1 c   ) can be configured to maintain a Family ID, a Branch ID, and a Volume ID for each data volume in a volume family stored in association with the data storage node  112 . As described herein, the namespace maintained by the namespace manager  126  can include a set of storage objects (e.g., VVOLs) organized as a collection of inodes. Such a collection of inodes can be organized with associated namespace metadata, including a namespace inode that can be configured to store information regarding the collection of inodes (including the Family ID, the Branch ID, and the Volume ID for each data volume in a volume family) in an inode file. 
       FIG. 3  depicts an exemplary namespace inode  302  that can be maintained by the namespace manager  126  of the data storage node  112 . As shown in  FIG. 3 , the namespace inode  302  can be configured to store a Family ID, a Branch ID, and a Volume ID for one or more data volumes stored in association with the data storage node  112 . For example, for an exemplary data volume “V 0 ” (“Volume  0 ”) in a volume family, the namespace inode  302  may store a Family ID, “FID- 0 ,” a Branch ID, “BID- 0 ,” and a Volume ID, “VID- 0 .” The namespace inode  302  can be further configured to store, for specific data volumes, a space accounting (SA) in-progress flag (T/F), and an indirect block pointer, “Indirect block- 0 ,” which points to an indirect block  304 . The indirect block  304  can be configured to store information pertaining to the set of volumes included in the namespace maintained by the namespace manager  126 . For example, the stored information may include an entry  306  that stores information pertaining to the Volume  0 , including a real inode number (“RN”) for the Volume  0 , as well as a virtual inode number (“VIN”) for the Volume  0 . It is further noted that, while the storage processing circuitry  118  services a storage IO request from one of the host computers  102 . 1 , . . . ,  102 . n  for reading/writing a data page “0” from/to the Volume  0 , the namespace manager  126  can incorporate the appropriate Family ID, Branch ID, and Volume ID into the storage IO request before it is forwarded along a write path to the mapping manager  128 . 
     To further facilitate the space accounting metadata recovery activities performed by the data storage node  112  (see  FIG. 1 c   ), the data storage node  112  can maintain an owner volume ID for each data page of a data volume stored in association with the data storage node  112 . As employed herein, the owner volume ID provides, for each data page, an indication of the data volume to which the data page was last written. In one embodiment, for each data page, the owner volume ID can be stored in a corresponding leaf page at a leaf level of a mapping hierarchy, which can be maintained by the mapping manager  128 . 
       FIG. 4  depicts an exemplary mapping hierarchy  400  for a plurality of data volumes (such as a volume  406  and a volume  408 ) in a volume family. As shown in  FIG. 4 , the mapping hierarchy  400  can be configured as a multi-level tree (e.g., a B+ tree) that includes at least a volume level  402  and a leaf level  404 . The volume level  402  can have nodes corresponding to at least the volume  406  and the volume  408 , and the leaf level  404  can have nodes corresponding to at least a leaf page  410 , a leaf page  412 , and a leaf page  414 . It is noted, however, that the multi-level tree of the mapping hierarchy  400  can include many more levels than the two levels  402 ,  404 . For example, the multi-level tree may include a multitude of volume levels above the volume level  402 . 
     As shown in  FIG. 4 , the node corresponding to the volume  406  can include information, attributes, or metadata corresponding to a parent  416 , a number of children  418 , a first child  420 , a previous sibling  422 , and a next sibling  424 . Likewise, the node corresponding to the volume  408  can include information, attributes, or metadata corresponding to a parent  434 , a number of children  436 , a first child  438 , a previous sibling  440 , and a next sibling  442 . The parent attributes  416 ,  434  correspond to pointers to locations of parent volumes of the respective volumes  406 ,  408 , if any. For example, the parent attribute  434  of the volume  408  may point to a location of the volume  406 , which may be the parent volume of the volume  408 . The number of children attributes  418 ,  436  provide indications of the number of child volumes of the respective volumes  406 ,  408 , if any. The first child attributes  420 ,  438  correspond to pointers to locations of first child volumes of the respective volumes  406 ,  408 , if any. For example, the first child attribute  420  of the volume  406  may point to a location of the volume  408 , which may be the first child volume (e.g., snapshot volume) of the volume  406 . It is noted that, once the first child volumes of the volumes  406 ,  408  are located, additional child volumes of the respective volumes  406 ,  408  may be located by following associated previous and/or next sibling pointers. The previous sibling attributes  422 ,  440  correspond to pointers to locations of previous sibling volumes for child volumes of the respective volumes  406 ,  408 , if any. The next sibling attributes  424 ,  442  correspond to pointers to locations of next sibling volumes for child volumes of the respective volumes  406 ,  408 , if any. As described herein, the owner volume ID for each data page of a data volume can be stored in a corresponding leaf page (such as the leaf page  410 ,  412 , or  414 ; see  FIG. 4 ) at the leaf level  404  of the mapping hierarchy  400 . The leaf page  410  can include an attribute or metadata corresponding to an owner volume ID  452 . Likewise, the leaf page  412  can include an attribute corresponding to an owner volume ID  454 , and the leaf page  414  can include an attribute or metadata corresponding to an owner volume ID  456 . 
     As further shown in  FIG. 4 , the node corresponding to the volume  406  can further include leaf pointers (such as a leaf pointer P 1   428  and a leaf pointer P 2   432 ) to locations of the leaf page  410 , the leaf page  412 , and/or the leaf page  414 . For example, the leaf pointer P 1   428  may point to a location of the leaf page  410 , and the leaf pointer P 2   432  may point to a location of the leaf page  412 . Likewise, the node corresponding to the volume  408  can further include leaf pointers (such as a leaf pointer P 1   446  and a leaf pointer P 2   450 ) to locations of the leaf page  410 , the leaf page  412 , and/or the leaf page  414 . For example, the leaf pointer P 1   446  may point to a location of the leaf page  410 , and the leaf pointer P 2   450  may point to a location of the leaf page  414 . In addition, each of the leaf pointer P 1   428 , the leaf pointer P 2   432 , the leaf pointer P 1   446 , and the leaf pointer P 2   450  can include a source (“S”) attribute or a copy (“C”) attribute. For example, the leaf pointer P 1   428  may include a source (S) attribute  426 , which indicates that the volume  406  is the source of a data page (e.g., 4 kilobytes (kb)) corresponding to the leaf page  410 ; and, the leaf pointer P 2   432  may include a source (S) attribute  430 , which indicates that the volume  406  is the source of a data page corresponding to the leaf page  412 . Further, the leaf pointer P 1   446  may include a copy (C) attribute  444 , which indicates that the volume  406  shares a copy of the data page corresponding to the leaf page  410  with the volume  408 ; and, the leaf pointer P 2   450  may include a source (S) attribute  448 , which indicates that the volume  408  is the source of a data page corresponding to the leaf page  414 . It is noted that each of the leaf pages  410 ,  412 , and  414  can further include pointers (not shown) to their respective corresponding data pages. It is further noted that the various information, attributes, metadata, and/or pointers contained in the mapping hierarchy  400 , which can describe mappings between physical blocks, virtual blocks, and/or logical blocks, are also referred to herein collectively as the “mapping metadata.” 
     As described herein, the space accounting metadata recovery activities performed by the data storage node  112  can include maintaining (i) a first counter that can track a first amount of physical storage space (the “VolumeCommittedCount”) committed to each data volume in each branch of a respective volume family, (ii) a second counter that can track an amount of physical storage space (the “FamilyCommittedCount”) committed to a respective volume family, and (iii) a third counter that can track an amount of physical storage space (the “FamilyUniqueCount”) unique to (or unshared by) a respective volume family. 
     In one embodiment, the mapping manager  128  can maintain, for each data volume in each branch of a respective volume family, the VolumeCommittedCount in a key-value store  504 , as illustrated in  FIGS. 5 a  and 5 b   . As shown in  FIG. 5 a   , the mapping superblock (SB)  132  can contain a pointer  502  (the “mapper SB pointer”) to the key-value store  504 . Further, as shown in  FIGS. 5 a  and 5 b   , the key-value store  504  can include a plurality of keys 0, 1, . . . , p, . . . , q that point to or are otherwise paired with a plurality of values 0, 1, . . . , p, . . . , q, respectively. In one embodiment, the “key” in a key-value pair can be implemented by a Family ID, a Branch ID, and a Volume ID of a data volume, while the “value” in the key-value pair can include the VolumeCommittedCount. For example, the key-0 (see  FIG. 5 b   ) of a 0 th  key-value pair may be implemented by the Family ID  506 , a Branch ID  510 , and a Volume ID  512  of a volume in a branch of a respective volume family, while the value-0 (see also  FIG. 5 b   ) of the 0 th  key-value pair may include a VolumeCommittedCount  518  of the volume in the branch of the respective volume family. Likewise, the key-p (see  FIG. 5 b   ) of an p th  key-value pair may be implemented by the Family ID  508 , a Branch ID  514 , and a Volume ID  516  of a volume in a branch of a respective volume family, while the value-0 (see also  FIG. 5 b   ) of the p th  key-value pair may include a VolumeCommittedCount  520  of the volume in the branch of the respective volume family. 
     In one embodiment, the mapping manager  128  can further maintain, for each volume family, both the FamilyCommittedCount and the FamilyUniqueCount in a key-value store  524 , as illustrated in  FIGS. 5 c  and 5 d   . As shown in  FIG. 5 c   , the mapping superblock (SB)  132  can further contain a pointer  522  (the “mapper SB pointer”) to the key-value store  524 . Further, as shown in  FIGS. 5 c  and 5 d   , the key-value store  524  can include a plurality of keys 0, 1, . . . , i, . . . , j that point to or are otherwise paired with a plurality of values 0, 1, . . . , i, . . . , j, respectively. In one embodiment, the “key” in a key-value pair can be implemented by a Family ID of a volume family, while the “value” in the key-value pair can include both the FamilyCommittedCount and the FamilyUniqueCount. For example, the key-0 (see  FIG. 5 d   ) of a 0 th  key-value pair may be implemented by the Family ID  506  of the respective volume family, while the value-0 (see also  FIG. 5 d   ) of the 0 th  key-value pair may include a FamilyCommittedCount  526  and a FamilyUniqueCount  528  of the respective volume family. Likewise, the key-i (see  FIG. 5 d   ) of an i th  key-value pair may be implemented by the Family ID  508  of the respective volume family, while the value-i (see also  FIG. 5 d   ) of the i th  key-value pair may include a FamilyCommittedCount  530  and a FamilyUniqueCount  532  of the respective volume family. 
     The disclosed techniques for performing recovery (i.e., checking and fixing) of space accounting metadata (including counters) while a data storage system is online for regular user data access will be further understood with reference to the following illustrative example, as well as the volume family VF 1  illustrated in  FIG. 2 . In this example, it is assumed that a file system checking (FSCK) utility has been executed or run, in an offline process, on the data storage node  112  to address and/or repair inconsistencies in the mapping metadata contained in the mapping hierarchy  400 . It is noted, however, that the disclosed techniques can be performed as a planned invocation of online space accounting metadata recovery activities, without first having executed the FSCK utility. 
     Having executed the FSCK utility in an offline process, the data storage node  112  is brought back online, allowing the data storage node  112  to service storage IO requests (e.g., read requests, write requests) received from the host computers  102 . 1 , . . . ,  102 . n . Further, while online, the data storage node  112  performs recovery of space accounting metadata of one or more data volumes and/or volume families stored on the storage unit(s)  114 . To that end, the data storage node  112  first performs recovery of space accounting metadata to address and/or repair a detected discrepancy in the VolumeCommittedCount for an exemplary specific data volume. For example, the specific data volume may be part of the volume family VF 1 , and may be assigned Family ID  506 , Branch ID  510 , and Volume ID  512 . Further, the VolumeCommittedCount for the specific data volume, for which a discrepancy has been detected, may be maintained in the key-value store  504  as the VolumeCommittedCount  518 . 
     Having detected the discrepancy in the VolumeCommittedCount  518  for the specific data volume, the data storage node  112  sets a starting point and an ending point for a tree walk through the mapping hierarchy  400  to verify the space accounting metadata for the respective volume. Further, the data storage node  112  stores indications of the starting and ending points in the namespace superblock  130 . It is noted that, upon receipt of a storage IO request while performing online space accounting metadata recovery activities, the data storage node  112  can check for any indications of such starting and ending points in the namespace superblock  130  to confirm that the space accounting metadata recovery activities are in-progress, and to subsequently service the storage IO request in an appropriate manner. Once the starting point and the ending point corresponding to the tree walk for the specific data volume are set, the data storage node  112  sets the space accounting (SA) in-progress flag for the specific data volume to “true” (SA in-progress flag, T/F, in namespace inode  302 ; see  FIG. 3 ), sets each step or increment of a logical offset from the starting point for the tree walk to a predetermined value (such as 2 megabytes (Mb) or any other suitable value), and stores the step or increment of the logical offset in the bookkeeping data structure  136 . In addition, the data storage node  112  creates, in the bookkeeping data structure  136 , a temporary key-value store like the key-value store  504 , but configured for storing a newly calculated VolumeCommittedCount for the specific data volume. 
     Now that on-line space accounting metadata recovery activities are in-progress, as indicated by the settings of the starting and ending points in the namespace superblock  130  and the SA in-progress flag in the namespace inode  302 , the data storage node  112  performs a tree walk through the mapping hierarchy  400  for the specific data volume from the starting point to the ending point, beginning at a logical offset equal to “0.” It is noted that an indication of the logical offset for the tree walk is updated at intervals in the bookkeeping data structure  136  based on the stored step or increment setting, e.g., at each 2 Mb step or increment of the tree walk. While performing the tree walk through the mapping hierarchy  400  for the specific data volume, the data storage node  112  maintains a count of the source (S) attributes (e.g., the source (S) attributes  426 ,  430 ; see  FIG. 4 ), each of which indicates that the specific data volume is the source of a data page pointed to by a corresponding leaf page. Further, the data storage node  112  updates the VolumeCommittedCount for the specific data volume in the temporary key-value store created in the bookkeeping data structure  136 , based on the count of the source (S) attributes in the mapping metadata for the specific data volume. 
     Upon receipt of a storage IO request (e.g., read request, write request), the data storage node  112  checks the indications of the starting and ending points of online space accounting metadata recovery activities stored in the namespace superblock  130  to confirm that such space accounting metadata recovery activities are in-progress. Further, the data storage node  112  checks the SA in-progress flag for the specific data volume stored in the namespace Mode  302 , and discovers that the SA in-progress flag for the specific data volume is set to “true.” In addition, the data storage node  112  checks a current logical offset from the starting point for the tree walk stored in the bookkeeping data structure  136 , and determines, based on the current logical offset, whether the space accounting metadata recovery activities and the storage IO request are accessing (or attempting to access) the same region of metadata. In one embodiment, if the space accounting metadata recovery activities are accessing the same region of metadata as the storage IO request, then any queries of the VolumeCommittedCount for the specific data volume can be serviced from the key-value store  504  to assure prompt access to the volume data. Further, if any errors are detected relating to the VolumeCommittedCount at the time of servicing the storage IO request, then such errors can be reported as appropriate. If, based on the current logical offset, it is determined that the space accounting metadata recovery activities have proceeded past the region of metadata being accessed by the storage IO request, then the VolumeCommittedCount for the specific data volume can be updated, as required, in both the temporary key-value store in the bookkeeping data structure  136  and in the key-value store  504 . If, based on the current logical offset, it is determined that the space accounting metadata recovery activities have not proceeded past the region of metadata being accessed by the storage IO request, then the VolumeCommittedCount for the specific data volume can be updated, as required, in the key-value store  504 . 
     Once the space accounting metadata recovery activities for addressing and/or repairing the detected discrepancy in the VolumeCommittedCount for the specific data volume have completed (e.g., the logical offset has reached the ending point for the tree walk), the temporary key-value store in the bookkeeping data structure  136  can replace the key-value store  504 , the mapper SB pointer  502  can be modified to point to the key-value store replacing the key-value store  504 , the indications of the starting and ending points in the namespace superblock  130  can be cleared, the SA in-progress flag for the specific data volume can be set to “false” (SA in-progress flag, T/F, in namespace inode  302 ; see  FIG. 3 ), and the prior key-value store  504  can be discarded. 
     Having addressed and/or repaired the detected discrepancy in the VolumeCommittedCount for the specific data volume, the data storage node  112  can further perform recovery of space accounting metadata to address and/or repair one or more detected discrepancies in the FamilyCommittedCount and/or the FamilyUniqueCount for the volume family VF 1 . For example, the volume family VF 1  may be assigned Family ID  506 , and the FamilyCommittedCount and the FamilyUniqueCount for the volume family VF 1  may each be maintained in the key-value store  524  as the FamilyCommittedCount  526  and the FamilyUniqueCount  528 , respectively. 
     Having detected the discrepancies in the FamilyCommittedCount  526  and/or the FamilyUniqueCount  528  for the volume family VF 1 , the data storage node  112  sets a starting point and an ending point for a tree walk through the mapping hierarchy  400  to verify the space accounting metadata for the volume family VF 1 . Further, the data storage node  112  stores indications of the starting and ending points in the namespace superblock  130 . It is noted that, upon receipt of a storage IO request while performing online space accounting metadata recovery activities, the data storage node  112  can check for any indications of such starting and ending points in the namespace superblock  130  to confirm that space accounting metadata recovery activities are in-progress, and to subsequently service the storage IO request in an appropriate manner. Once the starting point and the ending point corresponding to the tree walk for the volume family VF 1  are set, the data storage node  112  sets each step or increment of a logical offset from the starting point for the tree walk to a predetermined value (e.g., 2 Mb or any other suitable value), and stores the step or increment of the logical offset in the bookkeeping data structure  136 . Further, as the space accounting metadata is verified for each respective data volume in the volume family VF 1  (as described hereinabove for the exemplary specific data volume), the data storage node  112  sets the SA in-progress flag for the respective data volume to “true” (SA in-progress flag, T/F, in namespace inode  302 ; see  FIG. 3 ). In addition, the data storage node  112  creates, in the bookkeeping data structure  136 , a temporary key-value store like the key-value store  524 , but configured for storing a newly calculated FamilyCommittedCount, as well as a newly calculated FamilyUniqueCount, for the volume family VF 1 . 
     It is noted that the FamilyUniqueCount for the volume family VF 1  corresponds to the amount of physical storage space that would be freed if the volume family VF 1  were deleted. The amount of physical storage space allocated to the respective data volumes in the volume family VF 1  is deemed to be unique to the volume family VF 1 , so long as a deduplication domain for each deduplicated data page written to the respective data volumes consists of data segments within the volume family VF 1 . Such deduplication of a data page can cause one or more data segments of the data page to be shared among different logical addresses within the same data volume or across different data volumes. For example, each such data segment embodied as a data block may maintain a reference count to indicate a number of times that the data block is shared. Further, a reference count equal to “0” may indicate that the data block is not in use and may be reclaimed, a reference count equal to “1” may indicate that the data block is in use but not shared, and a reference count greater than “1” may indicate that the data block is in use and shared within a single data volume or between different data volumes. Indications of such reference counts relating to data block sharing within the volume family VF 1  can be stored in the VLB pages  134  for the volume family VF 1 , and can be considered as part of the metadata for the volume family VF 1 . In addition, during verification of the space accounting metadata for the volume family VF 1 , the data storage node  112  creates, in the bookkeeping data structure  136 , temporary VLB pages like the VLB pages  134 , but configured for storing newly calculated reference counts for the volume family VF 1 . 
     Upon receipt of a storage IO request (e.g., read request, write request), the data storage node  112  checks the indications of the starting and ending points of online space accounting metadata recovery activities stored in the namespace superblock  130  to confirm that such space accounting metadata recovery activities are in-progress. Further, the data storage node  112  checks the SA in-progress flag for each respective data volume stored in the namespace Mode  302 , as well as a current logical offset from the starting point for the tree walk stored in the bookkeeping data structure  136  to determine, based on the current logical offset, whether the space accounting metadata recovery activities are accessing the same region of metadata as the storage IO request, as described hereinabove. In addition, the data storage node  112  determines whether the storage IO request might change any metadata (e.g., the reference counts) stored in the VLB pages  134  for the volume family VF 1 . If it is determined that the storage IO request might change at least some of the metadata (e.g., the reference counts) stored in the VLB pages  134 , then such metadata can be updated, as required, in both the temporary VLB pages in the bookkeeping data structure  136  and in the VLB pages  134 . 
     Once the space accounting metadata recovery activities for addressing and/or repairing the detected discrepancies in the FamilyCommittedCount  526  and/or the FamilyUniqueCount  528  for the volume family VF 1  have completed (e.g., the logical offset has reached the ending point for the tree walk), the temporary key-value store in the bookkeeping data structure  136  can replace the key-value store  524 , the mapper SB pointer  522  can be modified to point to the key-value store replacing the key-value store  524 , the indications of the starting and ending points in the namespace superblock  130  can be cleared, all of the SA in-progress flags for the respective data volumes in the volume family VF 1  can be set to “false” (SA in-progress flag, T/F, in namespace Mode  302 ; see  FIG. 3 ), and the prior key-value store  524  can be discarded. 
     An exemplary method of performing recovery of space accounting metadata while a data storage system is online for regular user data access is described below with reference to  FIG. 6 . As depicted in block  602 , in an online process of the data storage system, space accounting metadata recovery of at least one data volume in a volume family is performed, in which the space accounting metadata recovery includes accessing a region of metadata pertaining to the at least one data volume in the volume family, and maintaining corresponding updated metadata pertaining to the at least one data volume in the volume family. As depicted in block  604 , a storage IO request is received for servicing at the data storage system. As depicted in block  606 , a determination is made as to whether the servicing of the storage IO request includes accessing the same region of metadata being accessed in the space accounting metadata recovery of the at least one data volume in the volume family. As depicted in block  608 , having determined that the servicing of the storage IO request includes accessing the same region of metadata as being accessed in the space accounting metadata recovery of the at least one data volume in the volume family, access to the region of metadata is permitted for servicing of the storage IO request, thereby assuring prompt access to data stored on the data storage system. 
     Having described the foregoing illustrative embodiments, other embodiments and/or variations may be made and/or practiced. For example, it was described herein that the clustered storage environment  100  of  FIG. 1  can include the plurality of host computers  102 . 1 ,  102 . 2 , . . . ,  102 . n  and the storage domain  104  interconnected by the network  106 , and that the disclosed techniques can be used to determine (i) an amount of physical storage space committed to each data volume in each branch of a respective volume family, (ii) an amount of physical storage space committed to a respective volume family, and (iii) an amount of physical storage space unique to (or unshared by) a respective volume family. In one embodiment, such information pertaining to committed and/or unique physical storage space for volumes and/or volume families can be used to perform data storage activities (such as data storage recovery and/or data migration) more efficiently among the respective data storage appliances  108 . 1 , . . . ,  108 . m  within the storage domain  104  of the clustered storage environment  100 . 
     While various embodiments of the disclosure have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the disclosure as defined by the appended claims.