Patent Publication Number: US-9424117-B1

Title: Virtual storage processor failover

Description:
BACKGROUND 
     A conventional data storage system, which is configured for failover operation, includes N+1 physical storage processors. That is, there are N physical storage processors that provide file-based access to host data by processing file-based host input/output (I/O) requests. Additionally, there is one spare physical storage processor in standby mode. 
     During operation, if one of the N physical storage processors providing file-based access to the host data fails, the spare physical storage processor transitions into operation from standby mode. That is, the spare physical storage processor takes over for the failed physical storage processor by processing file-based host I/O requests in place of the failed physical storage processor. 
     SUMMARY 
     Unfortunately, there are deficiencies to the above-described conventional failover approach which uses an N+1 failover model. In particular, the above-described conventional failover approach is very different from a block-based failover scheme in which block-based objects (e.g., volumes, logical units of storage or LUNs, etc.) which are owned by a failed physical storage processor are trespassed temporarily to a healthy physical storage processor. Accordingly, to provide a data storage system which supports both block-based and file-based access to host data, the data storage system must be configured with two very different failover models, i.e., an N+1 failover model for file and a trespass failover model for block. 
     In contrast to the above-described conventional file-based failover approach which uses an N+1 failover model, improved techniques are directed to performing virtual storage processor (VSP) failover in a manner similar to that of a trespass model for block-based failover. Along these lines, access to a VSP which is used to create an operating environment for a host file system is moved from a failed physical storage processor to a healthy physical storage processor. The healthy physical storage processor then accesses the VSP to recreate the operating environment for the host file system. Such failover operation takes place even though the failed physical storage processor may continue to be identified as the primary owner of the VSP (i.e., even though the VSP has been trespassed on to the healthy physical storage processor). Accordingly, such VSP failover enables file and block failover to share a common framework thus alleviating the need to support block-based and file-based access to host data using two very different failover models. 
     One embodiment is directed to a method of performing virtual storage processor (VSP) failover. The method includes accessing, by a first physical storage processor of the data storage apparatus, a VSP to create an operating environment for a host file system from the first physical storage processor (i.e., the primary owner of the VSP). The method further includes, after accessing the VSP to create the operating environment for the host file system from the first physical storage processor, detecting a failure of the first physical storage processor. The method further includes accessing, in response to detecting the failure of the first physical storage processor and by a second physical storage processor of the data storage apparatus, the VSP to re-create the operating environment for the host file system from the second physical storage processor. 
     In some arrangements, the VSP includes a VSP configuration file system which stores data defining the operating environment for the host file system. In these arrangements, accessing the VSP by the first physical storage processor includes controlling a root file system of the VSP and mounting the VSP configuration file system and the host file system to the root file system of the VSP by the first physical storage processor to access the VSP configuration file system and the host file system from the first physical storage processor. Likewise, accessing the VSP by the second physical storage processor includes controlling the root file system of the VSP and mounting the VSP configuration file system and the host file system to the root file system of the VSP by the second physical storage processor to access the VSP configuration file system and the host file system from the second physical storage processor. 
     In some arrangements, accessing the VSP by the first physical storage processor further includes processing, by the first physical storage processor, host input/output (I/O) requests on the host file system. Additionally, accessing the VSP by the second physical storage processor further includes processing, by the second physical storage processor, host I/O requests on the host file system. 
     In some arrangements, the first physical storage processor is considered a primary owner of the VSP within the data storage apparatus. Here, accessing the VSP by the second physical storage processor of the data storage apparatus includes providing the operating environment for the host file system from the second physical storage processor while the first physical storage processor remains considered the primary owner of the VSP. 
     In some arrangements, the data storage apparatus includes a configuration database containing a set of records which indicate that the first physical storage processor is considered the primary owner of the VSP. In these arrangements, providing the operating environment for the host file system from the second physical storage processor includes maintaining the set of records contained within the configuration database to continue to indicate that the first physical storage processor is considered the primary owner of the VSP while the second physical storage processor accesses the VSP. 
     In some arrangements, the method further includes, after the second physical storage processor accesses the VSP to re-create the operating environment for the host file system from the second physical storage processor, detecting restored availability of the first physical storage processor. In these arrangements, the method further includes, after detecting restored availability of the first physical storage processor, performing a failback operation to re-access the VSP, by the first physical storage processor of the data storage apparatus, to re-create the operating environment for the host file system from the first physical storage processor. 
     It should be understood that performing the failback operation may be carried out in response to a user entered command to failback the VSP from the second physical storage processor to the first physical storage processor. Alternatively, performing the failback operation may be carried out automatically upon detection of restored availability of the first physical storage processor. 
     In some arrangements, performing the failback operation includes relinquishing control of the root file system of the VSP and un-mounting the VSP configuration file system and the host file system from the root file system of the VSP by the second physical storage processor. Performing the failback operation further includes subsequently obtaining control of the root file system of the VSP and re-mounting the VSP configuration file system and the host file system to the root file system of the VSP by the first physical storage processor. After the failback operation has been completed, the method includes re-accessing the VSP by the first physical storage processor to process host I/O requests on the host file system by the first physical storage processor. 
     In some arrangements, the method further includes, in response to the detected failure of the first physical storage processor, trespassing, by the second physical storage processor of the data storage apparatus, a set of block-based data objects to the second physical storage processor to concurrently failover file-based host data access and block-based host data access from the first physical storage processor to the second physical storage processor. Such operation enables file-based failover to be handled together the same way a LUN may be trespassed to a remaining healthy physical storage processor under a failure condition (e.g., a software failure, a hardware failure, a firmware failure, combinations thereof, etc.). 
     It should be understood that, in the cloud context, electronic circuitry is formed by remote computer resources distributed over a network. Such a computing environment is capable of providing certain advantages such as enhanced fault tolerance, load balancing, processing flexibility, etc. 
     Other embodiments are directed to electronic systems and apparatus, processing circuits, computer program products, and so on. Some embodiments are directed to various methods, electronic components and circuitry which are involved in performing VSP failover. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features and advantages will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings, in which like reference characters refer to the same parts throughout the different views. In the accompanying drawings, 
         FIG. 1  is a block diagram showing an example data storage apparatus in an environment wherein improved techniques hereof may be practiced, the data storage apparatus including a storage processor having multiple virtualized storage processors (VSPs); 
         FIG. 2  is a block diagram showing example features of the front end of  FIG. 1  in additional detail; 
         FIG. 3  is a block diagram showing an example set of file systems of a VSP of  FIG. 1 ; 
         FIG. 4  is a table showing an example set of records stored in a configuration database that defines a VSP that can be run on the storage processor of  FIG. 1 ; 
         FIGS. 5A and 5B  are block diagrams showing example arrangements of virtualized storage processors; and 
         FIG. 6  is a flowchart showing an example process for managing host data using a VSP. 
         FIG. 7  is an example showing the locations of various VSPs among physical storage processors at a first operating time just prior to failover. 
         FIG. 8  is the example showing the locations of the various VSPs at a second operating time after failover. 
         FIG. 9  is a flowchart of a procedure which is performed to achieve VSP failover among physical storage processors. 
     
    
    
     DETAILED DESCRIPTION 
     Overview 
     Embodiments of the invention will now be described. It is understood that such embodiments are provided by way of example to illustrate various features and principles of the invention, and that the invention hereof is broader than the specific example embodiments disclosed. 
     An improved technique is directed to performing virtual storage processor (VSP) failover in a manner similar to that of a trespass model for block-based failover. That is, access to a VSP which is used to create an operating environment for a host file system is moved from a failed physical storage processor to a healthy physical storage processor. The healthy physical storage processor then accesses the VSP to recreate the operating environment for the host file system. Such failover operation takes place even though the failed physical storage processor may continue to be identified as the primary owner of the VSP (i.e., even though the VSP has been trespassed on to the healthy physical storage processor). As a result, such VSP failover enables file and block failover to share a common framework thus alleviating the need to support block-based and file-based access to host data using two very different failover models. 
     Data Storage Apparatus Details 
     Data storage systems typically include one or more physical storage processors (SPs) accessing an array of disk drives and/or electronic flash drives. Each SP is connected to a network, such as the Internet and/or a storage area network (SAN), and receives transmissions over the network from host computing devices (“hosts”). The transmissions from the hosts include “IO requests,” also called “host IOs.” Some IO requests direct the SP to read data from an array, whereas other IO requests direct the SP to write data to the array. Also, some IO requests perform block-based data requests, where data are specified by LUN (Logical Unit Number) and offset values, whereas others perform file-based requests, where data are specified using file names and paths. Block-based IO requests typically conform to a block-based protocol, such as Fibre Channel or iSCSI (Internet SCSI, where SCSI is an acronym for Small Computer System Interface), for example. File-based IO requests typically conform to a file-based protocol, such as NFS (Network File System), CIFS (Common Internet File System), or SMB (Server Message Block), for example. 
     In some data storage systems, an SP may operate one or more virtual data movers. As is known, a virtual data mover is a logical grouping of file systems and servers that is managed by the SP and provides a separate context for managing host data stored on the array. A single SP may provide multiple virtual data movers for different users or groups. For example, a first virtual data mover may organize data for users in a first department of a company, whereas a second virtual data mover may organize data for users in a second department of the company. Each virtual data mover may include any number of host file systems for storing user data. 
     In a typical virtual data mover arrangement, the SP has a root file system with mount points to which the host file systems of each virtual data mover are mounted. Thus, the SP and all its virtual data movers form a single, large directory and all share a common namespace. Hosts can access their virtual data mover-managed data by connecting to the SP over the network, logging on, and specifying paths relative to the SP&#39;s root where their data are kept. The typical arrangement thus requires hosts to access data of a virtual data mover using paths that are referenced to and dependent upon the root of the SP. 
     In addition, settings for prescribing virtual data mover operations are conventionally stored in the root file system of the SP. Many of these settings are global to all virtual data movers operating on the SP; others may be specific to particular virtual data movers. 
     Unfortunately, the intermingling of virtual data mover content within an SP&#39;s root file system impairs the ease of mobility and management of virtual data movers. For example, administrators wishing to move a virtual data mover (e.g., its file systems, settings, and servers) from one SP to another SP must typically perform many steps on a variety different data objects. File systems, server configurations, and other settings may need to be moved one at a time. Also, as the contents of different virtual data movers are often co-located, care must be taken to ensure that changes affecting one virtual data mover do not disrupt the operation of other virtual data movers. 
     In contrast to conventional approaches, an improved technique for managing host data in a data storage apparatus provides virtualized storage processors (VSPs) as substantially self-describing and independent entities. Each VSP has its own namespace, which is independent of the namespace of any other VSP. Each VSP also has its own network address. Hosts may thus access VSPs directly, without having to include path information relative to the SP on which the VSPs are operated. VSPs can thus be moved from one physical SP to another with little or no disruption to hosts, which may in many cases continue to access the VSPs on the new SPs using the same paths as were used to access the VSPs on the original SPs. 
     In some examples, each VSP includes within its namespace a configuration file system storing configuration settings for operating the VSP. These configuration settings include, for example, network interface settings and internal settings that describe the VSPs “personality,” i.e., the manner in which the VSP interacts on the network. By providing these settings as part of the VSP itself (e.g., within the file systems of the VSP), the VSP can be moved from one physical SP to another substantially as a unit. The increased independence of the VSP from its hosting SP promotes many aspects of VSP management, including, for example, migration, replication, failover, trespass, multi-tenancy, load balancing, and gateway support. 
     In some examples, the independence of VSPs is further promoted by storing data objects of VSPs in the form of respective files. These data objects may include, for example, file systems, LUNs, virtual storage volumes (vVols), and virtual machine disks (VMDKs). Each such file is part of a set of internal file systems of the data storage apparatus. Providing data objects in the form of files of a set of internal file systems promotes independence of VSPs and unifies management of file-based objects and block-based objects. 
     In accordance with improvements hereof, certain embodiments are directed to a method of managing host data on a data storage apparatus connected to a network. The method includes storing a network address and a set of host data objects accessible within a namespace of a virtualized storage processor (VSP) operated by a physical storage processor of the data storage apparatus. The namespace includes only names of objects that are specific to the VSP. The method further includes receiving, by the physical storage processor, a transmission over the network from a host computing device. The transmission is directed to a network address and includes an IO request designating a pathname to a host data object to be written or read. The method still further includes identifying the host data object designated by the IO request by (i) matching the network address to which the transmission is directed with the network address stored for the VSP, to identify the VSP as the recipient of the IO request, and (ii) locating the host data object within the namespace of the VSP using the pathname. The IO request is then processed to complete the requested read or write operation on the identified host data object. 
     Other embodiments are directed to computerized apparatus and computer program products. Some embodiments involve activity that is performed at a single location, while other embodiments involve activity that is distributed over a computerized environment (e.g., over a network). 
     An improved technique for managing host data in a data storage apparatus provides virtualized storage processors (VSPs) as substantially self-describing and independent constructs. 
       FIG. 1  shows an example environment  100  in which embodiments of the improved technique hereof can be practiced. Here, multiple host computing devices (“hosts”)  110 ( 1 ) through  110 (N), access a data storage apparatus  116  over a network  114 . The data storage apparatus  116  includes a physical storage processor, or “SP,”  120  and storage  180 . The storage  180  is provided, for example, in the form of hard disk drives and/or electronic flash drives. Although not shown in  FIG. 1 , the data storage apparatus  116  may include multiple SPs like the SP  120 . For instance, multiple SPs may be provided as circuit board assemblies, or “blades,” which plug into a chassis that encloses and cools the SPs. The chassis has a backplane for interconnecting the SPs, and additional connections may be made among SPs using cables. It is understood, however, that no particular hardware configuration is required, as any number of SPs (including a single one) can be provided and the SP  120  can be any type of computing device capable of processing host IOs. 
     The network  114  can be any type of network, such as, for example, a storage area network (SAN), local area network (LAN), wide area network (WAN), the Internet, some other type of network, and/or any combination thereof. In an example, the hosts  110 ( 1 -N) connect to the SP  120  using various technologies, such as Fibre Channel, iSCSI, NFS, SMB 3.0, and CIFS, for example. Any number of hosts  110 ( 1 -N) may be provided, using any of the above protocols, some subset thereof, or other protocols besides those shown. As is known, Fibre Channel and iSCSI are block-based protocols, whereas NFS, SMB 3.0, and CIFS are file-based protocols. The SP  120  is configured to receive IO requests  112 ( 1 -N) in transmissions from the hosts  110 ( 1 -N) according to both block-based and file-based protocols and to respond to such IO requests  112 ( 1 -N) by reading or writing the storage  180 . 
     The SP  120  is seen to include one or more communication interfaces  122 , control circuitry (e.g., a set of processors  124 ), and memory  130 . The communication interfaces  122  include, for example, adapters, such as SCSI target adapters and network interface adapters, for converting electronic and/or optical signals received from the network  114  to electronic form for use by the SP  120 . The set of processors  124  includes one or more processing chips and/or assemblies. In a particular example, the set of processors  124  includes numerous multi-core CPUs. The memory  130  includes both volatile memory (e.g., RAM), and non-volatile memory, such as one or more ROMs, disk drives, solid state drives (SSDs), and the like. The set of processors  124  and the memory  130  are constructed and arranged to carry out various methods and functions as described herein. Also, the memory  130  includes a variety of software constructs realized in the form of executable instructions. When the executable instructions are run by the set of processors  124 , the set of processors  124  are caused to carry out the operations of the software constructs. Although certain software constructs are specifically shown and described, it is understood that the memory  130  typically includes many other software constructs, which are not shown, such as various applications, processes, and daemons. 
     As shown, the memory  130  includes an operating system  134 , such as Unix, Linux, or Windows™, for example. The operating system  134  includes a kernel  136 . The memory  130  is further seen to include a container  132 . In an example, the container  132  is a software process that provides an isolated userspace execution context within the operating system  134 . In various examples, the memory  130  may include multiple containers like the container  132 , with each container providing its own isolated userspace instance. Although containers provide isolated environments that do not directly interact (and thus promote fault containment), different containers can be run on the same kernel  136  and can communicate with one another using inter-process communication (IPC) mediated by the kernel  136 . Containers are well-known features of Unix, Linux, and other operating systems. 
     In the example of  FIG. 1 , only a single container  132  is shown. Running within the container  132  is an IO stack  140  and multiple virtualized storage processors (VSPs)  150 ( 1 - 3 ). The IO stack  140  provides an execution path for host IOs (e.g.,  112 ( 1 -N)) and includes a front end  142  and a back end  144 . The VSPs  150 ( 1 - 3 ) each run within the container  132  and provide a separate context for managing host data. In an example, each VSP manages a respective set of host file systems and/or other data objects and uses servers and settings for communicating over the network  114  with its own individual network identity. Although three VSPs are shown, it is understood that the SP  120  may include as few as one VSP or as many VSPs as the computing resources of the SP  120  and storage resources of the storage  180  allow. 
     Although the VSPs  150 ( 1 - 3 ) each present an independent and distinct identity, it is evident that the VSPs  150 ( 1 - 3 ) are not, in this example, implemented as independent virtual machines. Rather, all VSPs  150 ( 1 - 3 ) operate in userspace and employ the same kernel  136  of the SP  120 . Although it is possible to implement the VSPs  150 ( 1 - 3 ) as independent virtual machines (each including a virtualized kernel), it has been observed that VSPs perform faster when the kernel  136  is not virtualized. 
     Also, it is observed that the VSPs  150 ( 1 - 3 ) all run within the container  132 , i.e., within a single userspace instance. Again, the arrangement shown reflects a deliberate design choice aimed at optimizing VSP performance. It is understood, though, that alternative implementations could provide different VSPs in different containers, or could be provided without containers at all. 
     The memory  130  is further seen to store a configuration database  170 . The configuration database  170  stores system configuration information, including settings related to the VSPs  150 ( 1 - 3 ) and their data objects. In other implementations, the configuration database  170  is stored elsewhere in the data storage apparatus  116 , such as on a disk drive separate from the SP  120  but accessible to the SP  120 , e.g., over a backplane or network. 
     In operation, the hosts  110 ( 1 -N) issue IO requests  112 ( 1 -N) to the data storage apparatus  116 . The IO requests  112 ( 1 -N) may include both block-based requests and file-based requests. The SP  120  receives the IO requests  112 ( 1 -N) at the communication interfaces  122  and passes the IO requests to the IO stack  140  for further processing. 
     At the front end  142  of the IO stack  140 , processing includes associating each of the IO requests  112 ( 1 -N) with a particular one of the VSPs  150 ( 1 - 3 ). In an example, each VSP stores a network address (e.g., an IP address) in a designated location within its file systems. The front end  142  identifies the network address to which each IO request is directed and matches that address with one of the network addresses stored with the VSPs  150 ( 1 - 3 ). The front end  142  thus uses the network address to which each IO request is sent to identify the VSP to which the IO request is directed. Further processing of the IO request is then associated (e.g., tagged) with an identifier of the matching VSP, such that the IO request is processed within a particular VSP context. Any data logging, metrics collection, fault reporting, or messages generated while the IO request is being processed are stored with the associated VSP (e.g., in a file system dedicated to the VSP). Also, any path information provided with the IO request (e.g., to a particular directory and file name) is interpreted within the namespace of the identified VSP. 
     Processing within the front end  142  may further include caching data provided with any write IOs and mapping host data objects (e.g., host file systems, LUNs, vVols, VMDKs, etc.) to underlying files stored in a set of internal file systems. Host IO requests received for reading and writing both file systems and LUNs are thus converted to reads and writes of respective files. The IO requests then propagate to the back end  144 , where commands are executed for reading and/or writing the physical storage  180 . 
     In an example, processing through the IO stack  140  is performed by a set of threads maintained by the SP  120  in a set of thread pools. When an IO request is received, a thread is selected from the set of thread pools. The IO request is tagged with a VSP identifier, and the selected thread runs with the context of the identified VSP. Typically, multiple threads from different thread pools contribute to the processing of each IO request (there are many processing layers). Multiple threads from the thread pools can process multiple IO requests simultaneously, i.e., in parallel, on the data objects of any one VSP or multiple VSPs. 
     Although  FIG. 1  shows the front end  142  and the back end  144  together in an “integrated” form, the front end  142  and back end  144  may alternatively be provided on separate SPs. For example, the IO stack  140  may be implemented in a “modular” arrangement, with the front end  142  on one SP and the back end  144  on another SP. The IO stack  140  may further be implemented in a “gateway” arrangement, with multiple SPs running respective front ends  142  and with a back end provided within a separate storage array. The back end  144  performs processing that is similar to processing natively included in many block-based storage arrays. Multiple front ends  142  can thus connect to such arrays without the need for providing separate back ends. In all arrangements, processing through both the front end  142  and back end  144  is preferably tagged with the particular VSP context such that the processing remains VSP-aware. 
       FIG. 2  shows portions of the front end  142  in additional detail. Here, and describing the architecture generally without regard to any particular VSP, it is seen that a set of lower-deck file systems  202  represents LUNs and host file systems in the form of files. Any number of lower-deck file systems  202  may be provided. In one arrangement, a single lower-deck file system may include, as files, any number of LUNs and/or host file systems, as well as their snaps (i.e., point-in-time copies). In another arrangement, a different lower-deck file system is provided for each primary object to be stored, e.g., for each LUN and for each host file system. Additional arrangements provide groups of host file systems and/or groups of LUNs together in a single lower deck file system. The lower-deck file system for any object may include a file storing the object itself, as well as files storing any snaps of the object. Each lower-deck file system  202  has an inode table (e.g.,  232 ,  242 ), which provides a unique inode for each file stored in the lower-deck file system. The inode table of each lower-deck file system stores properties of each file in the respective lower-deck file system, such as ownership and block locations at which the file&#39;s data are stored. Lower-deck file systems are built upon storage elements managed by a storage pool  204 . 
     The storage pool  204  organizes elements of the storage  180  in the form of slices  250 . A “slice” is an increment of storage space, such as 256 MB in size, which is obtained from the storage  180 . The pool  204  may allocate slices to lower-deck file systems  202  for use in storing their files. The pool  204  may also deallocate slices from lower-deck file systems  202  if the storage provided by the slices is no longer required. In an example, the storage pool  204  creates slices by accessing RAID groups formed from the storage  180 , dividing the RAID groups into FLUs (Flare LUNs), and further dividing the FLU&#39;s into slices. 
     Continuing with reference to the example shown in  FIG. 2 , a user object layer  206  includes a representation of a LUN  210  and of an HFS (host file system)  212 , and a mapping layer  208  includes a LUN-to-file mapping  220  and an HFS-to-file mapping  222 . The LUN-to-file mapping  220  maps the LUN  210  to a first file F 1  ( 236 ), and the HFS-to-file mapping  222  maps the HFS  212  to a second file F 2  ( 246 ). Through the LUN-to-file mapping  220 , any set of blocks identified in the LUN  210  by a host IO request is mapped to a corresponding set of blocks within the first file  236 . Similarly, through the HFS-to-file mapping  222 , any file or directory of the HFS  212  is mapped to a corresponding set of blocks within the second file  246 . The HFS  212  is also referred to herein as an “upper-deck file system,” which is distinguished from the lower-deck file systems  202 , which are for internal use. 
     In this example, a first lower-deck file system  230  includes the first file  236  and a second lower-deck file system  240  includes the second file  246 . Each of the lower-deck file systems  230  and  240  includes an inode table ( 232  and  242 , respectively). The inode tables  232  and  242  provide information about files in respective lower-deck file systems in the form of inodes. For example, the inode table  232  of the first lower-deck file system  230  includes an inode  234 , which provides file-specific information about the first file  236 . Similarly, the inode table  242  of the second lower-deck file system  240  includes an inode  244 , which provides file-specific information about the second file  246 . The information stored in each inode includes location information (e.g., block locations) where the respective file is stored, and may thus be accessed as metadata to identify the locations of the files  236  and  246  in the storage  180 . 
     Although a single file is shown for each of the lower-deck file systems  230  and  240 , it is understood that each of the lower-deck file systems  230  and  240  may include any number of files, each with its own entry in the respective inode table. In one example, each lower-deck file system stores not only the file F 1  or F 2  for the LUN  210  or HFS  212 , but also snaps of those objects. For instance, the first lower-deck file system  230  stores the first file  236  along with a different file for every snap of the LUN  210 . Similarly, the second lower-deck file system  240  stores the second file  246  along with a different file for every snap of the HFS  212 . 
     As shown, a set of slices  260  is allocated by the storage pool  204  for storing the first file  236  and the second file  246 . In the example shown, slices S 1  through S 4  are used for storing the first file  236 , and slices S 5  through S 7  are used for storing the second file  246 . The data that make up the LUN  210  are thus stored in the slices S 1  through S 4 , whereas the data that make up the HFS  212  are stored in the slices S 5  through S 7 . 
     In some examples, each of the lower-deck file systems  230  and  240  is associated with a respective volume, such as a sparse LUN. Sparse LUNs provide an additional layer of mapping between the lower-deck file systems  202  and the pool  204  and allow the lower-deck file systems to operate as file systems normally do, by accessing underlying volumes. Additional details about sparse LUNs and their relation to lower-deck file systems may be found in U.S. Pat. No. 7,631,155, which is hereby incorporated by reference in its entirety. The incorporated patent uses the term “container file system” to refer to a construct similar to the lower-deck file system disclosed herein. 
     Although the example of  FIG. 2  shows storage of a LUN  210  and a host file system  212  in respective lower-deck file systems  230  and  240 , it is understood that other data objects may be stored in one or more lower-deck file systems in a similar manner. These may include, for example, file-based vVols, block-based vVols, and VMDKs. 
       FIG. 3  shows an example set of components of the data storage apparatus  116  that are associated with a particular VSP  300  (i.e., any of the VSPs  150 ( 1 - 3 )). The components shown in  FIG. 3  include components that are managed in the context of the VSP  300  and components that form the “personality” of the VSP  300 . These components may be referred to herein as “included” within the VSP  300 , by which it is meant that the components are associated with the VSP  300  within the data storage apparatus  116  and are not associated with any other VSP. It is thus seen that the VSP  300  “includes” a number of lower-deck file systems hosting various host data objects, as well as internal data objects. 
     For example, the VSP  300  includes a first lower-deck file system  310  and a second lower-deck file system  320 . The first lower-deck file system  310  includes a file FA, which provides a file representation of a first host file system  312 . Similarly, the second lower-deck file system  320  includes a file FB, which provides a file representation of a second host file system  322 . The host file systems  312  and  322  are upper-deck file systems, which may be made available to hosts  110 ( 1 -N) for storing file-based host data. HFS-to-file mappings, like the HFS-to-file mapping  222 , are understood to be present (although not shown in  FIG. 3 ) for expressing the files FA and FB in the form of upper-deck file systems. Although only two host file systems  312  and  322  are shown, it is understood that the VSP  300  may include any number of host file systems. In an example, a different lower-deck file system is provided for each host file system. The lower-deck file system stores the file representation of the host file system, and, if snaps are turned on, any snaps of the host file system. In a similar manner to that described in connection with  FIG. 2 , each of the lower-deck file systems  310  and  320  includes a respective inode table, allowing the files FA and FB and their snaps to be indexed within the respective lower-deck file systems and accessed within the storage  180 . 
     In some examples, the VSP  300  also includes one or more lower-deck file systems for storing file representations of LUNs. For example, a lower-deck file system  330  stores a file FC, which provides a file representation of a LUN  332 . A LUN-to-file mapping (not shown but similar to the mapping  320 ) expresses the file FC in the form of a LUN, which may be made available to hosts  110 ( 1 -N) for storing block-based host data. In an example, the lower-deck file system  330  stores not only the file FC, but also snaps thereof, and includes an inode table in essentially the manner described above. 
     The VSP  300  further also includes a lower-deck file system  340 . In an example, the lower-deck file system  340  stores file representations FD and FE of two internal file systems of the VSP  300 —a root file system  342  and a configuration file system  344 . In an alternative arrangement, the files FD and FE are provided in different lower-deck file systems. In an example, the lower-deck file system  340  also stores snaps of the files FD and FE, and files are accessed within the lower-deck file system  340  via file system-to-file mappings and using an inode table, substantially as described above. 
     In an example, the root file system  342  has a root directory, designated with the slash (“/”), and sub-directories as indicated. Any number of sub-directories may be provided within the root file system in any suitable arrangement with any suitable file structure; the example shown is merely illustrative. As indicated, one sub-directory (“Local”) stores, for example, within constituent files, information about the local environment of the SP, such as local IP sub-net information, geographical location, and so forth. Another sub-directory (“Rep”) stores replication information, such as information related to any ongoing replication sessions. Another sub-directory (“Cmd Svc”) stores command service information, and yet another sub-directory (“MPs”) stores mount points. 
     In the example shown, the directory “MPs” of the root file system  342  provides mount points (e.g., directories) on which file systems are mounted. For example, the host file systems  312  and  322  are respectively mounted on mount points MP 1  and MP 2 , and the configuration file system  344  is mounted on the mount point MP 3 . In an example, establishment of the mount points MP 1 -MP 3  and execution of the mounting operations for mounting the file systems  312 ,  322 ,  344  onto the mount points MP 1 -MP 4  are provided in a batch file stored in the configuration file system  344  (e.g., in Host Objects). It is understood that additional mount points may be provided for accommodating additional file systems. 
     The root file system  342  has a namespace, which includes the names of the root directory, sub-directories, and files that belong to the root file system  342 . The file systems  312 ,  322 , and  344  also each have respective namespaces. The act of mounting the file systems  312 ,  322 , and  344  onto the mount points MP 1 , MP 2 , and MP 3  of the root file system  342  serves to join the namespace of each of the file systems  312 ,  322 , and  344  with the namespace of the root file system  342 , to form a single namespace that encompasses all the file systems  312 ,  322 ,  342 , and  344 . This namespace is specific to the VSP  300  and is independent of namespaces of any other VSPs. 
     Also, it is understood that the LUN  332  is also made available to hosts  110 ( 1 )- 110 (N) through the VSP  300 . For example, hosts  110   a - n  can send read and write IO requests to the LUN  332  (e.g., via Fibre Channel and/or iSCSI commands) and the SP  120  services the requests for the VSP  300 , e.g., by operating threads tagged with the context of the VSP  300 . Although  FIG. 3  shows both the LUN  322  and the host file systems  312  and  322  together in a single VSP  300 , other examples may provide separate VSPs for LUNs and for file systems. 
     Although the VSP  300  is seen to include file systems and LUNs, other host objects may be included, as well. These include, for example, file-based vVols, block-based vVols, and VMDKs. Such host objects may be provided as file representations in lower-deck file systems and made available to hosts ( 1 )- 110 (N). 
     As its name suggests, the configuration file system  344  stores configuration settings for the VSP  300 . These settings include settings for establishing the “personality” of the VSP  300 , i.e., the manner in which the VSP  300  interacts over the network  114 . Although the configuration file system  344  is shown with a particular directory structure, it is understood that any suitable directory structure can be used. In an example, the configuration file system  344  stores the following elements:
         IF ConFIG. Interface configuration settings of any network interface used for processing IO requests and tagged with a context of the VSP  300 . IF Config includes the IP address of the VSP, as well as related network information, such as sub-masks and related IP information.   CIFS. Configuration settings and names of one or more CIFS servers used in the context of the VSP  300 . The CIFS servers manage IO requests provided in the CIFS protocol. By including the CIFS configuration within the configuration file system  344 , the CIFS configuration becomes part of the VSP  300  itself and remains with the VSP  300  even as the VSP  300  is moved from one SP to another SP. This per-VSP configuration of CIFS also permits each VSP to have its own customized CIFS settings, which may be different from the settings of CIFS servers used by other VSPs.   NFS. Configuration settings and names of one or more NFS servers used in the context of the VSP  300 . The NFS servers manage IO requests provided in the NFS protocol. By including the NFS configuration within the configuration file system  344 , the NFS configuration becomes part of the VSP  300  itself and remains with the VSP  300  even as the VSP  300  is moved from one SP to another SP. This per-VSP configuration of NFS also permits each VSP to have its own customized NFS settings, which may be different from the settings of NFS servers used by other VSPs.   Exports. NFS exports, CIFS shares, and the like for all supported protocols. For security and management of host access, users are typically given access only to specified resources mounted to the root file system  342 , e.g., host file systems, sub-directories of those file systems, and/or particular LUNs. Access to these resources is provided by performing explicit export/share operations, which expose entry points to the resources for host access. In an example, these export/share operations are included within one or more batch files, which may be executed when the VSP  300  is started. Exports are typically VSP-specific, and depend upon the particular data being hosted and the access required.   CAVA/NDMP: CAVA (Celerra Anti-Virus Agent) configuration file, including location of external server for performing virus checking operations. NDMP (Network Data Management Protocol) provides backup configuration information. CAVA and NDMP settings are configurable on a per-VSP basis.   NIS/DNS/LDAP: Local configurations and locations of external servers for providing resolution of IP addresses. NIS (Network Information Service), DNS (Directory Name System), and LDAP (Lightweight Directory Access Protocol) settings are configurable on a per-VSP basis. The DNS configuration stores local host name and domain name of the VSP  300 , as well as the location of a DNS server for resolving host names.   Host Objects: Identifiers for all host file systems (e.g.,  312  and  322 ), LUNs (e.g., LUN  332 ), and other host objects included within the VSP  300 . Host objects may also include batch files and/or lists of instructions for establishing mount points in the root file system  342  and for mounting the host file system(s) and LUN(s) to the mount points.   Parameters: Low-level settings (e.g., registry settings) for configuring VSP  300 . These include cache settings and settings for specifying a maximum number of threads running on the SP  120  that may be used to service IO requests within the context of the VSP  300 . Parameters are configurable on a per-VSP basis.   Statistics: Metrics, log files, and other information pertaining to activities within the context of the VSP  300 . Statistics are updated as they accumulate.
 
Many configuration settings are established at startup of the VSP  300 . Some configuration settings are updated as the VSP  300  is operated. The configuration file system  344  preferably does not store host data.
       

     Although  FIG. 3  has been shown and described with reference to a particular VSP  300 , it is understood that all of the VSPs  150 ( 1 - 3 ) may include a root file system, a configuration file system, and at least one host file system or LUN, substantially as shown. Particular host objects and configuration settings differ, however, from one VSP to another. 
     By storing the configuration settings of VSPs within the file systems of the VSPs themselves and providing a unique namespace for each VSP, VSPs are made to be highly independent, both of other VSPs and of the particular SPs on which they are provided. For example, migrating a VSP from a first data storage system to a second data storage system involves copying its lower-deck file systems (or some subset thereof) from a source SP on the first data storage system to a target SP on the second, starting the VSP&#39;s servers on the target SP in accordance with the configuration settings, and resuming operation on the target SP. As the paths for accessing data objects on VSPs are not rooted to the SPs on which they are run, hosts may often continue to access migrated VSPs using the same instructions as were used prior to moving the VSPs. Similar benefits can be enjoyed when moving a VSP from one SP to another SP in the same data storage system. To move a VSP from a first SP to a second SP, The VSP need merely be shut down (i.e., have its servers stopped) on the first SP and resumed (i.e., have its servers started up again) on the second SP. 
       FIG. 4  shows an example record  400  of the configuration database  170 , which are used to define a particular VSP having a VSP identifier (ID)  410 . The VSP ID  410  may identify one of the VSPs  150 ( 1 - 3 ) or some other VSP of the data storage apparatus  116 . The record  400  specifies, for example, an owning SP (physical storage processor), authentication, and identifiers of the data objects associated with the listed VSP. The data object identifiers include identifiers of the root file system, configuration file system, and various host file systems (or other host objects) that may be accessed in the context of the listed VSP. The record  400  may also identify the lower-deck file system used to store each data object. The record  400  may further specify host interfaces that specify IO protocols that the listed VSP is equipped to handle. 
     Although  FIG. 4  shows only a single record  400  for a single VSP, it is understood that the configuration database  170  may store records, like the record  400 , for any number of VSPs, including all VSPs of the data storage apparatus  116 . During start-up of the data storage apparatus  116 , or at some other time, a computing device of the data storage apparatus  116  reads the configuration database  170  and launches a particular VSP or a group of VSPs on the identified SPs. As a VSP is starting, the SP that owns the VSP reads the configuration settings of the configuration file system  344  to configure the various servers of the VSP and to initialize its communication protocols. The VSP may then be operated on the identified SP, i.e., the SP may then be operated with the particular VSP&#39;s context. 
     It is understood that VSPs  150 ( 1 - 3 ) operate in connection with the front end  142  of the IO stack  140 . The VSPs  150 ( 1 - 3 ) thus remain co-located with their respective front ends  142  in modular and gateway arrangements. 
       FIGS. 5A and 5B  show two different example arrangements of VSPs. In  FIG. 5A , the VSPs  150 ( 1 - 3 ) access the storage pool  204 . Thus, the lower-deck file systems of the VSPs  150 ( 1 - 3 ) all derive the slices needed to store their underlying file systems and other data objects from the pool  204 . In  FIG. 5B , multiple storage pools  550 ( 1 - 3 ) are provided, one for each of the VSPs  150 ( 1 - 3 ), respectively. Providing different pools for respective VSPs promotes data isolation among the VSPs, and may be better suited for applications involving multiple tenants in which each tenant&#39;s data must be kept separate from the data of other tenants. 
       FIG. 6  shows an example method  600  for managing host data on a data storage apparatus connected to a network. The method  600  that may be carried out in connection with the data storage apparatus  116 . The method  600  is typically performed by the software constructs, described in connection with  FIGS. 1-3 , which reside in the memory  130  of the storage processor  120  and are run by the set of processors  124 . The various acts of the method  600  may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in orders different from those illustrated, which may include performing some acts simultaneously, even though the acts are shown as sequential in the illustrated embodiments. 
     At step  610 , a network address and a set of host data objects are stored in a data storage apparatus. The set of host data objects are accessible within a namespace of a virtualized storage processor (VSP) operated by a physical storage processor of the data storage apparatus. The namespace includes only names of objects that are specific to the VSP. For example, an IP address of the VSP  300  is stored in a file of a directory of the configuration file system  344 . The VSP  300  runs on the SP  120  of the data storage apparatus  116 . A set of host objects, including host file systems  312  and  322 , and LUN  332 , are also stored in the data storage apparatus  116 . These host objects are made accessible within the namespace of the VSP  300  by mounting these data objects to mount points MP 1 -MP 4  within the root file system  342  and thus merging their namespaces with that of the root file system  342 . The resulting merged namespace includes only names of objects that are specific to the VSP  300 . 
     At step  612 , a transmission is received by the physical storage processor over the network from a host computing device. The transmission is directed to a network address and includes an IO request designating a pathname to a host data object to be written or read. For example, the SP  120  receives a transmission over the network  114  from one of the hosts  110 ( 1 -N). The transmission is directed to a particular IP address and includes an IO request (e.g., one of  112 ( 1 -N)). The IO request designates a location of a host data object to be written or read (e.g., a pathname for a file-based object or a block designation for a block-based object). The location may point to any of the host file systems  312  or  322 , to the LUN  332 , or to any file or offset range accessible through the host file systems  312  or  322  or the LUN  332 , respectively. The location may also point to a vVol or VMDK, for example, or to any other object which is part of the namespace of the VSP  300 . 
     At step  614 , the host data object designated by the IO request is identified by (i) matching the network address to which the transmission is directed with the network address stored for the VSP, to identify the VSP as the recipient of the IO request, and (ii) locating the host data object within the namespace of the VSP using the pathname. For example, each of the VSPs  150 ( 1 - 3 ) stores an IP address in its configuration file system  344 . When an IO request is received, an interface running within the front end  142  of the IO stack  140  checks the IP address to which the IO request is directed and matches that IP address with one of the IP addresses stored for the VSPs  150 ( 1 - 3 ). The VSP whose IP address matches the IP address to which the IO request is directed is identified as the recipient of the IO request. The IO request arrives to the SP  120  with a pathname to the host data object to be accessed. The front end  142  looks up the designated pathname within the identified VSP to identify the particular data object to which the IO request is directed. 
     At step  616 , the IO request is processed to complete the requested read or write operation on the identified host data object. For example, the front end  142  and the back end  144  process the IO request to perform an actual read or write to the designated host data object on the storage  180 . 
     An improved technique has been described for managing host data in a data storage apparatus. The technique provides virtualized storage processors (VSPs) as substantially self-describing and independent entities. Each VSP has its own namespace, which is independent of the namespace of any other VSP. Each VSP also has its own network address. Hosts may thus access VSPs directly, without having to include path information relative to the SP on which the VSP is operated. VSPs can thus be moved from one physical SP to another with little or no disruption to hosts, which may continue to access the VSPs on the new SPs using the same paths as were used when the VSPs were running on the original SPs. 
     As used throughout this document, the words “comprising,” “including,” and “having” are intended to set forth certain items, steps, elements, or aspects in an open-ended fashion. Also, and unless explicitly indicated to the contrary, the word “set” as used herein indicates one or more of something. Although certain embodiments are disclosed herein, it is understood that these are provided by way of example only and the invention is not limited to these particular embodiments. 
     Having described certain embodiments, numerous alternative embodiments or variations can be made. For example, embodiments have been shown and described in which host file systems, LUNs, vVols, VMDKs, and the like are provided in the form of files of underlying lower-deck file systems. Although this arrangement provides advantages for simplifying management of VSPs and for unifying block-based and file-based operations, the use of lower-deck file systems is merely an example. Indeed, host file systems, LUNs, vVols, VMDKs, and the like may be provided for VSPs in any suitable way. 
     Also, although the VSPs  150 ( 1 - 3 ) are shown and described as userspace constructs that run within the container  132 , this is also merely an example. Alternatively, different VSPs may be provided in separate virtual machines running on the SP  120 . For example, the SP  120  is equipped with a hypervisor and a virtual memory manager, and each VSP runs in a virtual machine having a virtualized operating system. 
     Also, the improvements or portions thereof may be embodied as a non-transient computer-readable storage medium, such as a magnetic disk, magnetic tape, compact disk, DVD, optical disk, flash memory, Application Specific Integrated Circuit (ASIC), Field Programmable Gate Array (FPGA), and the like (shown by way of example as medium  650  in  FIG. 6 ). Multiple computer-readable media may be used. The medium (or media) may be encoded with instructions which, when executed on one or more computers or other processors, implement the various methods described herein. Such medium (or media) may be considered an article of manufacture or a machine, and may be transportable from one machine to another. 
     VSP Failover Details 
       FIGS. 7 and 8  illustrate VSP failover among two physical SPs  120 (A),  120 (B) (collectively, physical SPs  120 ) of a data storage apparatus  116  by way of example (also see  FIG. 1 ).  FIG. 7  shows the two physical SPs  120  at a first operating time, T 1 , just prior to failover.  FIG. 8  shows the two physical SPs  120  at a second operating time, T 2 , after the first operating time, T 1 , i.e., after failover. 
     By way of example only, there is a storage pool  204 , a user interface  800 , and a configuration database  170 . The storage pool  204  is formed from a set of storage units and, as mentioned earlier, contains a set of lower deck file systems  202  (also see  FIG. 2 ). The user interface  800  takes input from and provides output to a user (e.g., an administrator) and may take the form of a user workstation or terminal in communication with the processing circuitry (i.e., the physical SPs  120 ) of the data storage apparatus  116  to provide the user with a command line interface or GUI. 
     As shown in  FIG. 7 , the storage pool  204  provides storage for VSPs  150 (A)( 1 ),  150 (A)( 2 ) which are owned by the physical SP  120 (A). In particular, a lower-deck file  802 (A)( 1 ) contains a VSP configuration file system  344 (A)( 1 ) which defines a personality for the VSP  150 (A)( 1 ) (also see  FIG. 3 ). Similarly, another lower-deck file  802 (A)( 2 ) contains another VSP configuration file system  344 (A)( 2 ) which defines a personality for the VSP  150 (A)( 2 ). 
     Additionally and as shown in  FIG. 7 , a lower-deck file  804 (A)( 1 ) contains a host file system  806 (A)( 1 ) for use by a host. Similarly, another lower-deck file  804 (A)( 2 ) contains another host file system  806 (A)( 2 ) for use by a host. The personalities, or operating environments in which the host file systems  806 (A)( 1 ),  806 (A)( 2 ) reside, are defined by the VSP configuration file systems  344 (A)( 1 ),  344 (A)( 2 ), respectively. Recall that the VSP configuration file systems  344  and host file systems  806  are mounted to the respective root file systems (or root structures) of the VSPs  150  (see dashed lines in FIGS.  FIGS. 7 and 8 , and also see  FIG. 3 ). 
     Furthermore and as shown in  FIG. 7 , the storage pool  204  further provides storage for a VSP  150 (B)( 1 ) which is owned by the physical SP  120 (B). In particular, a lower-deck file  802 (B)( 1 ) contains a VSP configuration file system  344 (B)( 1 ) which defines a personality for the VSP  150 (B)( 1 ), and a lower-deck file  804 (B)( 1 ) contains a host file system  806 (B)( 1 ) for use by a host. Again, the VSP configuration file system  344 (B)( 1 ) and the host file system  806 (B)( 1 ) are mounted to the root file system of the VSP  150 (B)( 1 ). 
     It should be understood that the configuration database  170  includes a set of records  400  (also see  FIG. 4 ) which is used to manage and track ownership of various constructs/objects of the data storage apparatus  116 . Along these lines, the configuration database  170  indicates, for each VSP  150 , a particular physical SP  120  that owns that VSP  150 . Likewise, the configuration database  170  indicates, for each lower-deck file  802 ,  804 , a particular VSP  150  that owns that that lower-deck file  802 ,  804  (i.e., the particular VSP to which that lower-deck file  802 ,  804  is mounted), and so on. 
     In this example, the physical SP  120 (A) owns VSPs  150 (A)( 1 ),  150 (A)( 2 ). Similarly, the physical SP  120 (B) owns VSP  150 (B)( 1 ). 
     During operating time T 1  which is prior to failover ( FIG. 7 ), it should be understood that the physical SP  120 (A) processes host input/output (I/O) requests directed to the host file systems  806 (A)( 1 ),  806 (A)( 2 ) which are mounted to the VSPs  150 (A)( 1 ),  150 (A)( 2 ), respectively. Similarly, the physical SP  120 (B) processes host I/O requests directed to the host file system  806 (B)( 1 ) which is mounted to the VSP  150 (B)( 1 ). 
     Now, suppose that the physical SP  120 (A) enters a failure. Examples for such a failure include a hardware failure or a software failure that prevents the physical SP  120 (A) from properly processing further host I/O requests. At this point, physical SP  120 (B) takes over the VSPs  150 (A)( 1 ),  150 (A)( 2 ) owned by failed physical SP  120 (A) on behalf of failed physical SP  120 (A). That is, the VSPs  150 (A)( 1 ),  150 (A)( 2 ) owned by failed physical SP  120 (A) are trespassed on to healthy physical SP  120 (B). 
     It should be understood that are variety of mechanisms are capable of detecting the failure of physical SP  120 (A). For instance, physical SP  120 (A) could be completely unresponsive and physical SP  120 (B) considers physical SP  120 (A) to have failed due to lost contact, e.g., loss of a heartbeat signal, timeout of communications, etc. Alternatively, the physical SP  120 (A) may be able to still communicate and thus signal the physical SP  120 (B) that it has failed in some manner and that the physical SP  120 (B) must failover the VSPs  150 (A)( 1 ),  150 (A)( 2 ) for high/continuous availability purposes, and so on. 
     In response to detecting the failure of the physical SP  120 (A), the physical SP  120 (B) takes control of the VSPs  150 (A)( 1 ),  150 (A)( 2 ). In particular, the physical SP  120 (B) accesses the records  400  of the configuration database  170  (also see  FIG. 4 ) to determine which VSPs  150  list the physical SP  120 (A) as their primary owner. The physical SP  120 (B) then identifies the VSP root file systems of these VSPs  150  and identifies which file systems were mounted to these VSP root file systems. 
     The physical SP  120 (B) then takes control of the identified VSPs  150  which in the example are VSPs  150 (A)( 1 ),  150 (A)( 2 ) (illustrated by the arrows  808 ( 1 ),  808 ( 2 ) in  FIG. 7 ). Along these lines, the physical SP  120 (A) may need to un-mount the upper-deck file systems that are mounted to the root file systems of the VSPs  150 (A)( 1 ),  150 (A)( 2 ), namely, VSP configuration file systems  344 (A)( 1 ),  344 (A)( 2 ) and host file systems  806 (A)( 1 ),  806 (A)( 2 ). Additionally, the physical SP  120 (B) fix aspects of the lower deck files  802 ,  804  containing these upper-deck file systems, e.g., close/unlock/etc. the files, adjust metadata/configuration data associated with the files, and so on. Additionally, the physical SP  120 (B) may attempt to retrieve any queued or uncompleted I/O requests. 
     Next and as illustrated in  FIG. 8 , the physical SP  120 (B) obtains control of the VSP root file system of the VSPs  150 (A)( 1 ),  150 (A)( 2 ) and the lower-deck files  802 (A)( 1 ),  804 (A)( 1 ),  802 (A)( 2 ),  804 (A)( 2 ). Once the physical SP  120 (B) is able to fully access these lower-deck files, the physical SP  120 (B) mounts the VSP configuration file system  344 (A)( 1 ) and host file system  806 (A)( 1 ) to the VSP root file system of the VSP  150 (A)( 1 ), and mounts the VSP configuration file system  344 (A)( 2 ) and host file system  806 (A)( 2 ) to the VSP root file system of the VSP  150 (A)( 2 ) (also see  FIG. 3 ). 
     It should be understood that such operation takes place even though, according to the records of the configuration database  170 , the physical SP  120 (A) still owns the VSPs  150 (A)( 1 ),  150 (A)( 2 ). That is, although the physical SP  120 (B) may temporarily own the VSPs  150 (A)( 1 ),  150 (A)( 2 ), the physical SP  120 (A) still remains the primary owner of the VSPs  150 (A)( 1 ),  150 (A)( 2 ) and the configuration database  170  continues to reflect this. 
     In some arrangements, the physical SP  120 (B) updates the configuration database  170  to indicate that it is a temporary owner, but not the primary owner, of the VSPs  150 (A)( 1 ),  150 (A)( 2 ). Accordingly, if the physical SP  120 (B) were to fail, the configuration database  170  would indicate that the physical SP  120 (B) had access to the VSPs  150 (A)( 1 ),  150 (A)( 2 ) prior to the failure. 
       FIG. 8  shows the load balanced situation at operating time T 2  following operating time T 1 . Here, the VSPs  150 (A)( 1 ),  150 (A)( 2 ) have successfully failed over from the physical SP  120 (A) to the physical SP  120 (B). Accordingly, the remaining physical SP  120 (B) now processes host I/O requests directed to the host file systems  806 (A)( 1 ),  806 (A)( 2 ),  806 (B)( 1 ) which are mounted to the VSPs  150 (A)( 1 ),  150 (A)( 2 ),  150 (B)( 1 ), respectively. 
     It should be understood that movement of the VSP configuration and host file systems was not required during failover. Rather, these upper-deck file systems were simply re-mounted to the VSP following movement of only VSP root objects from the physical SP  120 (A) to the physical SP  120 (B). 
     At this point, it should be understood that failback may be effectuated via a user command  810  or automatically. Along these lines, the data storage apparatus  116  may be preconfigured to wait until the user enters a failback command  810  before the data storage apparatus  116  restores VSPs  150  to their owner physical SPs  120 . Alternatively, the data storage apparatus  116  may be preconfigured to automatically restore VSPs  150  to their owner physical SPs  120  upon detection that a failed physical SP  120  is available again (e.g., for a software issue, the failed physical SP  120  may simply reboot which may take just a few minutes). 
     During failback, the processing circuitry of the data storage apparatus  116  performs a failback operation to enable the primary owner physical SP  120  to re-access the VSPs  150  (i.e., the physical SP  120 (A) in  FIGS. 7 and 8 ). In particular, the physical SP  120 (B) un-mounts the VSP configuration file systems  344 (A)( 1 ),  344 (A)( 2 ) and the host file systems  806 (A)( 1 ),  806 (A)( 2 ) from the root file systems of the trespassing VSPs  150 (A)( 1 ),  150 (A)( 2 ), and relinquishes control of their root file systems. Subsequently, the physical SP  120 (A) obtains control of the root file systems of the VSPs  150 (A)( 1 ),  150 (A)( 2 ) and re-mounting the VSP configuration file systems  344 (A)( 1 ),  344 (A)( 2 ) and the host file systems  806 (A)( 1 ),  806 (A)( 2 ) to the root file systems. 
     As part of the failback operation, the physical SP  120 (A) updates the configuration database  170  indicate that the physical SP  120 (B) no longer temporarily owns the VSPs  150 (A)( 1 ),  150 (A)( 2 ). In particular, the physical SP  120 (A) is both the primary owner and the current owner of the VSPs  150 (A)( 1 ),  150 (A)( 2 ). Now that the failback operation has been completed, the physical SP  120 (A) re-accesses the VSPs  150 (A)( 1 ),  150 (A)( 2 ) to create the operating environments for processing host I/O requests to the host file systems  806 (A)( 1 ),  806 (A)( 2 ), respectively. 
     It should be understood that, in some arrangements, the physical SPs  120  simultaneously failover both file-based operations and block-based operations. That is, the physical SPs  120  concurrently trespass a set of block-based data objects with file-based data objects. Such operation is available when the physical SPs  120  share a common framework for handling failover. 
       FIG. 9  is a flowchart of a procedure  900  which is performed by the data storage apparatus  116  for VSP failover. At  902 , a first physical storage processor  120  accesses a VSP to create an operating environment for a host file system from the first physical storage processor. During this time, the first physical storage processor  120  normally processes host I/O requests on the host file system (e.g., see the physical SP  120 (A) in  FIG. 7 ). 
     At  904 , a second physical storage processor  120  detects a failure of the first physical storage processor  120 . For example, the first physical storage processor  120  may have experienced a software malfunction or a hardware failure preventing the first physical storage processor  120  from continuing to process host I/O requests on the host file system. 
     At  906 , the second physical storage processor  120  accesses the VSP in response to detecting the failure of the first physical storage processor. With such access, the second physical storage processor re-creates the operating environment for the host file system from the second physical storage processor. During this time, the second physical storage processor  120  processes host I/O requests on the host file system (e.g., see the physical SP  120 (B) in  FIG. 8 ). 
     As described above, improved techniques are directed to performing VSP failover in a manner similar to that of a trespass model for block-based failover. Along these lines, access to a VSP  150  (i.e., a VSP configuration file system) which is used to create an operating environment for a host file system is moved from a failed physical SP  120  to a healthy physical SP  120 . The healthy physical SP  120  then accesses the VSP  150  to recreate the operating environment for the host file system. Such failover operation takes place even though the failed physical storage processor  120  may continue to be identified as the primary owner of the VSP  150  (i.e., even though the VSP  150  has been trespassed on to the healthy physical SP  120 ). Accordingly, such VSP failover enables file and block failover to share a common framework thus alleviating the need to support block-based and file-based access to host data using two very different failover models. While various embodiments of the present 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 spirit and scope of the present disclosure as defined by the appended claims. 
     Further, although features are shown and described with reference to particular embodiments hereof, such features may be included in any of the disclosed embodiments and their variants. Thus, it is understood that features disclosed in connection with any embodiment can be included as variants of any other embodiment, whether such inclusion is made explicit herein or not. Those skilled in the art will therefore understand that various changes in form and detail may be made to the embodiments disclosed herein without departing from the scope of the disclosure. Such modifications and enhancements are intended to belong to various embodiments of the disclosure.