Patent Publication Number: US-8984239-B2

Title: Hardware and operating system support for persistent memory on a memory bus

Description:
RELATED APPLICATION 
     This application claims priority to, and is a continuation of, the U.S. Non-Provisional application Ser. No. 12/503,480 to Condit et al., entitled “Hardware and Operating System Support for Persistent Memory On A Memory Bus,” filed Jul. 15, 2009, which claims priority to U.S. Provisional Application No. 61/108,400 to Condit et al., entitled, “Cache Hierarchy, File System, and Operating System for Byte-Addressable, Non-Volatile Memory,” filed Oct. 24, 2008. 
    
    
     BACKGROUND 
     A traditional computing device directly connects a volatile memory component, such as dynamic random access memory (DRAM), to a memory bus; however, persistent storage devices, including disk and flash, are connected to a slow, low bandwidth input/output (I/O) bus. To achieve acceptable performance, a file system aggressively buffers data in the DRAM at the risk of data loss or file system corruption in the event of a failure (e.g., system crash or power loss). The file system, which may be a part of an operating system, includes responsibility for managing the data on the persistent storage. 
     In an implementation, the file system in the computing device may provide consistency guarantees, temporal safety guarantees, correctness, and performance for data and data accesses stored on a device. The consistency implemented by the file system assures that data stored in the persistent storage has not been corrupted, so that the data may describe a valid file system. The temporal safety may limit an amount of time that the data—once written by an application—resides in the volatile memory before being transferred to the non-volatile memory. In other words, the temporal safety (guarantee) defines the time between a write issued by an application, and the write becoming persistent. The application may include a program designed to perform a specific task such as reading or writing data. The correctness describes whether writes are reflected to the persistent storage after the writes were issued by the application. The consistency, the correctness, and the temporal safety may be sacrificed to a certain degree, in order to overcome performance limitations of the persistent storage. 
     SUMMARY 
     The following presents a simplified summary in order to provide a basic understanding of some aspects of the disclosed subject matter. This summary is not an extensive overview of the disclosed subject matter, and is not intended to identify key/critical elements or to delineate the scope of such subject matter. A purpose of the summary is to present some concepts in a simplified form as a prelude to the more detailed description that is presented later. 
     In an implementation, a computing device may include a non-volatile memory (to provide a persistent storage) that is directly connected to a memory bus (i.e., directly addressable), and placed side by side with a volatile memory. To this end, a file system supported by such a hardware setup may be implemented to improve correctness, temporal safety, consistency, and performance in the computing device, through the use of atomic updates and maintaining the order of writes. The file system may implement a tree structure, which allows large amounts of data to be atomically changed. 
     To accomplish the foregoing and other related ends, certain illustrative aspects are described herein in connection with the following description and the annexed drawings. These aspects are indicative of various ways in which the disclosed subject matter may be practiced, all of which are intended to be within the scope of the disclosed subject matter. Other advantages and novel features may become apparent from the following detailed description when considered in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description is described with reference to accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to reference like features and components. 
         FIG. 1  is a block diagram of an exemplary computing device architecture. 
         FIG. 2  is a block diagram of an exemplary control processing unit (CPU). 
         FIG. 3  is a block diagram of an exemplary cache controller. 
         FIG. 4  is a block diagram of an exemplary memory controller. 
         FIG. 5  is a block diagram of an exemplary non-volatile memory file system structure. 
         FIG. 6  is a block diagram illustrating an example of in-place append. 
         FIG. 7  is a block diagram illustrating an example of atomically updating a pointer to a copied and modified data block. 
         FIG. 8  is a block diagram illustrating an example of a partial copy-on write. 
         FIG. 9  is a block diagram illustrating an example of a partial copy-on-write involving multiple data blocks. 
         FIG. 10  is a flow diagram of a method for atomically updating a data structure. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure is directed towards systems, components, techniques and methods for implementing a byte-addressable, bit alterable persistent storage (i.e., provided by non-volatile memory) that is directly connected to a memory bus, and placed side by side with a dynamic random access memory (DRAM) component. In an implementation, a phase change memory (PCM) may be the byte-addressable, bit alterable persistent storage that is directly connected to the memory bus. In order to address the issues of consistency, safety, correctness, and performance, a file system (which is a part of an operating system) may manage data on the non-volatile memory. The file system may be used to organize the data in the non-volatile memory while improving consistency, safety, correctness, and performance. The improvement may be implemented by the file system through “ordering” and “atomicity” provided in the hardware setup. For example, the “ordering” provided by the hardware setup allows software to declare important ordering constraints between writes to non-volatile memory, ensuring that any writes reordered by a cache controller or a memory controller do not violate these constraints. The “atomicity” may assure correctness and the safety by writing data of a particular size (e.g., 8 bytes) either completely to the non-volatile memory or not at all. 
     A file system may be utilized with the non-volatile memory component to facilitate consistency and safety guarantees, as well as improved performance relative to disk-based file systems running on the same storage medium. For example, the file system can facilitate reading and writing of its data structures directly from the non-volatile memory component, as opposed to conventional file systems that maintain copies of data structures in volatile memory and then reflect changes to those data structures to non-volatile memory. 
     Furthermore, an operating system (OS) may utilize the non-volatile memory component to facilitate performing an “instant boot” (of the operating system). In an implementation, the instant boot uses the non-volatile memory component to provide persistent storage for OS data state. For instance, a snapshot (e.g., memory image) of the DRAM can be taken after the computer has booted and can be stored in the non-volatile memory component. On the next boot, the snapshot can be copied from the non-volatile memory component to the DRAM to facilitate bypassing at least a portion of the boot sequence. 
     In accordance with various other aspects and embodiments, the non-volatile memory component may be employed to facilitate efficiently updating a file system tree when a block (e.g., data block) in the file system tree is updated; to facilitate employing a fast-append operation to take advantage of any available unused space in a last data block of a file, where an append operation that can fit within an unused portion of a data block can be written directly to that part of the data block, and the file size can be updated atomically in order to commit the change; and/or to facilitate employing a fast copy operation, where a copy-on-write operation can be improved by copying a desired portion of the data (e.g., only copying the data that is to be preserved). In another implementation, the non-volatile memory component is utilized with regard to a set of uses for applications. For example, the non-volatile memory component is utilized to facilitate checkpointing application state, where an application can reflect the state of the application (e.g., heap, stack, registers) directly to the non-volatile memory component. The application can utilize such checkpoints to facilitate reliability and security. 
     Computing Device Architecture 
       FIG. 1  is a computing device architecture  100  that includes central processing unit or CPU component  102 , cache controller component  104 , northbridge component  106 , memory controller component  108 , non-volatile memory component  110 , dynamic random access memory (DRAM) component  112 , southbridge component  114 , and a hard disk component  116 . The CPU component  102  is a control unit in the computing device  100 . In an implementation, the CPU component  102  includes the cache controller component  104 , and the northbridge component  106  includes the memory controller component  108 . In other implementations, the CPU component  102  may be an integrated system that includes the cache controller component  104  and the memory controller  108 ; such an integrated system would connect the non-volatile memory component  110 , DRAM component  112 , and southbridge component  114  directly to CPU component  102 . During application writes (i.e., writes) for which the writes may include specific tasks, such as writing data to implement or execute a program, the computing device  100  implements a file system to assure temporal safety, correctness, consistency, and performance. The file system may manage data in the computing device  100 . 
     The CPU component  102  may include a system that integrates the cache controller component  104  and the memory controller  108 . The file system may be implemented by enforcing “ordering” and “atomicity” in a memory system of the computing device  100 . The memory system may include the CPU component  102  that may directly access the non-volatile memory component  110 . In an implementation, data from the non-volatile memory component  110  may be read into the CPU component  102  without being reflected in the DRAM component  112 . 
     In an implementation, the “ordering” includes a sequence of writes (in a certain order) that are issued by the file system. The computing device  100  stores the sequence of writes temporarily in the cache controller component  104 , and then flushes the sequence of writes from the cache controller component  104  into the memory controller component  108 , possibly in a different order to improve performance. In other implementations, the cache controller component  104  and the memory controller component  108  may preserve ordering for certain data writes, ensuring that the data writes are received by the non-volatile memory component  110  in the same order that the data writes were issued by the file system. The file system uses the “ordering” guarantee to enforce safety, consistency, and correctness guarantees. Furthermore, the “ordering” may include modification of the CPU component  102  e.g., using epoch identification (epoch ID), persistence bits, etc. as further discussed below. 
     The “atomicity” may include atomic write to non-volatile memory component  110  that may either be completed entirely, or not at all. In an implementation, the cache controller component  104  identifies atomic writes and send this information to the memory controller component  108 , which enforces atomicity for these data writes. In an implementation, the file system uses the atomicity to help enforce safety, consistency, and correctness guarantees. 
     The CPU component  102  includes a device or component (not shown) that interprets and executes instructions (such as writes) issued by an application. Epochs indicate ordering constraints on writes; specifically, the hardware must preserve ordering between two writes that occur in different epochs (i.e., writes in the same epoch may be reordered arbitrarily). The CPU component  102  includes a processor core that may provide the epoch ID. The epoch ID is used as a reference to maintain the “ordering” in the cache controller component  104  and the memory controller component  108 . The epoch ID indicates a current epoch, for which the CPU component  102  is writing to the non-volatile memory component  110 . The current epoch includes all writes issued to the non-volatile memory component  110  since the previous epoch boundary, as indicated by the application. In an implementation, the CPU component  102  issues writes to a cache (not shown) in the cache controller component  104  (e.g., CPU component  102  writes “A” to the cache in the cache controller component  104  where “A” is a variable). In turn, the cache controller component  104  stores the data to cache blocks identified by the epoch ID. Subsequently, the cache controller component  104  may perform writes to the memory controller component  108  (e.g., cache controller component  104  writes “A” to memory controller component  108 ). The transfer and processing of the data from the cache controller component  104  to the memory controller component  108  may be performed according to the ordering indicated by the epoch. The memory controller component  108  may write the data to the non-volatile memory component  110 , or to the DRAM component  112 . The writes performed by the cache controller component  104  and the memory controller component  108  may maintain “ordering” according to the epoch boundaries given by the application running in the computing device  100 . 
     The cache controller component  104  may include a memory subsystem in which frequently used data values may be duplicated for quick access. The cache controller component  104  includes the cache block that stores content of frequently accessed random access memory (RAM) locations, and addresses where the RAM locations may be stored. The cache controller component  104  also stores the content of non-volatile memory locations. When the CPU component  102  references an address in the non-volatile memory component  110 , the cache controller component  104  may verify if the address is held in the cache block. If the address is held in the cache block, then the contents at that address (i.e., data) may be returned to the CPU component  102 . 
     The cache controller component  104  implements mechanisms to enforce the “ordering” and the “atomicity” in the computing device  100 . As further discussed below, the cache controller component  104  may use persistence bits, epoch IDs, and atomic bits in order to enforce “ordering” within the cache controller component  104 . 
     The northbridge component  106  may include an interface between the CPU component  102 , the nonvolatile memory component  110 , and the DRAM component  112 . In an implementation, the northbridge component  106  is a system that includes the memory controller component  108 . The memory controller component  108  may include flexibility of access to the volatile memory (e.g., DRAM component  112 ) and the non-volatile memory component  110 . Data may be accessed at the same time on both components (i.e., non-volatile memory component  110  and DRAM component  112 ). In addition, the memory controller component  108  may be responsible for differentiating non-volatile component  110  (e.g., PCM) operations from DRAM component  112  operations by handling distinct timing characteristics of both technologies (i.e., non-volatile memory component  110 , and DRAM component  112 ). These properties of the memory controller component  108  remain true in an alternative implementation where the memory controller component  108  is integrated into the CPU component  102 , and the northbridge component  106  is not present. 
     In the same manner that the cache controller component  104  implements the mechanism to enforce “ordering” and “atomicity,” the memory controller component  108  implements the “ordering” and the “atomicity” during operation. The “ordering” in the memory controller component  108  is implemented by the use of the epoch ID, the persistence bits, and a timing provided by a scheduler component (as discussed below). In an implementation, the scheduler component performs scheduling of data accesses, or provides the timing in data access within the computing device  100 . 
     In another implementation, for correct operation (using “atomicity”), the memory controller component  108  provides support for atomic writes (i.e., “atomicity”) in the file system (or other software components). For example, in case of a power outage or failure, in-flight writes may be either completed in their entirety or fail completely. The in-flight writes may include persistent data in the cache blocks of the cache controller component  104  that were not yet reflected in the non-volatile memory component  110 . In an implementation, the atomic writes in the file system are obtained by inserting a capacitor device(s) (not shown) in the memory controller component  108 , to assure that there is enough energy to complete a maximum number of write transactions ongoing within a subsystem of the non-volatile memory component  110 . As a result, although the memory controller component  108  may fail to issue further commands, in-flight writes within the non-volatile memory component  110  may be guaranteed to complete. 
     The non-volatile memory component  110  may include a reliable storage system (i.e., handles more memory requests) that does not lose data when power is removed. The non-volatile memory component  110  may be a byte-addressable and bit alterable non-volatile memory directly connected to a memory bus (i.e., memory bus  118 ) to obtain direct access or exposure to the CPU component  102 . The byte-addressability may include the ability of the non-volatile memory component  110  to perform small, random writes as compared to large data transfers in traditional disk or flash. In contrast to non-volatile flash memories operating on coarse blocks (e.g., kilobytes of data), the byte addressability function may improve performance and reduce power costs. The bit alterable function may include data writing in the non-volatile memory component  110  without separate data erasures. In an implementation, a phase change memory (PCM) may be implemented as the non-volatile memory component  110 . The non-volatile memory component  110  may support the file system that optimizes the properties of the byte-addressable and bit alterable non-volatile memory component  110 . In addition, the file system may exploit small, random writes at a given time in the non-volatile memory component  110 . This file system may optimize the use of the small, random writes whenever possible to reduce memory bus traffic and unnecessary writes in the non-volatile memory component  110 . 
     In an implementation, the non-volatile memory component  110  is a type of non-volatile memory that provides non-volatile, byte-addressable storage (i.e., persistent storage). Unlike the DRAM component  112 , the non-volatile memory component  110  may store data by using resistivity as opposed to electrical charge. The non-volatile memory component  110  may also use some other physical property that allows it to expose byte addressable, bit-alterable persistent storage. In an embodiment, byte-addressable non-volatile memory component  110  simultaneously improves performance and strengthens temporal safety, correctness, and consistency guarantees. Trade-offs that traditional file systems make between these factors (i.e., temporal safety, correctness, etc.) may be based on the properties of hard disks (e.g., hard disk  116 ), which greatly favor sequential access of large data blocks. With the byte addressable non-volatile memory component  110 , a different set of trade-offs in the file system may be explored. The different set of trade-offs in the file system may simultaneously improve the performance and strengthen temporal safety, correctness, and consistency guarantees by enforcing the “ordering” and “atomicity” in the CPU component  102 . 
     The non-volatile memory component  110  and the DRAM component  112  may be directly connected to a memory bus  118 . The memory bus  118  may carry data to or from the northbridge component  106 . The northbridge component  106  may further connect to the CPU component  102  through a signal path  120 . The signal path  120  may carry the data to or from the CPU component  102 . In other implementations, the northbridge component  106  is not included in the computing device  100 . To this end, the memory controller component  108  may be integrated to the CPU component  102  with direct signal paths between the NVM component  110  and the DRAM component  112 . 
     The DRAM component  112  may be used for the heap and the stack to further protect lifetime wear of the non-volatile memory component  110 . In addition, the use of the DRAM component  112  (i.e., for the heap and the stack) may provide power reduction consumption in the computing device  100 . The heap may include a portion of memory reserved for a program to use for the temporary storage of data structures whose existence or size cannot be determined until a program/application is running. The stack may store data such as procedure and function call addresses, passed parameters, and sometimes local variables. 
     The southbridge component  114  may include an interface that connects the northbridge component  106  and I/O devices such as hard disk component  116 . The southbridge component  114  may pass data to or from the northbridge component  106  through a signal path  122 . At the other side of the southbridge component  114  is the hard disk component  116 . Although the hard disk component  116  is shown, other implementations may use different devices or no devices at all. The hard disk component  116  may be a non-volatile storage device that stores digitally encoded data. In an implementation, the hard disk component  116  passes or receives data through a signal path  124  that is connected to the southbridge component  114 . This data may subsequently be accessed through the signal path  122  by the northbridge component  106 . 
     Central Processing Unit 
       FIG. 2  is an exemplary implementation of a central processing unit (CPU) component  102  ( FIG. 1 ) that includes processor cores  200 - 1 ,  200 - 2 , . . . ,  200 - n  (hereinafter referred to as processor  200 ), and epoch ID counters  202 - 1 ,  202 - 2 , . . .  202 - n  (hereinafter referred to as epoch ID counter  202 ). The processor  200  may include an electronic circuit that can execute write operations (writes). To maintain ordering among writes, each processor cores  200 - 1 ,  200 - 2 , . . .  200 - n , may use an epoch ID counter  202  that provides an epoch ID used as a reference to enforce ordering in the non-volatile memory component  110 . The epoch ID may indicate the writes that the processor  200  may be writing to the non-volatile memory component  110 . The epoch ID may be supplied to the cache controller component  104 , and the memory controller component  108 , in order to detect and prevent ordering violations. As discussed further below, the epoch ID may be used by the file system to enforce the ordering that is supported by the computing device  100 . 
     In an implementation, the epoch ID counter  202  may be incremented by one (1) each time the processor cores  200 - 1 ,  200 - 2 , . . .  200 - n  encounters a memory barrier that marks the end of the epoch ID. To this end, the epoch ID may allow the file system (as further discussed below) to detect a write that may be safely reordered (because of the memory barrier). When the write commits to any address in the cache controller component  104 , the write may be tagged with the value of the epoch ID provided by the epoch ID counter  202 . The value of the epoch ID may be propagated with the write throughout the cache controller component  104  and memory controller component  108  to enforce the ordering. 
     Cache Controller Component 
     Ordering 
       FIG. 3  is an exemplary implementation of a cache controller component  104  ( FIG. 1 ). Caching persistent data may increase performance because the non-volatile memory component  110  writes slower than the cache controller component  104 . In addition, since non-volatile memory component  110  cells may sustain a limited number of writes before wearing out, reducing write traffic through caching of the persistent data may extend the lifetime of the non-volatile memory component  110 . The cache controller component  104  may include cache blocks  300 - 1 ,  300 - 2 , . . . and  300 - n  (hereinafter referred to as cache  300 ) that stores data. 
     Each cache blocks  300 - 1 ,  300 - 2 , . . .  300 - n  may be associated respectively with persistence bits  302 - 1 ,  302 - 2 , . . .  302 - n  (hereinafter referred to as persistence bit  302 ). The persistence bit  302  (or vector bit) may be set appropriately at the time the cache  300  is filled based on cache&#39;s address. The persistence bit  302  may be used to identify the cache  300  data referenced to the non-volatile memory component  110  address ranges. In other words, if the cache  300  (e.g., cache block  300 - 1 ) includes the data to be written into the non-volatile memory component  110 , then the persistence bit  302  may be required to identify the cache  300  that contains the data (i.e., persistence bit is equal to one). Otherwise, the persistence bit  302  may not be required to be associated with the cache  300  (i.e., persistence bit is equal to zero), if the data may be referenced to the DRAM component  112 . 
     Epoch IDs  304 - 1 ,  304 - 2 , . . .  304 - n  (hereinafter referred to as epoch ID  304 ) may refer to a defined memory barrier (for each epoch ID  304 ) as provided by the epoch ID counter  202  in the CPU component  104 . In an implementation, the epoch ID  304  is used to identify the cache  300  when the persistence bit  302  is set to one. In other words, the data in the epoch ID  304  may be referenced to the non-volatile memory component  110 . For a given epoch ID  304  (e.g., epoch ID  304 - 1 ), the epoch ID  304  may refer to one or more writes of persistent data before a defined memory barrier that were not yet reflected to the non-volatile memory component  110 . These writes of persistent data may be referred to, collectively, as belonging to an in-flight epoch identified by epoch ID  304 . The one or more dirty persistent data in the in-flight epoch ID  304  may be tracked by a dirty block counter  306 . The dirty counter  306  may include dirty counters  306 - 1 ,  306 - 2 , . . .  306 - n  (hereinafter referred to as dirty block counter  306 ) that may be associated respectively with the in-flight epoch ID  304  to implement the ordering in the cache controller component  104 . 
     In another implementation, the dirty block counter  306  may track the number of persistent dirty data residing in the cache  300  for each in-flight epoch ID  304  at each point in time. In a first in first out (FIFO) buffer, the oldest in-flight epoch ID  304  may include the data that were written or entered earlier. The cache controller component  104  tracks which epoch ID is assigned to the oldest in-flight epoch. The dirty block counter  306  may be incremented, whenever the data is updated (i.e., a new write application provides a new data), and the dirty block counter  306  may be decremented when the dirty persistent data is reflected to the non-volatile memory component  110 . When the dirty block counter  306  associated with the oldest in-flight epoch ID  304  reaches a zero value, then the in-flight epoch ID previously identifying the oldest epoch ID no longer resides in cache controller component  104 ; the cache controller component  104  then identifies the next oldest epoch ID. 
     In another embodiment, the cache controller component  104  performs an update to the oldest in-flight epoch ID  304  in the cache  300 . At each point in time, any of the in-flight epoch ID  304  that are younger than the oldest in-flight epoch ID  304  in the cache  300  may not be replaced. To this end, a replacement is performed in a manner that respects ordering in the cache  300 . 
     Addresses mapped to the DRAM component  112  and persistent blocks from the oldest epoch in the cache  300  may all be eligible for replacement. If the cache controller component  104  does not find data blocks mapped to the DRAM component  112  for replacement, then the cache controller component  104  may attempt to replace the oldest in-flights epoch ID  304  in the cache  300 . To this end, all of the earlier in-flight epoch ID  304  may be flushed from that level of cache hierarchy first, and in program order. 
     Memory Controller Component 
     Ordering 
       FIG. 4  is an exemplary implementation of a memory controller component  108  ( FIG. 1 ). A memory controller  400  may include memory controller transactions  400 - 1 ,  400 - 2 , . . . and  400 - n  (hereinafter referred to as memory controller  400 ) that may contain metadata. The memory controller  400 , following the cache  300 , may be respectively associated with persistence bit  402  (i.e., persistent bits  402 - 1 ,  402 - 2 , . . .  402 - n ), epoch ID  404  (i.e., epoch IDs  404 - 1 ,  404 - 2 , . . .  404 - n ), dirty block counter  406  (i.e., dirty block counters  406 - 1 ,  406 - 2 , . . .  406 - n ), and atomic bits  408  (i.e., atomic bits  408 - 1 ,  408 - 2 , . . .  408 - n ). 
     In an implementation, the memory controller component  108  may assure that a write (e.g., dirty data) may not be reflected to the non-volatile memory component  110  (e.g., PCM) before in-flight writes associated with all of the earlier epochs are performed. As such, the memory controller component  108  may record the epoch ID  304  associated with each persistent write in the memory controller&#39;s transaction queue, and maintain a count of the in-flight writes from each epoch that is queued up at a given point in time (i.e., by using the dirty counter block  406 ). The persistence bit  402 , the epoch ID  404 , and the dirty block counter  406  may include the same functions and operations as the persistence bit  302 , the epoch ID  304 , and the dirty block counter  306 , which were discussed under the cache controller component  104 . In another implementation, among persistent writes, only those persistent writes associated with the oldest epoch (e.g., in epoch ID  404 ) may be eligible for scheduling in the memory controller component  108  at any point by a scheduler component  410 . 
     The memory controller  400  may further include scheduler component  410  that schedules memory accesses according to timing constraints defined by a particular memory technology. The scheduler component  410  may be used in the memory controller  400  to guarantee correctness by following the program order to enforce ordering in the cache controller component  104 . The scheduler component  410  may further support the enforcement of the ordering by the file system (or other software components) through integrated access (i.e., at the same time) of the non-volatile memory component  110 , and the DRAM component  112 . Since the memory controller  400  follows a queue of requests from the cache  300 , the scheduler component  410  assures that the writes cannot be reflected in the non-volatile memory component  110  before in-flight writes with all of earlier epochs are performed. The queue of requests from the cache  300  may include flushing out data information (i.e., reflecting information to the memory controller  400  and then deleting this information from the cache  300 ) received by the memory controller component  108  according to the timing as designed in the file system. The scheduler component  410  may include an added state for tracking of the persistence bit  402  and the epoch ID  404 . The timing, persistence bit  402 , and the epochs ID  404  may be referred to as restraints (or conditions) used to enforce ordering in the file system while scheduling memory accesses. 
     Atomicity 
     The atomicity may be enforced at the memory controller component  108  to assure safety and correctness guarantees during enforcement of the “ordering.” To prevent data corruption during unexpected failures, the file system atomically updates certain memory location in the non-volatile memory component  110 . In an implementation, in case of a power outage or failure, in-flight writes must be either completed in their entirety or must fail completely, and not update the non-volatile memory component  110 . In certain implementations, a capacitor device (not shown) may be inserted in the memory controller component  108 . The capacitor device may hold enough energy to complete the maximum number of write transactions ongoing within the non-volatile memory component  110 . 
     In another implementation, atomic persistence writes in the memory controller component  108  are provided via hardware journaling. In other words, a write to a predetermined address may signal to the memory controller component  108  that the next write in the program order may be performed atomically. The hardware journaling may be implemented using the atomic bit  408  associated to further identify each memory controller transactions  400 - 1 ,  400 - 2 , . . .  400 - n . The atomic bit  408  may provide a timing of the atomic write in the program order in the memory controller component  108 . The atomic bit may be propagated throughout the cache controller component  106 , and may be interpreted appropriately by the memory controller component  108 . 
     Design Principles for a Non-Volatile Memory File System 
     Using the non-volatile memory component  110  instead of a hard disk  116  may provide performance benefits based on the speed of the non-volatile memory component  110 . The non-volatile memory component  110  also provides an opportunity to further improve both performance and reliability by designing a non-volatile memory file system (PFS) optimized for the unique properties of the byte-addressable, non-volatile memory component  110 . 
     In an implementation, the PFS may be based on three design principles. The first design principle is to exploit small, random writes. Instead of writing large blocks of data at a time, PFS is optimized to use small, random writes whenever possible, to reduce memory bus traffic and unnecessary writes to the non-volatile memory component  110 . 
     The second design principle is to avoid using the DRAM component  112  for file system data and metadata. Instead, PFS stores data and metadata in the non-volatile memory component  110 . This frees the DRAM component  112  for other uses, and provides the opportunity to reduce power consumption by reducing the amount of the DRAM component  112  required by the computer device architecture  100 . Furthermore, the operating system does not need to manage two tiers of storage, which simplifies the task of ensuring reliability. 
     The third design principle is to rely on hardware assistance to implement guarantees. Since the non-volatile memory component  110  provides the opportunity to eliminate the layer of the DRAM component  112  between the CPU component  104  and persistent storage, interposing on application operations to enforce ordering or safety guarantees could impede performance. To this end, the PFS can be designed on the premise that hardware enforces the ordering and temporal safety guarantees of data written into the CPU cache. PFS uses write barriers to denote the required ordering between sets of operations and to mark certain writes as atomic. The underlying cache controller component  104  and the memory controller component  108  are then free to issue writes between two write barriers in any order while still preserving consistency guarantees. 
     The above design principles, coupled with the architectural design for the non-volatile memory component  110 , allow for the design of a file system that provides strong consistency, correctness, and temporal safety guarantees. PFS provides a strong consistency guarantee by assuring that a crash or power failure will not result in a corrupted file system image. PFS also provides a strong correctness guarantee by leveraging architectural support to reflect application writes to the non-volatile memory component  110  atomically and in the order they were issued by the application. Finally, PFS may improve temporal safety guarantees by reducing the window of vulnerability for data loss from seconds to the number of cycles required make data persistent in PRAM. 
     A File System Layout for Non-Volatile Memory 
     In an embodiment, file system data and metadata may be stored in a tree structure in the non-volatile memory component  110 , accessible from a root pointer stored at a predetermined location. Consistency is assured throughout the file system by performing intermediate operations in unused portions of the non-volatile memory component  110  and then using an atomic operation to commit them. For example, when changing a page of user data, PFS copies the existing user data to a freshly allocated block of the non-volatile memory component  110 , updates the new block, and then atomically overwrites the pointer to the old block with the pointer to the new block. In many cases, updates can be done entirely in-place through careful ordering of writes. In an embodiment, PFS marks epoch boundaries before and after each 64-bit “commit” of file system state, which assures that the committing operation will be written to the non-volatile memory component  110  only after the write operations upon which the committing operation depends have been made persistent. 
     As a result, there is a very strong consistency guarantee: all file system operations are either committed completely or not at all. In addition, there are strong safety guarantees. Since updates can be applied to the non-volatile memory component  110  synchronously, data will arrive in the non-volatile memory component  110  in the time it takes to flush the cache. Since most common file operations can be performed in-place, high performance gains may be achieved. 
     In an embodiment, persistent data structures within PFS include at least three kinds of files. First, an inode file is a single file containing an array of fixed-size inodes, each uniquely representing a file or directory in the file system. The root of the inode file represents the root of the file system as a whole, and this root pointer is stored in a predetermined location in the non-volatile memory component  110 . Inodes contain file metadata including the root pointer and size of the associated file. An entry in the inode file is only considered valid if it is referred to by a valid directory entry. Second, directory files contain an array of directory entries that include an inumber (i.e., the index of an inode in the inode file) and the name of the corresponding file. Directory entries are only considered valid if they contain a non-zero inumber. Third, data files contain user data only. 
       FIG. 5  is an exemplary block diagram  500  of the non-volatile memory component  110  ( FIG. 1 ) file system structure. The top half of the file system is the inode file  502 , which starts from a root pointer  504  at a predetermined location. The dashed box shows the “data” for the inode file  502 , which includes an array of inodes  506 . Each inode points to a directory file or a data file.  FIG. 5  shows three such files, whose data is also stored in a hierarchal structure. 
     In an embodiment, each kind of file is represented with the same basic data structure: a tree consisting entirely of, e.g., 4 KB blocks. The leaves of the tree represent a file&#39;s data (i.e., user data, directory entries, or inodes), and the interior nodes of each tree contain 512 64-bit pointers to the next level of the tree. In  FIG. 5 , the leaves of each file are shown in a dashed box; taken in sequence, the blocks in this dashed box represent the file&#39;s contents. For simplicity, only two pointers per block are shown. 
     The height of each tree is indicated by the low-order bits of the root pointer  504 , which allows the file system to determine whether a given block is an interior (pointer) block or a leaf (data) block by remembering the number of hops taken from the root pointer  504 . For example, with a tree of height of zero, the root pointer  504  points directly to a data block which can contain up to, e.g., 4 KB of file data. With a tree of height one (1), the root pointer  504  points to an interior block of 512 pointers, each of which points to a 4 KB data block, for a total of, e.g., 2 MB. A tree of height three (3) can store 1 GB of data, and a tree of height 5 can store, e.g., 256 TB of data. It is to be noted that a given tree is of uniform height. For example, if a tree has height of three (3), then all file data will be found three hops down from the root pointer  504 , and no file data is stored at interior nodes. It is also noted that because the root pointer  504  and its height are stored in one 64-bit value, they can be updated atomically. 
     At any level of the tree, a null pointer represents zero data for the entire range of the file spanned by that pointer. For example, if the root pointer  504  is a null pointer with height five (5), then it represents an empty (i.e., zeroed) 256 TB file. Null pointers can also appear at interior nodes, so a write to the end of this 256 TB file will not cause us to write 256 TB of zeros; rather, it will result in a chain of five pointers down to a single data block, with null pointers in the remainder of the interior nodes. Thus, the file representation can achieve very compact representations of large, sparse files. 
     Trees can have varying height. For example, data file 1  508  has height one (1), directory file 2  510  has height 2, and data file 3  512  has height three (3). Data blocks may be at the same level of each tree. For example, in directory file 2  510 , the third data block is still located three hops from the root, even though the parent of the third data block only has one pointer. It is noted that data file 3  512  is missing a second block due to a null pointer in the parent—this block is assumed to be entirely zero. Components of the tree can store pointers  514 , file metadata  516 , directory information  518 , or user data  520 . 
     The size of each file is stored along with each root pointer. For the inode file  502 , the file size is stored in a predetermined location (root node  504 ). For all other files, the file size is stored in each file&#39;s inode next to the root pointer  504 . If the file size exceeds the amount of data encompassed by the current tree height, then the tail of the file is assumed to be zero. Therefore, the 256 TB zero file may also be represented by a tree of height of zero (0) and a file size of 256 TB. If the file size is less than the amount of data represented by the current tree height, then data in the tree beyond the end of the file is ignored and may contain garbage. For example, if a tree has a height 1 (with a maximum of 2 MB) and a file size of 1 MB, then the first 256 pointers of the interior node point to valid data, and the last 256 pointers are ignored and may contain arbitrary bits. 
     In an implementation, persistent data can be updated in three ways: in-place updates, in-place appends, and partial copy-on-write. These approaches to updating data represent distinct advantages over disk-based file systems, which are not able to modify persistent storage at a byte granularity. 
     Updating Persistent Data Via In-Place Update 
     In-place updates are an efficient approach. In an embodiment, for data files, in-place updates can be performed for writes of 64 bits or less, since the hardware guarantees that these updates are atomic. The file system can be built around any granularity of atomic writes. For the purposes of discussion, we will assume an atomic write size of 64 bits. For metadata structures, file system invariants can be used to do in-place updates. For example, when adding an entry in a directory file, the file system can find an unoccupied (i.e., zeroed) directory entry, write the name of the new entry, and then write the entry&#39;s number. Since an entry is only considered valid when it contains a non-zero inumber, this final write commits the change to the file system. Similarly, inodes are not considered valid until a directory entry points to them, so inodes that are not yet live can be modified in-place. For example, the file system may be writing to a “dead” inode in preparation for creating a directory entry that points to the “dead” inode. 
       FIG. 6  is an exemplary implementation  600  of an in-place update. In one embodiment, the non-volatile memory component  110  ( FIG. 1 ) contains file data stored in a tree structure, with a root  602  at the beginning of the tree. The root  602  has a root pointer  604  that points to pointer block 1  606 . Pointer block 1  606  contains multiple pointers, including pointer  608 , which points to pointer block 2  610 . Pointer block 2  610  has pointer 1  612  which points to data block 1  614  and pointer 2  616  which points to data block 2  618 . Other branches of the tree may contain additional pointers and data blocks. For example, pointer block 3  620 , data block 3  622 , and data block 4  624  are in another branch. The data for File 1  626  consists of the data stored in data blocks  622 ,  624 ,  614 , and  618 . The size of modified data  628  is less than or equal to the maximum size that can be atomically written. A particular amount of data can be atomically written if there is a guarantee that the entire write operation will either succeed or fail (i.e. 8 bits, 64 bits, 8 bytes). Since modified data  628  is changed through an atomic write, no other modifications to the tree structure are needed when the changes are made to Modified Data  628 . 
     Updating Persistent Data via In-Place Appends 
     In-place appends take advantage of the file size variable that accompanies the root pointer for each file. Since all data beyond the file size is ignored, the file system can safely write to these locations in-place, and once all of the data has been written, the file system can atomically update the file size to extend the valid data range. 
       FIG. 7  is an exemplary implementation  700  of an in-place append to a file with root pointer  602 . In one embodiment, File 1  626  consists of data blocks  622 ,  624 ,  614 , and  618 . Pointer block 2  610  has pointer 2  616  which points to data block 2  618  and pointer 1  612  points to data block 1  514 . Appended data  702  is added to data block 2  618 , which is part of File 1  626 . A file size variable accompanies the file to the root pointer  602 . Once all of the appended data  702  is written to data block 2  618 , the file size variable is atomically updated to a larger file size. This extends the valid data range. Any data beyond the file size is ignored. Therefore, if a crash occurs before the file size is updated or while the append is in progress, the appended data is simply ignored. 
     Updating Persistent Data Via Partial Copy-on-Write 
     Partial copy-on-write is a technique for updating persistent data, allowing an atomic update to an arbitrarily large portion of the file system. In this approach, the file system performs a copy-on-write on all portions of the tree that will be affected by the write operation, up to the lowest point at which a change can be committed with a single write. 
       FIG. 8  is an exemplary implementation  800  of partial copy-on write to a file with root pointer  602 . File 1  626  consists of data blocks  622 ,  624 ,  614 , and  618 . Pointer block 2  610  includes pointer 2  616 , which points to data block 2  618 . When data block 2  618  needs to be modified, data block 3  802  is created by copying data block 2  618  to a newly-allocated location in non-volatile memory. In certain embodiments, only some of the data in data block 2  618  is copied (i.e., data that will not be modified). As shown, data block 3  802  is changed in two areas: modified data 1  804  and modified data 2  806 . After the data in data block 3  802  is updated, pointer 2  616  is atomically updated to pointer 3  808 , such that pointer block 2  610  points to data block 3  802  instead of data block 2  618 . If a crash occurs before pointer 2  616  is updated, pointer block 2  610  will continue to point to data block 2  618 . Therefore, pointer 2  616  will not point to “dirty data” if a crash occurs. 
       FIG. 9  is an exemplary implementation  900  of a partial copy-on-write involving multiple data blocks. File 1  626  consists of data blocks  622 ,  624 ,  614 , and  618 . Pointer block 1  606  includes pointer  608 , which points to pointer block 2  610 . Pointer block 2  610  includes pointer 1  612 , which points to data block 1  614  and pointer 2  616 , which points to data block 2  618 . When data is modified in both data block 1  614  and data block 2  618 , a Subtree  902  is created by copying pointer block 2  610  to pointer block 3  904 , copying data block 1  614  to data block 3  906 , and copying data block 2  618  to data block 4  908 , where data blocks  904 ,  906 , and  908  are newly-allocated blocks in non-volatile memory. In this example, a user wants to write data that spans both the third and fourth data blocks of the file. To do so, the file system allocates new space in the non-volatile memory component  110  for new blocks, copies any existing data that will not be overwritten (e.g., the beginning of the third data block), and then updates the new blocks as appropriate. The file system also copies and updates any pointer blocks that cannot be updated atomically. 
     Pointer 3  912  points to data block 3  906  and pointer 4  914  points to data block 4  908 . Once the file system has created these data blocks and copied any data that will not be modified, it writes modified data  916  to the new blocks. In addition, it writes modified pointers  912  and  914  to block  904 , so that block  904  now points to blocks  906  and  908  instead of  614  and  618 . There may still be pointers in pointer block  904  that point to old data blocks (not shown in this example), as long as those data blocks have not been modified. After modified data  916  has been updated in the Subtree  902 , pointer A  608  is atomically updated to pointer B  918 , so that pointer block 1  606  points to pointer block 3  904  instead of pointer block 2  610 . If a crash occurs before pointer A  608  is updated, pointer A  608  will continue to point to pointer block 2  610 . Only when all of the updates in Subtree  902  are complete does the file system commit the change by performing an atomic update of the pointer A  608 . Therefore, pointer block 1  606  will not point to “dirty data” if a crash occurs. 
     Updating Persistent Data 
       FIG. 10  is a flow diagram of an exemplary process  1000  for atomically updating a data structure. Process  1000  may be implemented by CPU component  102  of computing device architecture  100 . Process  1000  is illustrated as a collection of blocks in a logical flow diagram, which represents a sequence of operations that can be implemented in hardware, software, or a combination thereof. In the context of software, the blocks represent computer instructions that, when executed by one or more processors, perform the recited operations. 
     The order in which the method is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any order to implement the method, or alternate method. Additionally, individual blocks may be deleted from the method without departing from the spirit and scope of the subject matter described herein. Furthermore, the process can be implemented in any suitable hardware, software, firmware, or a combination thereof, without departing from the scope of the invention. 
     At step  1002 , an application issues a write command. In one embodiment, e.g., 8 bytes of data can be written atomically. Other embodiments may atomically write smaller or larger amounts of data. At step  1004 , if the amount of data to be written is less than or equal to 8 bytes, then an atomic write is issued at step  1006 . The method  1000  then ends at step  1008 . 
     At step  1002 , if the amount of data to be written is more than 8 bytes, then at step  1010  the file system determines if the write is limited to one data block. If so, then at step  1012  the file system determines if the write is limited to appending data to a file. If so, then at step  1014  the files system appends data to the data block. At step  1016 , after the data is appended to the data block, the file system atomically updates a file size variable associated with the appended file. The method  1000  then ends at step  1008 . 
     At step  1012 , if the write is not limited to simply appending data, then at step  1018 , a copy of the data block is created. In some embodiments, only data that will remain unchanged is copied into the newly created data block. At step  1020 , the file system writes to the data block. At step  1022 , after all writes to the data block are complete, the file system atomically updates the pointer from the original data block to the new data block. The method  1000  then ends at step  1008 . 
     At step  1010 , if the write is not limited to one data block, then at step  1024  the file system creates a copy of the subtree that contains data to be modified. At step  1026 , the file system writes data to the data blocks. At step  1028 , after all writes to the data blocks are complete, the file system atomically updates the pointer from the original subtree to the new subtree. The method  1000  then ends at step  1008 . 
     Using Non-Volatile Memory with Volatile Data Structures 
     In an embodiment, for speed and convenience a number of data structures may be maintained in volatile memory. First, the file system may have a list of free blocks of the non-volatile memory component  110  as well as freed and allocated inumbers. Because these data structures are stored in volatile memory (i.e., the DRAM component  112 ), they are reconstructed from the file system at every boot; however, this can be done in a fraction of a second, even on a moderately full file system. Storing this data in the DRAM component  112  provides that there is no need to worry about consistency guarantees between the free list and the file system itself. 
     Second, the file system stores a list of freed and allocated blocks from an in-flight copy-on-write operation. For example, while performing a write, the file system will keep track of any newly allocated blocks (i.e.,  FIG. 9 , block  904 , block  906 , block  908 ) as well as any blocks that will need to be freed if the operation succeeds (i.e.,  FIG. 9 , block  610 , block  614 , block  618 ). When the operation is complete, the file system iterates over either the freed list or the allocated list (depending on the success of the operation) and adds these blocks to the global free list. Because commits are atomic, this data never needs to be stored in NVM  110  or reconstructed. 
     Third, the file system stores a cache of directory entries from each directory that has been opened by the user. Each directory entry in the cache is stored simultaneously in a list and a hash table so that the file system can support quick, ordered directory listings as well as quick individual name lookups. Any updates to directories are immediately reflected to the non-volatile memory component  110  as well. 
     Because these data structures are only found in the DRAM component  112 , the file system need not use atomic writes to update them; rather, they are synchronized with the file system updates using only conventional locks. An alternate design might place some of these structures in the non-volatile memory component  110  directly. 
     Non-Volatile Memory File System Operations 
     This section presents details of an embodiment of a file system implementation. Since files in the file system use the same basic tree data structure, the implementation has a core set of routines, called the crawler, which is designed to traverse these trees and perform reads and writes. To implement a file system operation, the crawler is given a root pointer  602 , the height of the tree, a range of file offsets, and a callback function. Because the system can compute the file offsets spanned by each pointer, the crawler only needs to visit the pointers included in the specified range of offsets. Once the crawler gets to the leaf nodes, it will invoke the callback with the appropriate addresses. 
     The crawler is responsible for updating the tree height and any internal pointers. To update the tree height, the crawler looks to see if the requested file offsets are beyond the offsets spanned by the current file tree. If so, it increases the height of the tree by an appropriate amount. Each increase in the height of the tree is a simple operation: the crawler allocates a new pointer block, sets the first pointer in this block to the old tree, and then sets the root pointer  602  to point to this new block (along with the new height, encoded as low-order bits). These updates can all be performed atomically, independent of the write operation that is about to be performed. 
     At leaf nodes, the crawler invokes a callback, and if the callback wishes to perform a copy-on-write operation, it will allocate a new block, perform any necessary updates, and return the pointer to that new block. The crawler then updates any internal nodes (i.e., pointer blocks  514 ) as appropriate. If no modifications are made by the callbacks, the crawler returns the existing pointer block untouched. If only one pointer is modified by the callbacks, then the crawler commits that operation in-place. If more than one pointer is modified, the crawler makes a complete copy of that pointer block, deferring the commit to a higher level in the tree. 
     Sometimes only copy-on-write is allowed. For example, when a write operation proceeds down two branches of the tree, neither branch is allowed to commit in-place, since any commits need to happen at a common ancestor. This case also arises when the user performs a write that will update existing data and extend the end of the file. Because the file system needs to update both the file size and the root pointer  602  atomically, the file system needs to perform a copy-on-write on the inode  516  itself, and the file system needs to disallow in-place commits during the file write. 
     Because the file system has two levels of tree data structures (i.e., the inode file  502  and everything below it), many operations invoke the crawler twice: once to find an inode  516  in the inode file  502 , and a second time to perform some operation on that inode  516 . The callback for the top level crawl invokes the crawler a second time for the bottom-level file. Copy-on-writes can be propagated upward through both invocations of the crawler. 
     In an embodiment, the file system is implemented in the Microsoft Windows® Operating System Driver Model, but the following presents a simplified view of these operations. When a file is opened, the file system operation parses the path and uses the directory entry cache to look up the target file or directory. Because the directory entry cache stores complete directory information in the DRAM component  112 , this operation only needs to access the non-volatile memory component  110  if a directory is being opened for the first time. 
     If the file does not exist and a new file is created, the file system claims a new inumber from the free list and then writes a new inode to a block  516  of the inode file  502  at the appropriate offset. Because inodes within blocks  516  are invalid unless referenced by a directory entry, these updates can be performed in-place. Once the inode  516  is ready, the file system writes a new directory entry into the parent directory. Once again, this update can be done in-place, because the directory entry is not valid until a nonzero inumber is written to the appropriate field. Finally, the file system updates the directory entry cache in the DRAM component  112 . 
     It is noted that this entire operation can effectively be performed with in-place updates to metadata; thus, file creation is consistent, synchronous, and extremely fast. A few extra writes may be required when the inode file  502  or directory file  518  is extended. 
     When a file is read, the file system invokes the crawler on the appropriate range of the file. The read callback copies data from the data block  520  into a user-supplied buffer. No updates to file system data may be required, although the access time can be updated with an in-place atomic write. 
     When a file is written, the file system may perform a copy-on-write of the inode  516  itself, such that the operation uses a two-level crawl. The top level crawl operates on the inode file  502  and locates the target file&#39;s inode  516 . Then the file system invokes the write crawler on the appropriate range of this file. The write callback determines whether an in-place write is possible, and if so, the write callback performs that write. If not, the write callback makes a copy of the block, updates the copy, and returns the copy to the crawler. The crawler then updates the internal nodes using the logic described above. 
     The file system atomically updates either the file size or the root pointer  602  within the inode  516  as necessary. If both are updated, then a copy-on-write is performed on the inode block  516  itself, and the new version is returned to the inode file crawler to be committed higher up in the tree. For efficiency, the file system updates the file modification time separately. If atomicity is required, the file system can force a copy-on-write on every write operation. 
     When a directory is read, the file system loads the directory into the directory entry cache, if the directory is not already cached. The file system searches for the requested name, looks up all relevant inodes in the inode file  502  from the non-volatile memory component  110 , and fills the application&#39;s buffer. Loading a directory into the directory entry cache may also be performed with the crawler. The crawler is invoked with the entire file range, so that the crawler receives callbacks for the entire directory. At each data block  518 , the crawler reads the relevant directory entries and enters them in the cache. 
     When a file or directory is closed the file system checks to see whether the file or directory has been marked for deletions by a separate call not shown. If so, the file system deletes the file or directory by crawling the directory file to the location of the directory entry and writing a zero to the inumber field in-place. Because a zero inumber indicates an invalid directory entry, this atomic write instantly invalidates both the directory entry and the inode  516  to which it refers. Finally, the file system updates the volatile data structures, including the free block list and the free inumber list. 
     This implementation exhibits many of the benefits of redesigning a file system for use on the non-volatile memory component  110  ( FIG. 1 ). Through use of byte-level accesses, the file system can perform in-place updates for a large number of operations, and through use of an atomic write, the file system can provide strong consistency and safety guarantees for arbitrarily large changes to the file system. On a system that requires block-based updates, the file system would not be able to achieve the same combination of high performance and strong guarantees. 
     In certain embodiments, file write times are not updated atomically with respect to the write itself, because doing so would require all write operations to be propagated up to the inode itself using copy-on-write. Therefore, if a crash occurs between the write and the timestamp update, it is possible that the timestamp will not be up to date. This problem could be addressed by implementing a wider atomic write primitive or by squeezing the modification time and the root pointer into a single 64-bit value. 
     Instant Boot 
     An application enabled by non-volatile, byte-addressable memory is “instant boot.” Operating systems may take several minutes to boot from a completely powered-down state. For example, certain operating systems offer “sleep” and “hibernate” features for partially shutting down a computer. In sleep mode, an operating system can power down all devices except for the DRAM component  112 , and in hibernate mode, the contents of the DRAM component  112  are copied to the hard disk  116  so that power can be turned off completely. Hibernating saves more power, but also takes more time to enter and leave the hibernating state. Fundamentally, startup may be a problem that is I/O bound to persistent storage (i.e., hard disk  116 ). A faster hard disk  116  results in faster boot time, but data structures and file system objects may be to be copied across a bus and into the DRAM component  112  to be made useful. 
     A simple form of instant boot may keep operating system data structures in PRAM  110  instead of the DRAM component  112 . In this case, sleep mode would not require any power. Therefore, such method provides the power savings of hibernate mode with the speed of sleep mode. Existing operating systems may include all the necessary code for managing hardware devices during sleep mode. 
     A second form of instant boot may use the non-volatile memory component  110  as a backing store for operating system data state. The file system can take a snapshot of the DRAM component  112  after the computer (i.e., computer device architecture  100 ) has booted and store this snapshot in the non-volatile memory component  110 . On the next boot, this snapshot can be copied from the non-volatile memory component  110  to the DRAM component  112  in order to skip past portions of the boot sequence. Components and devices should be placed into the correct state, and update performed on the saved memory image to account for changes in time, hardware configuration, and etc. It is noted that the specific operating system data structures required to support this scenario depends upon the operating system. 
     Instant boot benefits in two ways from a cache hierarchy design. First, the time to read OS data structures from the non-volatile memory component  110  is reduced, because the non-volatile memory component  110  might be faster than other storage mediums, and because the non-volatile memory component  110  is located closer to the CPU component  104 . Second, since the non-volatile memory component  110  is byte addressable from the CPU component  104 , OS data structures do not need to be marshaled before they are written, since they must be when they are written to other mediums such as disk. Pointers can be preserved in the non-volatile memory component  110 , and then restored when they are read back in the DRAM component  112  at the time of boot. 
     Application Checkpoints 
     The non-volatile memory component  110  opens up a new set of uses to applications, including checkpointing application state. Applications could reflect the state of the application (e.g., heap, stack, registers) directly to the non-volatile memory component  110 . Applications could use these checkpoints for reliability and security. 
     Extending Storage Capacity with Traditional Storage Media 
     Although the non-volatile memory component  110  provides many advantages over hard disks  116  or solid-state drives for storing file system data, in certain cases, such a use maybe more expensive per byte than these other media. Therefore, an approach to this problem may be to store metadata and small files in the non-volatile memory component  110  based file system. For larger files that do not fit within the non-volatile memory component  110  based file system, the file system can store a symbolic link to a file stored on a hard disk  116  or solid-state drive using a traditional file system such as NTFS. The operating system can implement one of many policies for moving files between the non-volatile memory component  110  based and disk-based file systems in order to speed up access to commonly-used files while providing the high capacity of a hard disk  116 . 
     The non-volatile memory component  110  based file system may be periodically copied to the hard disk  116  itself so that recovery of the full file system can be performed if only the hard disk  116  is removed from the system. 
     CONCLUSION 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as exemplary forms of implementing the claims. For example, the systems described could be configured as wireless communication devices, computing devices, and other electronic devices.