Patent Publication Number: US-8127096-B1

Title: High capacity thin provisioned storage server with advanced snapshot mechanism

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. provisional patent application No. 60/950,664, filed on Jul. 19, 2007, entitled “A Novel Method of Implementing Low-Footprint High Capacity Thin Provisioned Storage Server with Advanced Snapshots Mechanism,” which is expressly incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Enterprise information systems typically utilize large-scale storage systems to provide data storage facilities to employees, vendors, customers, or other users. Due to the massive scale of such systems, the task of administering a large-scale storage system can be challenging. In particular, system administrators responsible for large-scale storage systems perform a number of tasks, such as partitioning of the storage system into multiple volumes to support the users and applications of the system. However, determining the amount of storage space to be allocated to each volume is complicated since an administrator cannot know, in advance, how much space will be utilized by each storage volume over a long period of time. An incorrect allocation of space may lead to the starving of some volumes for space, requiring the addition of new capacity to the volumes, while other volumes go underutilized. Traditionally, the underutilized volumes cannot be easily reassigned to volumes needing additional storage space. 
     A thin provisioning solution can assist storage administrators in the allocation of storage capacity. A thin provisioning system can support creating storage volumes irrespective of the amount of physical capacity actually present. This can reduce the amount of system capacity purchased at the initial deployment time of the system. Thin provisioning can also relieve an administrator from figuring out an exact allocation of storage pools at design or deployment time. Instead, an administrator can make an arbitrary thin provisioning of the virtual storage space among all of the applications to start. Later, actual physical capacity behind the storage system can be added as appropriate. 
     Storage administrators are also usually responsible for making frequent data backups to be used at times of disaster or other data loss. Snapshots have become a preferred method of protecting a data storage volume against inadvertent data loss and for performing background backups. A read-only snapshot is a non-writable volume that is a point-in-time image of a data storage volume that can be created, mounted, deleted, and rolled back onto the data storage volume arbitrarily. Such snapshots are utilized extensively in the data storage industry for security, backup, and archival purposes. A writeable snapshot is initially an image of a read-only parent snapshot. The writeable snapshot may be written to and modified without affecting the read-only parent snapshot. 
     Both thin provisioning and snapshot management, within a data storage system, generally use various tables or data structures to store details related to the storage provisioning and snapshots. The size and access-complexity of these tables or data structures can limit the capacity of, or reduce the performance of, a data storage system. This can be particularly true as storage capacities become very large or the number of snapshots increases. 
     It is with respect to these considerations and others that the disclosure made herein is presented. 
     SUMMARY 
     Technologies are described herein for supporting high capacity storage servers that provide thin provisioning and snapshots while using a reduced memory footprint. The associated technologies for tracking territories, provisions, and chunks within a storage system can support an increased storage capacity and an increased number of snapshots within a data storage system. 
     According to one embodiment, flexible virtual address translation can support direct translation from a virtual address to an address in physical storage. The flexible virtual address translation can also support indirect translation from a virtual address through an intermediate structure and from the intermediate structure to an address in physical storage. The intermediate structure can provide provision tracking and support snapshot provisions. Both read-only and writeable snapshots may be supported. A data structure, referred to as a volume table, may be provided for supporting the virtual to physical address translation. In the instance where address mapping is direct, a simplified provision tracking function can be provided within the volume table. 
     According to another embodiment, volume tables associated with the various volumes within a data storage system can be stored together in a global volume table. A global volume table header can serve as an index of volume tables within the global volume table. An entry in the global volume table header can be associated with a single volume table and specify the size of the volume table as well as the offset of the volume table within the global volume table. 
     According to yet another embodiment, granularities of the territories, provisions, and chunks within a data storage system can be reduced to improve efficiencies in the operation of the storage system and reduce storage space waste. Processes for handling volume and snapshot I/O operations using the various data structures discussed herein may also contribute to improved efficiencies while supporting increased storage capacities and an increased number of snapshots. 
     It should be appreciated that the above-described subject matter may also be implemented as a computer-controlled apparatus, a computer process, a computing system, or as an article of manufacture such as a computer-readable medium. These and various other features will be apparent from a reading of the following Detailed Description and a review of the associated drawings. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended that this Summary be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a network architecture diagram illustrating aspects of a storage system according to one exemplary embodiment; 
         FIG. 2  is a block diagram illustrating the division of physical capacity within a data storage server into provisions and territories according to one exemplary embodiment; 
         FIG. 3  is a data structure diagram illustrating a global volume table and a global volume table header according to one exemplary embodiment; 
         FIG. 4  is a data structure diagram illustrating a system volume table segment and system volume table entries according to one exemplary embodiment; 
         FIG. 5  is a data structure diagram illustrating a volume table mapping both directly into physical storage and indirectly via system volume table segments according to one exemplary embodiment; 
         FIG. 6  is a logical flow diagram illustrating a process for providing thin provisioning in a high capacity data storage system according to one exemplary embodiment; 
         FIG. 7  is a logical flow diagram illustrating a process for handling writes to data volumes according to one exemplary embodiment; 
         FIG. 8  is a logical flow diagram illustrating a process for handling writes to snapshots according to one exemplary embodiment; and 
         FIG. 9  is a computer architecture diagram illustrating a computer hardware architecture for a computing system capable of high capacity data storage. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is directed to technologies for providing large capacity storage servers that can support thin provisioning and snapshots while maintaining a reduced memory footprint. Through the use of the embodiments presented herein, technologies for tracking territories, provisions, and chunks within a storage system can support an increased storage capacity and an increased number of snapshots. A thin provisioning storage system can use various tables or data structures to store details related to the various storage provisioning elements and snapshots. The tables may be maintained within the main memory of the storage system to support quick access during I/O operations. As a storage system approaches several terabytes (TB) in capacity, efficient management of these tables can reduce operating delays and memory overruns. Technology presented herein can support increased storage capacities efficiently. For example, up to 256 TB, or more, or storage capacity may be supported. Additionally, thousands of snapshots may be supported within a storage system. 
     Technologies presented herein relate to U.S. patent application Ser. No. 11/254,347, filed on Oct. 20, 2005, and entitled “Method, System, Apparatus, and Computer-Readable Medium for Provisioning Space in a Data Storage System,” and to U.S. Pat. No. 7,373,366 entitled “Method, System, Apparatus, and Computer-Readable Medium for Taking and Managing Snapshots of a Storage Volume” both of which are expressly incorporated herein by reference in their entirety. 
     While the subject matter described herein is presented in the general context of program modules that execute in conjunction with the execution of an operating system and application programs on a computer system, those skilled in the art will recognize that other implementations may be performed in combination with other types of program modules. Generally, program modules include routines, programs, components, data structures, and other types of structures that perform particular tasks or implement particular abstract data types. Moreover, those skilled in the art will appreciate that the subject matter described herein may be practiced with other computer system configurations, including hand-held devices, multiprocessor systems, microprocessor-based or programmable consumer electronics, minicomputers, mainframe computers, and the like. 
     In the following detailed description, references are made to the accompanying drawings that form a part hereof, and which are shown by way of illustration specific embodiments or examples. Referring now to the drawings, in which like numerals represent like elements through the several figures, aspects of a computing system and methodology for large capacity storage servers with thin provisioning, snapshots, and reduced memory footprints will be described. 
     Turning now to  FIG. 1 , details will be provided regarding an illustrative operating environment for the implementations presented herein, as well as aspects of several software components that provide the functionality described herein for continuous data protection. In particular,  FIG. 1  is a network architecture diagram showing aspects of a storage system  100  that includes several virtualized clusters  5 A- 5 B. A virtualized cluster is a cluster of different storage nodes that together expose a single storage device. In the example storage system  100  shown in  FIG. 1 , the clusters  5 A- 5 B (collectively, clusters  5 ) include storage server computers  2 A- 2 G (also referred to herein as “storage nodes” or a “node”, collectively nodes  2 ) that are operative to read and write data to one or more mass storage devices, such as hard disk drives. The cluster  5 A includes the nodes  2 A- 2 D and the cluster  5 B includes the nodes  2 E- 2 G. All of the nodes  2  in a cluster  5  can be physically housed in the same rack, located in the same building, or distributed over geographically diverse locations, such as various buildings, cities, or countries. 
     According to implementations, the nodes within a cluster may be housed in a one rack space unit storing up to four hard disk drives. For instance, the node  2 A is a one rack space computing system that includes four hard disk drives  4 A- 4 D (collectively, disks  4 ). Alternatively, each node may be housed in a three rack space unit storing up to fifteen hard disk drives. For instance, the node  2 E includes fourteen hard disk drives  4 A- 4 N. Other types of enclosures may also be utilized that occupy more or fewer rack units and that store fewer or more hard disk drives. In this regard, it should be appreciated that the type of storage enclosure and number of hard disk drives utilized is not generally significant to the implementation of the embodiments described herein. Any type of storage enclosure and virtually any number of hard disk devices or other types of mass storage devices may be utilized. 
     As shown in  FIG. 1 , multiple storage nodes may be configured together as a virtualized storage cluster. For instance, the nodes  2 A- 2 D have been configured as a storage cluster  5 A and the nodes  2 E- 2 G have been configured as a storage cluster  5 B. In this configuration, each of the storage nodes  2 A- 2 G is utilized to handle I/O operations independently, but are exposed to the initiator of the I/O operation as a single device. It should be appreciated that a storage cluster may include any number of storage nodes. A virtualized cluster in which each node contains an independent processing unit, and in which each node can field I/Os independently (and route them according to the cluster layout) is called a horizontally virtualized or peer cluster. A cluster in which each node provides storage, but the processing and mapping is done completely or primarily in a single node, is called a vertically virtualized cluster. 
     Data may be striped across the nodes of each storage cluster. For instance, the cluster  5 A may stripe data across the storage nodes  2 A,  2 B,  2 C, and  2 D. The cluster  5 B may similarly stripe data across the storage nodes  2 E,  2 F, and  2 G. Striping data across nodes generally ensures that different I/O operations are fielded by different nodes, thereby utilizing all of the nodes simultaneously, and that the same I/O operation is not split between multiple nodes. Striping the data in this manner provides a boost to random I/O performance without decreasing sequential I/O performance. 
     According to embodiments, each storage server computer  2 A- 2 G includes one or more network ports operatively connected to a network switch  6  using appropriate network cabling. It should be appreciated that, according to embodiments of the invention, Ethernet or Gigabit Ethernet may be utilized. However, it should also be appreciated that other types of suitable physical connections may be utilized to form a network of which each storage server computer  2 A- 2 G is a part. Through the use of the network ports and other appropriate network cabling and equipment, each node within a cluster is communicatively connected to the other nodes within the cluster. Many different types and number of connections may be made between the nodes of each cluster. Furthermore, each of the storage server computers  2 A- 2 G need not be connected to the same switch  6 . The storage server computers  2 A- 2 G can be interconnected by any type of network or communication links, such as a LAN, a WAN, a MAN, a fiber ring, a fiber star, wireless, optical, satellite, or any other network technology, topology, protocol, or combination thereof. 
     Each cluster  5 A- 5 B is also connected to a network switch  6 . The network switch  6  is connected to one or more client computers  8 A- 8 N (also referred to herein as “initiators”). It should be appreciated that other types of networking topologies may be utilized to interconnect the clients and the clusters  5 A- 5 B. It should also be appreciated that the initiators  8 A- 8 N may be connected to the same local area network (“LAN”) as the clusters  5 A- 5 B or may be connected to the clusters  5 A- 5 B via a distributed wide area network, such as the Internet. An appropriate protocol, such as the Internet Small Computer Systems Interface (“iSCSI”) protocol may be utilized to enable the initiators  8 A- 8 D to communicate with and utilize the various functions of the storage clusters  5 A- 5 B over a wide area network such as the Internet. 
     Two or more disks  4  within each cluster  5 A- 5 B or across clusters  5 A- 5 B may be mirrored for data redundancy and protection against failure of one, or more, of the disks  4 . Examples of the disks  4  may include hard drives, spinning disks, stationary media, non-volatile memories, or optically scanned media; each, or in combination, employing magnetic, capacitive, optical, semiconductor, electrical, quantum, dynamic, static, or any other data storage technology. The disks  4  may use IDE, ATA, SATA, PATA, SCSI, USB, PCI, Firewire, or any other bus, link, connection, protocol, network, controller, or combination thereof for I/O transfers. 
     Storage volume snapshots and continuous data protection features may be provided on one or more storage server computers  2 A- 2 G, one or more storage clusters  5 A- 5 B, or one or more client computers  8 A- 8 N. Furthermore, the processes for implementing CDP or snapshots for CDP may execute on any of these systems or may operate in a distributed fashion with components executing across two or more of these systems. 
     Referring now to  FIG. 2 , a block diagram  200  illustrates division of the physical capacity within a storage server computer  2  into provisions  220 A- 220 N and territories  210 A- 210 N according to one exemplary embodiment. The available physical capacity of the computer  2  can be made up of a number of hard disk drives  4 A- 4 D. It should be appreciated that other storage nodes connected to the computer  2  may also contribute physical capacity to the available physical capacity of the computer  2 . The available physical capacity can be divided into a number of unique, equally sized areas, called territories  210 A- 210 N. According to embodiments, the size of a territory may be a reduced size of 8 megabytes (MB). However, it should be appreciated that territories of other sizes may be utilized. 
     The available physical capacity may be further subdivided into units referred to herein as provisions  220 A- 220 N. The provisions  220 A- 220 N can comprise unique, equally sized areas of the available physical capacity. As one example, the provisions  220 A- 220 N may be a reduced size of 512 kilobytes (KB). A 512 KB provision can support a reduction of the storage used upon creation of a snapshot by up to half, compared to a system using 1 MB provisions. It should be appreciated that provisions of other sizes may also be utilized. 
     A provision can be further subdivided into chunks  230 A- 230 C. Chunk size can be specified at the time of volume creation. According to one embodiment, the chunk size can be selected as 64 KB, or a reduced size of 8 KB. Selecting an 8 KB chunk granularity can benefit volumes serving online transaction processing (OLTP) applications such as email or database systems. Such applications may often serve 8 KB random I/O operations. When a volume using 64 KB chunk granularity receives an 8 KB write to a snapshot, a wasteful series of operations may ensure. For example, a previous 64 KB chunk may have to be read, the appropriate 8 KB of new data modified, and finally the whole 64 KB chunk can be written to the new provision allocation. Alternatively, if the chunk granularity is the same size as the write (8 KB in the given example), then a single write may suffice and the read-modify-write (RMW) operation may be avoided. 
     A thin provisioning system can support provision tracking. Provision tracking is the ability to track “unused” and “written” portions of the storage space with very fine granularities so that snapshot actions may minimize wasted space. The size of a chunk  230 A- 230 C can imply a chunk storage granularity for provision tracking. After a snapshot event, data writes may entail allocating a new provision  220 A- 220 N for the new data to be written. Thus, for every chunk  230 A- 230 C that was written in a previous snapshot lifetime and written anew in a subsequent snapshot lifetime, a new provision is allocated. The provision  220 A- 220 N can be allocated at the reduced provision granularity 512 KB. A provision allocation involving a new territory can cause a territory to be allocated on demand. The territory allocation can be made at a reduced granularity of 8 MB. Once the provision  220 A- 220 N is allocated, the new data write can be made to a chunk  230 A- 230 C within the new provision  220 A- 220 N. The chunk can be written at either a 64 KB granularity or a reduced 8 KB granularity. Thus, allocation of new physical storage space occurs at the territory granularity, for example the reduced 8 MB granularity. However, after a snapshot is created, write allocations can occur at the provision granularity, for example the reduced provision granularity of 512 KB. 
     Turning now to  FIG. 3 , a data structure diagram  300  illustrates a global volume table  310  and a global volume table header  320  according to one exemplary embodiment. For a given storage volume, an array called a volume table (VT) can maintain a mapping of logical addresses to physical territories. The various volume tables for the various volumes within a data storage system can be stored together in a global volume table (GVT)  310 . Each volume table within the GVT  310  can be specified by a pointer or offset into the GVT  310  where the respective VT begins. 
     The size of a given VT within the GVT  310  depends upon the size of the associated volume. As an example, a GVT can store 32 million VT entries. If each entry can support 8 MB, then 32 million entries can address up to 256 TB. According to this example, a 256 TB storage system with two volumes, each of size 128 TB, may be supported by two VTs within the GVT  310 . Then first VT can map 16 million territories and the second VT can map the remaining 16 million territories. 
     A GVT header  320  structure can serve as an index of VTs within the GVT  310 . An entry within the GVT header  320  can be associated with a given storage volume, and thus a VT. An entry in the GVT header  320  can specify the size of the associated VT and the offset of that VT within the GVT  310 . The entry in the GVT header  320  can also specify whether the associated volume belongs to a container or not. According to one embodiment, if a storage container can support a maximum of 64 volumes, a GVT header  320  may be an array of 64 entries, one for each volume. It should be appreciated that a different number of possible volumes may be supported. 
     A volume table entry  330  is the element within a VT, and thus within a GVT  310 . The VT entry  330  can map an 8 MB logical territory to an 8 MB physical territory, either directly or through another level of mapping. An example VT entry  330  can be eight bytes in length. An eight byte VT entry  330  can include a 30 bit territory pointer field, an 11 bit sequence number field for snapshots, 7 reserved bits, and a 16 bit provision tracking bitmap. The provision tracking bitmap can support tracking written and unwritten provisions directly within the VT. The size of the VT entry  330  and the fields within the VT entry  330  are only an example, the fields, the sizes of the fields, and the size of the VT entry  330  may differ from these examples. 
     The 30 bit territory pointer field may be further broken down as a 1 bit field and a 29 bit field. The 1 bit field can be used to indicate whether the address translation is one or two level. That is, this 1 bit field may indicate whether the entry directly points to a physical address or points indirectly through a system volume table (SVT) as discussed in further detail with respect to  FIG. 5 . The 29 bit field can store either a physical territory address or an SVT pointer depending upon the setting of the 1 bit field. 
     Turning now to  FIG. 4 , a data structure diagram  400  illustrates a system volume table segment  410  and system volume table entries  420  according to one exemplary embodiment. Volume tables (VT) alone, as discussed with respect to  FIG. 3  may not resolve volumes with active snapshots. Volumes with active snapshots can have physical territories containing data from both current and previous snapshot lifetimes. In order to track the current lifetime (volume) location, an intermediate data structure can be used to provide accounting on a provision level. A system volume table (SVT) can serve as such a data structure. An SVT can link provisions belonging to the same address, but that also belonging to different snapshot lifetimes. An SVT can contain an SVT entry  420  for each provision in the system. According to examples discussed herein, a provision can store 512 KB. An SVT segment  410  can have 16 SVT entries  420  each addressing a 512 KB provision, thus totaling 8 MB (or one territory) per SVT entry  420 . Other provision or territory sizes may also be supported. 
     An SVT entry  420  can store several pieces of information related to each provision. According to the illustrated example, an SVT entry  420  can store an 11 bit sequence number to specify the lifetime that the provision was allocated in, a 29 bit SVT down pointer  430 , and a new writes bitmap. The new writes bitmap can contain 8 bits or 64 bits depending on whether the volume uses 64 KB chunks or 8 KB chunks. The smaller chunk size of 8 KB can divide a provision into more chucks, so more bits are required in the new writes bitmap to associate with each chunk. The new writes bitmap can provide a bit for each chunk within the provision. These bits can represent whether a particular chunk is valid within the provision or not. Thus, the bit indicates if a given provision has the relevant data written to it, or if an early lifetime snapshot is to be referenced to find the data. The SVT entry  420  can be six bytes for a 64 KB chunk volume, or 13 bytes for an 8 KB chunk volume. 
     An example SVT entry down pointer  430  can have 29 bits that can be again split into various fields. These fields of the SVT down pointer  430  can be a 2 bit segment type, a 16 bit page number, a 7 bit segment number, and a 4 bit entry number. The fields, and field lengths of the SVT entry  420  and the SVT down pointer  430  may differ from these nonlimiting examples. The SVT down pointer  430  can be used as a pointer to the next physical provision belonging to the same volume and with the same logical provision number. During a read, if the new writes bitmap is not set for the provision being sought, the SVT down pointer  420  can be used to traverse into the SVT until an SVT entry  420  is identified with the bit set indicating that it has data for that provision. 
     Turning now to  FIG. 5 , a data structure diagram  500  illustrates a volume table  510  mapping both directly into a physical storage  520  and indirectly via system volume table segments  410  according to one exemplary embodiment. A volume table (VT)  510  can support directing I/O operations from a virtual address to a correct physical address either directly or indirectly. Data may be stored in a scattered fashion over one or more physical disks  4 . Thin provisioning may be considered a mapping from a virtual address associated within a volume to a physical address associated with physical storage  520 . The physical storage  520  can include one or more disks  4 , or any other physical data storage media. As discussed with respect to  FIG. 3 , a VT  510  can be stored within a GVT  310  and can contain one or more VT entries  330 . 
     As illustrated in  FIG. 5 , a VT entry  330  can point directly to physical storage  520 . In such an instance, a virtual address can be used to index into the VT  510  and recover the information for accessing the physical storage  520 . A VT entry  330  can also have an indirect relationship to the physical storage  520 . In the indirect instance, the VT entry  330  may point to an SVT segment  410 . The SVT segment  410  may, in turn, include a physical pointer, or addressing information, for accessing the physical storage  520 . The SVT can be considered an intermediate structure, as it is mapped between the VT and the physical storage  520 . Each VT entry  330  can directly, or indirectly, address one territory. In an example discussed with respect to  FIG. 2 , a territory can store 8 MB of storage capacity. In an indirect addressing instance, the SVT segment  410  associated with the VT entry  330  can have 16 SVT entries  420  each addressing 512 KB. Each SVT segment  410  can include a 31 bit physical territory pointer along with a 1 bit flag to specify if the segment is free. 
     As discussed with respect to  FIG. 3 , the volume table entry  330  can directly track unwritten provisions. This can be used when the VT entry  330  directly points to physical storage  520 . However, such mapping may not support snapshots. Thus, a territory involving snapshots can use indirect addressing from a VT entry  330  via an SVT segment  410 . Territories that have been allocated for snapshots can also be tracked by a data structure similar to a volume table, called a snapshot territory table (STT)  530 . A separate STT  530  can be maintained for each storage container. The STT  530  can be stored to disk independently from the GVT  310 . 
     According to one embodiment, the SVT may be limited to 300 MB to maintain efficient use of memory within the storage server  2 . SVT segments  410  can be allocated in 8 KB blocks called aligned SVT segments as needed. The allocation can cease if the SVT space reaches the 300 MB limit. After the 300 MB limit is reached, VT entries  330  may no longer indirectly map through an SVT and may directly map to the physical storage  520 . In this instance, snapshots may no longer be supported in association with the volume. 
     Turning now to  FIG. 6 , additional details will be provided regarding the embodiments presented herein for large capacity storage servers with thin provisioning, snapshots, and reduced memory footprints. In particular,  FIG. 6  is a flow diagram showing a routine  600  that illustrates aspects of a process for providing thin provisioning in a high capacity data storage system according to one exemplary embodiment. It should be appreciated that the logical operations described herein are implemented (1) as a sequence of computer implemented acts or program modules running on a computing system and/or (2) as interconnected machine logic circuits or circuit modules within the computing system. The implementation is a matter of choice dependent on the performance and other requirements of the computing system. Accordingly, the logical operations described herein are referred to variously as operations, structural devices, acts, or modules. These operations, structural devices, acts and modules may be implemented in software, in firmware, in special purpose digital logic, and any combination thereof. It should also be appreciated that more or fewer operations may be performed than shown in the figures and described herein. These operations may also be performed in parallel, or in a different order than those described herein. 
     The routine  600  can begin with operation  610  where chunks, provisions, and territories having reduced granularity sizes may be supported. As discussed with respect to  FIG. 2 , examples of reduced granularities can include territory allocations of 8 MB, provisions of 512 KB, and chunks of either 64 KB or 8 KB. Other sizes or combinations of sizes may also be used. Reduced allocation granularities can support more efficient storage system operations and reduction in wasted space during various storage operations. 
     At operation  620 , a global volume table (GVT)  310  can be provided for storing and organizing one or more volume tables (VT). At operation  630 , territory mapping from a VT can be supported in two ways. First, territories may be mapped from the VT, or GVT  310 , directly to physical storage. Alternatively, territories may be mapped from the VT, or GVT  310 , indirectly via a system volume table segment (SVT segment)  410 . This flexibility can support the bypassing of an SVT when snapshots are not being used in a given volume. Thus, the memory that may have been used to maintain the unused SVT entries can be more efficiently used for some other function. 
     At operations  640 , a snapshot territory table (STT)  530  can be provided for tracking territories that have been allocated to snapshots. Routine  700  can support processing write operations to storage volumes. Additional details related to routine  700  are discussed with respect to  FIG. 7 . Routine  800  can support processing write operations to storage snapshots. Additional details related to routine  800  are discussed with respect to  FIG. 8 . 
     At operation  650 , increased storage capacity and an increased number of snapshots can be supported. Through technologies discussed herein, a huge number of volumes and a very large capacity of storage may be supported. For example, thousands of snapshots and 256 TB, or more, of total capacity. These technologies can be implemented with a minimum impact on CPU and memory resources within the storage server  2  while maintaining a very high level of system performance. 
     Turning now to  FIG. 7 , additional details will be provided regarding the embodiments presented herein for large capacity storage servers with thin provisioning, snapshots, and reduced memory footprints. In particular,  FIG. 7  is a flow diagram illustrating a routine  700  that shows aspects of a process for handling writes to data volumes according to one exemplary embodiment. The routine  700  can begin with operation  702  where a write operation is received for a storage volume. In operation  704 , a lookup is performed at the corresponding VT entry  330 . At operation  706 , it is determined if a valid territory pointer exists in the VT entry  330 . 
     If it is determined at operation  706  that there is not a valid territory pointer in the VT entry  330  corresponding to the write address, the routine  700  can continue to operation  708  to handle a first write arriving to that 8 MB territory associated with the write operation received at operation  702 . A direct mapping to the new territory can be established by populating the VT entry  330  with a pointer to the physical territory within the physical storage  520 . At operation  710 , it is determined if an SVT segment  410  is available for converting the physical pointer to an indirect pointer. If no free SVT segment  410  is available, the routine  700  can transition to operation  716  where the write request can be satisfied to the territory with a direct mapping from the VT entry  330 . The routine  700  can return after the write I/O is performed in operation  716 . 
     If it is determined in operation  710  that a free SVT segment  410  is available, the routine can continue to operation  712  where the direct mapping created in operation  708  can be converted to an indirect mapping. This conversion can occur by redirecting the pointer in the VT entry  330  to a newly allocated SVT segment  410  and then pointing the SVT segment  410  to the physical storage  520 . Effectively, the SVT segment  410  is inserted between the VT entry  330  and the physical storage  520  as illustrated in  FIG. 5 . At operation  714 , the write I/O received at operation  702  can be satisfied to the new indirectly addressed physical storage  520 . The routine  700  can return after the write I/O is performed in operation  714 . 
     If it is determined at operation  706  that a valid territory pointer exists in the VT entry  330  corresponding to the write operation, the routine  700  can continue to operation  718  where it can be determined if the snapshot lifetime associated with the existing territory is for the current lifetime or a previous snapshot. This can be determined, for example, by comparing the sequence number in the VT entry  330  with the sequence number of the current volume. 
     If it is determined at operation  718  that the snapshot lifetime associated with the existing territory is for the current lifetime (generally referred to as the volume) then the write operation can be performed in the existing provision. As such, the routine  700  can continue to operation  720  where the write I/O received at operation  702  can be satisfied to the current provision by overwriting the existing data. This overwrite may be acceptable since the provision is in the current lifetime so no snapshot was created between the overwritten data and the data being written presently. This write can occur either directly or indirectly through an SVT segment  410  as determined by the pointer in the VT entry  330  that was identified in operation  704 . The routine  700  can return after the write I/O is performed in operation  720 . 
     If it is determined at operation  718  that the snapshot lifetime associated with the existing territory is for a previous lifetime (generally referred to as a previous snapshot) then new storage can be allocated to preserve the data stored in the previous lifetime as a snapshot provision. Such techniques for preserving data from previous snapshots from being overwritten may be dictated by the operation of the snapshot management mechanism within the storage server  2 . As such, the routine  700  can proceed to operation  722  where it can be determined if the VT mapping is direct to a physical storage  520  or indirect via an SVT entry  330 . 
     If it is determined at operation  722  that the VT mapping is direct to a physical storage  520 , it may be desirable to reroute the direct mapping through a newly allocated SVT segment  410 . Thus, the routine  700  can continue to operation  724  where it can be determined if a free SVT segment  410  is available for converting the physical pointer to an indirect pointer. If it is determined at operation  724  that no free SVT segments  410  are available, the routine  700  can transition to operation  726  where the sequence number in the VT entry  330  is overwritten to the sequence number of the current volume. This operation can take the territory that was being used by a previous snapshot and reuse it for the current volume which can take priority. In operation  728 , snapshots associated with the sequence number overwritten in operation  726  are invalidated. This may be done because the snapshots in question may become flawed by the loss of the territory that was repurposed in operation  726 . At operation  730 , the write I/O received at operation  702  can be satisfied to the repurposed territory. The routine  700  can return after the write I/O is performed in operation  730 . 
     If it is determined in operation  724  that a free SVT segment  410  is available, the routine can continue to operation  732  where the direct mapping of the territory can be converted to an indirect mapping. This conversion can occur by redirecting the pointer in the VT entry  330  to a newly allocated SVT segment  410  and then pointing the SVT segment  410  to the physical storage  520 . Effectively, the SVT segment  410  is inserted between the VT entry  330  and the physical storage  520  as illustrated in  FIG. 5 . At operation  734 , the write I/O received at operation  702  can be satisfied to the new indirectly addressed physical storage  520 . The routine  700  can return after the write I/O is performed in operation  734 . 
     If it is determined at operation  722  that the VT mapping is indirect via an SVT entry  330 , a new provision within the SVT segment  410  may be allocated if available. Thus, the routine  700  can continue to operation  736  where it can be determined if there is an available provision for allocation. 
     If it is determined at operation  736  that a provision is available for allocation, the routine  700  may continue to operation  738  where a new provision is allocated. In operation  740 , the write I/O received at operation  702  can be satisfied to the newly allocated provision from operation  738 . The routine  700  can return after the write I/O is performed in operation  740 . 
     If it is determined at operation  736  that a provision is not available for allocation, the routine  700  may continue to operation  742  where a previous snapshot provision may be overwritten. The sequence number of a previous entry within the chain of the SVT can be overwritten to the current volume sequence number. This operation can take the provision that was being used by a previous snapshot and reuse it for the current volume which can take priority. In operation  744 , snapshots associated with the sequence number overwritten in operation  742  are invalidated. This may be done because the snapshots in question may become flawed by the loss of the territory that was repurposed in operation  742 . At operation  746 , the write I/O received at operation  702  can be satisfied to the repurposed provision. The routine  700  can return after the write I/O is performed in operation  746 . It should be appreciated then when a snapshot is overwritten in operation  742 , the SVT segment  410  can be traversed to find a writable snapshot to be overwritten. If no writeable snapshot is identified, a read-only snapshot may be overwritten. As discussed, snapshots that rely on the particular overwritten sequence number are marked as invalid. A background monitoring service may receive a notification of invalidated snapshots. The background monitoring service can then delete the invalidated snapshot to reclaim the snapshot provision space. 
     Turning now to  FIG. 8 , additional details will be provided regarding the embodiments presented herein for large capacity storage servers with thin provisioning, snapshots, and reduced memory footprints. In particular,  FIG. 8  is a flow diagram illustrating a routine  800  that shows aspects of a process for handling writes to snapshots according to one exemplary embodiment. The routine  800  can begin with operation  802  where a write operation is received for a storage snapshot. In operation  804 , a lookup is performed at the corresponding VT entry  330 . At operation  806 , it is determined if a valid territory pointer exists in the VT entry  330 . 
     If it is determined at operation  806  that there is not a valid territory pointer in the VT entry  330  corresponding to the write address, the routine  800  can continue to operation  808  to handle a first write arriving to that 8 MB territory associated with the write operation received at operation  802 . A direct mapping to the new territory can be established by populating the VT entry  330  with a pointer to the physical territory within the physical storage  520 . At operation  810 , it can be determined if an SVT segment  410  is available for converting the physical pointer within the VT to an indirect pointer. If no free SVT segment  410  is available, the routine  800  can transition to operation  816  where the write request can be satisfied to the territory with a direct mapping from the VT entry  330 . The routine  800  can return after the write I/O is performed in operation  816 . 
     If it is determined in operation  810  that a free SVT segment  410  is available, the routine  800  can continue to operation  812  where the direct mapping created in operation  808  can be converted to an indirect mapping. This conversion can occur by redirecting the pointer in the VT entry  330  to a newly allocated SVT segment  410  and then pointing the SVT segment  410  to the physical storage  520 . Effectively, the SVT segment  410  can be inserted between the VT entry  330  and the physical storage  520  as illustrated in  FIG. 5 . At operation  814 , the write I/O received at operation  702  can be satisfied to the new indirectly addressed physical storage  520 . The routine  800  can return after the write I/O is performed in operation  814 . 
     If it is determined at operation  806  that a valid territory pointer exists in the VT entry  330  corresponding to the write operation, the routine  800  can continue to operation  822  where it can be determined if the VT mapping is direct to a physical storage  520  or indirect via an SVT entry  330 . 
     If it is determined at operation  822  that the VT mapping is direct to a physical storage  520 , it may be desirable to reroute the direct mapping through a newly allocated SVT segment  410 . Thus, the routine  800  can continue to operation  824  where it can be determined if a free SVT segment  410  is available for converting the physical pointer to an indirect pointer. If it is determined at operation  824  that no free SVT segments  410  are available, the routine  800  can transition to operation  829  where a failure error is returned for the write I/O request received at operation  802 . The routine  800  can return after the error message is generated in operation  829 . 
     If it is determined in operation  824  that a free SVT segment  410  is available, the routine can continue to operation  832  where the direct mapping of the territory can be converted to an indirect mapping. This conversion can occur by redirecting the pointer in the VT entry  330  to a newly allocated SVT segment  410  and then pointing the SVT segment  410  to the physical storage  520 . Effectively, the SVT segment  410  is inserted between the VT entry  330  and the physical storage  520  as illustrated in  FIG. 5 . At operation  834 , the write I/O received at operation  802  can be satisfied to the new indirectly addressed physical storage  520 . The routine  800  can return after the write I/O is performed in operation  834 . 
     If it is determined at operation  822  that the VT mapping is indirect via an SVT entry  330 , a new provision within the SVT segment  410  may be allocated if available. Thus, the routine  800  can continue to operation  836  where it can be determined if there is an available provision for allocation. 
     If it is determined at operation  836  that a provision is available for allocation, the routine  800  may continue to operation  838  where a new provision is allocated. In operation  840 , the write I/O received at operation  802  can be satisfied to the newly allocated provision from operation  838 . The routine  800  can return after the write I/O is performed in operation  840 . 
     If it is determined at operation  836  that a provision is not available for allocation, the routine  800  may continue to operation  845  where a failure error is returned for the write I/O request received at operation  802 . The routine  800  can return after the error message is generated in operation  845 . 
     It should be appreciated that a significant difference between processing volume writes, as discussed with respect to  FIG. 7 , and processing snapshot writes, as discussed with respect to  FIG. 8 , is the handling of allocation failures. In the snapshot case, failure to allocate an SVT segment  410  or failure to allocate a provision can result in the I/O operation failing, while a previous snapshot can be overwritten to accommodate the write in the case of a volume write. This difference may be due to a volume write representing a write to the current state of a storage volume and thus may be provided priority over previous snapshots. In contrast, there is no default reason to overwrite one snapshot for another snapshot and generally it would not be desirable to risk loss of a volume for the sake of a write to a previous snapshot. 
       FIG. 9  and the following discussion are intended to provide a brief, general description of a suitable computing environment in which the embodiments described herein may be implemented. While the technical details are presented herein in the general context of program modules that execute in conjunction with the execution of an operating system, those skilled in the art will recognize that the embodiments may also be implemented in combination with other program modules. 
     Generally, program modules include routines, programs, components, data structures, and other types of structures that perform particular tasks or implement particular abstract data types. Moreover, those skilled in the art will appreciate that the embodiments described herein may be practiced with other computer system configurations, including hand-held devices, multiprocessor systems, microprocessor-based or programmable consumer electronics, minicomputers, mainframe computers, and the like. The embodiments described herein may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote memory storage devices. 
     In particular,  FIG. 9  shows an illustrative computer architecture for a storage server computer  2  that may be utilized in the implementations described herein. Such an illustrative computer system may also describe a client computer system  8 A- 8 N. The storage node computer  2  includes a baseboard, or “motherboard”, which is a printed circuit board to which a multitude of components or devices may be connected by way of a system bus or other electrical communication paths. In one illustrative embodiment, a CPU  22  operates in conjunction with a chipset  52 . The CPU  22  is a standard central processor that performs arithmetic and logical operations necessary for the operation of the computer. The storage node computer  2  may include a multitude of CPUs  22 . 
     The chipset  52  includes a north bridge  24  and a south bridge  26 . The north bridge  24  provides an interface between the CPU  22  and the remainder of the computer  2 . The north bridge  24  also provides an interface to a random access memory (“RAM”) used as the main memory  54  in the computer  2  and, possibly, to an on-board graphics adapter  30 . The north bridge  24  may also include functionality for providing networking functionality through a gigabit Ethernet adapter  28 . The gigabit Ethernet adapter  28  is capable of connecting the computer  2  to another computer via a network. Connections which may be made by the network adapter  28  may include LAN or WAN connections. LAN and WAN networking environments are commonplace in offices, enterprise-wide computer networks, intranets, and the internet. The north bridge  24  is connected to the south bridge  26 . 
     The south bridge  26  is responsible for controlling many of the input/output functions of the computer  2 . In particular, the south bridge  26  may provide one or more universal serial bus (“USB”) ports  32 , a sound adapter  46 , an Ethernet controller  60 , and one or more general purpose input/output (“GPIO”) pins  34 . The south bridge  26  may also provide a bus for interfacing peripheral card devices such as a graphics adapter  62 . In one embodiment, the bus comprises a peripheral component interconnect (“PCI”) bus. The south bridge  26  may also provide a system management bus  64  for use in managing the various components of the computer  2 . Additional details regarding the operation of the system management bus  64  and its connected components are provided below. 
     The south bridge  26  is also operative to provide one or more interfaces for connecting mass storage devices to the computer  2 . For instance, according to an embodiment, the south bridge  26  includes a serial advanced technology attachment (“SATA”) adapter for providing one or more serial ATA ports  36  and an ATA 100 adapter for providing one or more ATA 100 ports  44 . The serial ATA ports  36  and the ATA 100 ports  44  may be, in turn, connected to one or more mass storage devices storing an operating system  40  and application programs, such as the SATA disk drive  38 . As known to those skilled in the art, an operating system  40  comprises a set of programs that control operations of a computer and allocation of resources. An application program is software that runs on top of the operating system software, or other runtime environment, and uses computer resources to perform application specific tasks desired by the user. 
     According to one embodiment of the invention, the operating system  40  comprises the LINUX operating system. According to another embodiment of the invention the operating system  40  comprises the WINDOWS SERVER operating system from MICROSOFT CORPORATION. According to another embodiment, the operating system  40  comprises the UNIX or SOLARIS operating system. It should be appreciated that other operating systems may also be utilized. 
     The mass storage devices connected to the south bridge  26 , and their associated computer-readable media, provide non-volatile storage for the computer  2 . Although the description of computer-readable media contained herein refers to a mass storage device, such as a hard disk or CD-ROM drive, it should be appreciated by those skilled in the art that computer-readable media can be any available media that can be accessed by the computer  2 . By way of example, and not limitation, computer-readable media may comprise computer storage media and communication media. Computer storage media includes volatile and non-volatile, removable and non-removable media implemented in any method or technology for storage of information such as computer-readable instructions, data structures, program modules or other data. Computer storage media includes, but is not limited to, RAM, ROM, EPROM, EEPROM, flash memory or other solid state memory technology, CD-ROM, DVD, HD-DVD, BLU-RAY, or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by the computer. 
     A low pin count (“LPC”) interface may also be provided by the south bridge  6  for connecting a “Super I/O” device  70 . The Super I/O device  70  is responsible for providing a number of input/output ports, including a keyboard port, a mouse port, a serial interface  72 , a parallel port, and other types of input/output ports. The LPC interface may also connect a computer storage media such as a ROM or a flash memory such as a NVRAM  48  for storing the firmware  50  that includes program code containing the basic routines that help to start up the computer  2  and to transfer information between elements within the computer  2 . 
     As described briefly above, the south bridge  26  may include a system management bus  64 . The system management bus  64  may include a BMC  66 . In general, the BMC  66  is a microcontroller that monitors operation of the computer system  2 . In a more specific embodiment, the BMC  66  monitors health-related aspects associated with the computer system  2 , such as, but not limited to, the temperature of one or more components of the computer system  2 , speed of rotational components (e.g., spindle motor, CPU Fan, etc.) within the system, the voltage across or applied to one or more components within the system  2 , and the available or used capacity of memory devices within the system  2 . To accomplish these monitoring functions, the BMC  66  is communicatively connected to one or more components by way of the management bus  64 . In an embodiment, these components include sensor devices for measuring various operating and performance-related parameters within the computer system  2 . The sensor devices may be either hardware or software based components configured or programmed to measure or detect one or more of the various operating and performance-related parameters. The BMC  66  functions as the master on the management bus  64  in most circumstances, but may also function as either a master or a slave in other circumstances. Each of the various components communicatively connected to the BMC  66  by way of the management bus  64  is addressed using a slave address. The management bus  64  is used by the BMC  66  to request and/or receive various operating and performance-related parameters from one or more components, which are also communicatively connected to the management bus  64 . 
     It should be appreciated that the computer  2  may comprise other types of computing devices, including hand-held computers, embedded computer systems, personal digital assistants, and other types of computing devices known to those skilled in the art. It is also contemplated that the computer  2  may not include all of the components shown in  FIG. 9 , may include other components that are not explicitly shown in  FIG. 9 , or may utilize an architecture completely different than that shown in  FIG. 9 . 
     Based on the foregoing, it should be appreciated that technologies for large capacity storage servers with thin provisioning, snapshots, and reduced memory footprints are presented herein. Although the subject matter presented herein has been described in language specific to computer structural features, methodological acts, and computer readable media, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features, acts, or media described herein. Rather, the specific features, acts and mediums are disclosed as example forms of implementing the claims. 
     The subject matter described above is provided by way of illustration only and should not be construed as limiting. Various modifications and changes may be made to the subject matter described herein without following the example embodiments and applications illustrated and described, and without departing from the true spirit and scope of the present invention, which is set forth in the following claims.