Abstract:
A method and apparatus for storing transactional information in persistent memory. In one embodiment, the invention features a persistent volatile memory and an intermediary program in communication with the persistent volatile memory. The intermediary program receives transactional information and stores the information in the persistent volatile memory. A computer uses the intermediary program to enable the contents of the persistent volatile memory to remain unaltered during a failure of the computer. Additionally, the intermediary program may determine whether the transactional information meets a predetermined criteria before storing the information in the persistent volatile memory.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/550,108, filed Apr. 14, 2000, the entire disclosure which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to storing information and more specifically to storing transactional information in persistent memory on a computer. 
     BACKGROUND OF THE INVENTION 
     Information accessed by a computer system is often preserved for later retrieval by the computer system. In some circumstances, a computer system prevents access to a particular file stored on a disk when writing information (e.g., transactional information) to that file until such writing completes. This occurs to avoid corrupted transactions. For example, a database management system (DBMS) generally requires that all updates to files stored in non-volatile storage (e.g., a disk) are completed before that file is made available for access by an application. 
     In particular, a “database commit” is the final step in the successful completion of an update to a file made as a part of handling a transaction. For example, when a single transaction includes several steps, then all of the steps must be completed before the transaction is deemed successful and the database file is actually changed to reflect the transaction. When a transaction completes successfully, the changes to the file are said to be “committed”. 
     Since a read or write transaction requires a read from or a write to non-volatile storage (e.g., disk) and access to the disk is limited until the previous transaction is committed, access to the database file is slowed by the delays in committing the data. Consequently, the performance of the DBMS decreases. 
     One prior art solution to this problem is to store the information in a temporary log file in volatile cache memory so that the DBMS can commit the information to disk at a later time. Further, the performance decrease of the computer system when writing the information is typically reduced when the information is first written to the log file rather than non-volatile storage (e.g., disk). However, if the computer system failures (i.e., crashes) before the computer system updates the disk with the data updates stored in the volatile cache memory, all update information stored in the volatile cache memory is deleted and typically irretrievable. 
     Thus, there remains a need to store transactional information in a manner that allows the information to be available following a computer system failure without having to perform the same transaction again. Additionally, there remains a need to have the transactional information available after a system failure in a more efficient manner than currently available. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to ensure availability of transactional information following a failure of a computer system without having to perform the same transaction again. Additionally, another object of the invention is to provide availability to the transactional information following a system failure in a more efficient manner than currently available. The invention features a persistent volatile memory and an intermediary program in communication with the persistent volatile memory. The computer uses the intermediary program to enable the contents of the persistent volatile memory to remain unaltered during a failure of the computer. 
     In one aspect, the invention features a method for storing transactional information in a computer. The method comprises the steps of: (a) receiving transactional information; (b) storing the particular transactional information in a persistent volatile memory on the computer; and (c) retrieving the transactional information after a computer failure by accessing the transactional information stored in the persistent volatile memory on the computer. 
     Additionally, the method may also comprise flushing the persistent volatile memory to a persistent mass storage device. In one embodiment, the flushing occurs when the transactional information stored in the persistent volatile memory exceeds some predetermined threshold. In another embodiment, the flushing occurs when a predefined amount of time has elapsed since the storage of the transactional information in the persistent volatile memory. In yet another embodiment, the flushing occurs when a program, such as the operating system of the computer, is not busy. Alternatively, the flushing occurs when a file is closed or when the computer is shut down. 
     In another aspect, the invention features a method for providing persistent mass storage of transactional information. The method comprises the steps of: (a) receiving transactional information; (b) determining whether the transactional information meets a predetermined criteria; and (c) storing the transactional information that meets the predetermined criteria in a persistent cache. In one embodiment, the transactional information comprises unbuffered writes to disk, which are writes requested by an application and in which notification to the application of the completion of the write is necessary. For example, an unbuffered write can include copying a file from one directory to another directory, backing up and/or updating a file, and initializing a file. 
     In still another aspect, the invention features a persistent volatile memory and an intermediary program. The intermediary program receives transactional information and stores the transactional information in the persistent volatile memory. The contents of the persistent volatile memory remain unaltered through a system failure. In one embodiment, the intermediary program is a filter driver module that identifies particular transactional information to store in the persistent volatile memory. In another embodiment, the invention includes a flushing thread to flush the contents of the persistent volatile memory to a persistent non-volatile memory. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages of the invention described above, together with further advantages, may be better understood by referring to the following description taken in conjunction with the accompanying drawings. In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. 
         FIG. 1A  is a block diagram of an embodiment of a computer constructed in accordance with the invention. 
         FIG. 1B  is a flowchart depicting an embodiment of a process for writing a buffer atomically performed in accordance with the invention. 
         FIG. 1C  is a flowchart depicting an embodiment of the steps for recovery of completed transactional information during a boot cycle performed in accordance with the invention. 
         FIG. 1D  is a flowchart depicting an embodiment of the operation of the computer shown in FIG.  1 A. 
         FIG. 2A  is a flowchart depicting an embodiment of the operation of the computer of  FIG. 1A  to store unbuffered writes in persistent memory. 
         FIG. 2B  is a flowchart depicting an embodiment of the steps of flushing persistent memory as shown in FIG.  2 A. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1A , an embodiment of a computer  4  constructed in accordance with the invention is depicted. The computer  4  can be any personal computer (e.g.,  286 ,  386 ,  486 , Pentium, Pentium II, Macintosh computer), Windows™-based terminal (developed by Microsoft Corporation of Redmond, Wash.), Network Computer, wireless device, information appliance, RISC Power PC, X-device, workstation, mini computer, main frame computer, personal digital assistant, or other computing device that has a Windows™-based desktop and sufficient persistent mass storage. 
     The computer  4  includes a microprocessor  6 , a memory  8  for storing programs and/or data, an input/output (I/O) controller  10 , and a communications bus  12  allowing communication among these components. In one embodiment, the microprocessor  6  is a Pentium Classic/MMX CPU, developed by Intel Corporation of Austin, Tex., an AMD-K6 CPU, developed by AMD of Sunnyvale, Calif., and the like. 
     The computer  4  (i.e., the I/O controller  10 ) is additionally in communication with a persistent mass storage  22 , such as a magnetic disk or magneto-optical drive. In one embodiment, the persistent mass storage  22  is an internal component of the computer  4 . In another embodiment, a persistent mass storage  22 ′ (not shown) is an external component of the computer  4 . In particular, some computers  4  have redundant arrays of independent disks (RAID arrays) used as failure-tolerant persistent mass storage  22 . The computer  4  can also be in communication with a peripheral device (not shown), such as a mouse, printer, alphanumeric keyboard, and display. In some embodiments, the computer  4  also includes a network connection. 
     The memory  8  in such a computer  4  typically includes random-access memory (RAM)  14 , read-only memory (ROM)  16 , and non-volatile random-access memory (NVRAM)  20 . The RAM  14  typically contains one or more application programs  18  and an operating system  24 . Examples of the OS include, but are not limited to, Windows NT developed by Microsoft Corporation of Redmond, Wash., OS/2 developed by IBM Corporation of Armonk, N.Y., Netware developed by Novell, Incorporated of San Jose, Calif., and the like. In addition, the RAM  14  includes one or more intermediary programs  28 . In one embodiment and as described further below, an intermediary program  28  is a filter driver module that stores certain types of I/O transactional information in a persistent memory to ensure stability after a computer failure (e.g., from a computer crash). 
     In one embodiment, the RAM  14  is additionally partitioned into a volatile memory  32  and a persistent volatile memory  36 . The volatile memory  32  is directly accessible to the operating system  24  and is typically initialized or modified during a boot cycle of the computer  4 . The intermediary program  28  handles requests such as read request(s)  38  and/or write request(s)  39  from the operating system  24  which are directed to the persistent volatile memory  36 . In one embodiment, the persistent volatile memory  36  is a persistent cache memory. In a further embodiment, the persistent volatile memory  36  includes a log file in persistent cache memory (i.e., log cache). 
     The ROM  16  includes a modified basic input-output system (BIOS)  40  that handles the boot process of the computer  4 . The modified BIOS  40  prevents the operating system  24  from directly accessing the contents of the persistent volatile memory  36 . The persistent volatile memory  36  is not directly accessible to the operating system  24  and therefore is not modified or initialized by the operating system  24  during a boot cycle. In one embodiment, configuration information  44  regarding the location and size of the persistent volatile memory component  42  is stored in an entry in NVRAM  20 . 
     In general and during a normal boot operation of a typical computer system, a computer usually invokes a BIOS that provides low-level access to peripheral devices; identifies RAM available to the processor of the computer; initializes this RAM, typically destroying its contents; and then installs the operating system into RAM, giving the operating system access to the entire RAM to move information into and out of memory as necessary. If the computer is started after having been powered down, all of its memory will have been initialized. 
     In contrast and referring again to  FIG. 1A , during a normal boot operation the computer  4  invokes the modified BIOS  40 . The modified BIOS  40  retrieves configuration information  44  from NVRAM  20 . This configuration information  44  includes the start address and the size of persistent volatile memory  36 . The modified BIOS  40  then separates the RAM  14  into the volatile memory  32  and the persistent volatile memory  36 . The BIOS  40  then initializes the volatile memory  32 . The modified BIOS  40  provides low-level access to peripherals (not shown), installs the operating system  24  into the volatile memory  32  of RAM  14 , and prevents the operating system  24  from directly accessing the persistent volatile memory  36  during the boot cycle and normal computer operation. The operating system  24  is, in effect, unaware of the persistent volatile memory  36 . The operating system  24  then typically initializes or installs its own programs into the volatile memory  32 , often modifying the contents of the volatile memory  32 , but does not modify the contents of the persistent volatile memory  36 . This renders the contents of the persistent volatile memory  36  constant through a boot cycle. 
     In one embodiment of the invention, the intermediary program  28  is aware of the persistent volatile memory  36  and is able to access its contents. After reading the configuration information  44 , the intermediary program  28  serves as a link between the operating system  24  and the persistent volatile memory  36 . The intermediary program  28  receives a read request  38  from the operating system  24  to access the persistent volatile memory  36  and returns information to the operating system  24  from the appropriate location in the persistent volatile memory  36 . Similarly, the intermediary program  28  receives a write request  39  from the operating system  24  and stores information at the appropriate location in the persistent volatile memory  36 . 
     For example, in one embodiment of the invention the operating system  24  is the Windows 2000 operating system. Under Windows 2000, the persistent volatile memory  36  accessible through the intermediary program  28  appears to the operating system  24  as a RAM disk, in contrast to the invention, although the contents of a normal RAM disk do not survive a boot cycle. A Windows 2000 read request  38  or write request  39  includes an offset value (in bytes) from the start of the persistent volatile memory  36  and a length value (in bytes) of the data to read or the data to write. The intermediary program  28  computes the appropriate location in the persistent volatile memory  36  by adding the offset value in the request to the start address of the persistent volatile memory  36 . In one embodiment, the persistent volatile memory  36  includes 1 MB of configuration information at the beginning of the persistent volatile memory  36 , so the appropriate location is actually the sum of the offset value, the start address of the persistent volatile memory  36 , and 1 MB. 
     For a read request  38 , the intermediary program  28  copies a number of bytes equal in size to the length value from the computed location in the persistent volatile memory  36  to the user&#39;s buffer. For a write request  39 , the intermediary program  28  copies a number of bytes equal in size to the length value passed by the operating system  24  from the user&#39;s buffer to the computed location in the persistent volatile memory  36 . This interaction permits the operating system  24  to indirectly access the persistent volatile memory  36  without threatening the integrity of the contents of the persistent volatile memory  36  during a boot cycle. In another embodiment where Windows 2000 is the operating system  24 , the intermediary program  28  invokes the functionality of the operating system  24  to map the computed location onto the virtual address space of the operating system  24  for the copy operation. Other operating system  24  functionality completes the copy operation and unmaps the computed location from the virtual address space of the operating system  24 . 
     It is possible for the operating system  24  to crash while a write request  39  to persistent volatile memory  36  is being executed. In that case, an incomplete version of the request would be stored in the persistent volatile memory  36 . This can cause problems during subsequent operation, because a computer application may attempt to restore its state based on this incomplete information, potentially crashing the application and necessitating time-consuming reconstruction of the information lost during the crash. 
     To prevent this problem, the invention in one embodiment creates a table in the persistent volatile memory  36  and uses the table to describe the transactional information stored in the persistent volatile memory  36 . In particular, the intermediary program  28  creates and maintains the table. The intermediary program  28  creates and initializes the table when the computer  4  loads the intermediary program  28 . Examples of parameters that the table includes are, without limitation, a file descriptor, an offset into the file (i.e., a file byte location), a length value which indicates the number of bytes in the I/O request, a pointer that points to the location in the persistent volatile memory  36  at which the unbuffered write is stored, and a status field. The status field may include a free state that denotes that the location in the table is available to store information. The status field may also include a reserved state to denote that the intermediary program  28  is copying the transactional information to the persistent volatile memory  36 . In yet another embodiment, the status field may include an in-use state to denote that the table entry contains valid information and is not available for storage of new information. Although several examples of the parameters included in the table are described above, it should be noted that the table may also include additional parameters not described. 
     Alternatively, the invention creates look-aside buffer in the persistent volatile memory  36  and uses it for the atomic update and storage of transactional information; only when the write request  39  has been buffered and completed is it transferred out of the look-aside buffer. The intermediary program  28  may use the look-aside buffer to complete an unfinished write to the persistent volatile memory  36  (i.e., a computer failure before the write to the persistent volatile memory  36  completes). In greater detail, a look-aside buffer includes a set of bits that describe its state.  FIG. 1B  shows how the state of the look-aside buffer changes to reflect various stages in the processing of a write request  39 . 
     When no information is in the buffer, for example at the creation and initialization of the buffer, the buffer state is 0 (Step  10 ). When a write request  39  is received by the intermediary program  28 , the intermediary program  28  stores the computed location and the length (in bytes) of the request in the look-aside buffer and the state of the buffer becomes 1 (Step  12 ). At this point, the actual contents of the write request are copied into the buffer (Step  14 ). If the copy is successfully completed, the buffer state becomes 2 (Step  16 ). If the copy fails because of, for example, a system crash, the buffer state remains at 1. Once the buffer state is set to 2, the contents of the write request are copied out of the look-aside buffer to their computed location in the persistent volatile memory  36  (Step  18 ). When this is successfully completed, the buffer state returns to 0 (Step  10 ). 
     The effect of the value of the state of the look-aside buffer on the subsequent boot process is depicted in FIG.  1 C. At system reboot (Step  20 ), the intermediary program  28  locates all the look-aside buffers in the persistent volatile memory  36 . If there are no more look-aside buffers to check (Step  22 ), the system boot process continues (Step  24 ). If there are more look-aside buffers (Step  22 ), the intermediary program  28  proceeds to examine the state of each look-aside buffer, one at a time, in the persistent volatile memory  36  (Step  26 ). If the state of the buffer presently under examination is 0, the intermediary program  28  knows that there is no information stored in the look-aside buffer and the intermediary program  28  checks the next look-aside buffer (Step  22 ). If the buffer state is 1, the intermediary program  28  knows that the information in the look-aside buffer is the result of an incomplete transaction and should not be moved into the persistent volatile memory  36  for recovery by a computer application. The intermediary program  28  sets the state of this buffer to 0 (Step  20 ) and checks the next look-aside buffer (Step  22 ). If the buffer under examination is in state  2 , then the intermediary program  28  knows that the contents of the look-aside buffer are the result of a completed transaction that did not get copied into the persistent volatile memory  36 . The intermediary program  28  copies the contents of the look-aside buffer to the computed location in the persistent volatile memory  36  (Step  28 ). When the copy is completed, the buffer state is set to 0 (Step  20 ) and the intermediary program  28  checks the next look-aside buffer (Step  22 ). Eventually the intermediary program  28  will have checked the state of all the look-aside buffers, and the system boot will continue (Step  24 ). 
     Although described above and below as a table or a buffer, it should be noted that any data structure can be used to provide information about the state of the transactional information that is stored in the persistent volatile memory  36 . 
     Referring to  FIG. 1D , during a boot cycle the computer  4  loads the programs implementing the invention into the memory  8  at Step  40 . In one embodiment, the programs are the intermediary program  28  and the modified BIOS  40 . The programs  28 ,  40  divide the RAM  14  into two portions: the volatile memory  32  directly accessible to the operating system  24  in Step  42  and the persistent volatile memory  36  that is not directly accessible to the operating system  24  in Step  44 . This is accomplished through modifications to the BIOS  40 . The inaccessibility to the operating system  24  renders the contents of the persistent volatile memory  36  resistant to initialization or modification during a boot cycle. Again, one skilled in the art will recognize that the invention permits multiple persistent and non-persistent memory regions, but for the sake of simplicity of discussion and depiction, the present discussion assumes one volatile memory  32  and one persistent volatile memory  36 . 
     Once the memory partitioning has been achieved, the intermediary program  28  provides indirect access to the persistent volatile memory  36  to the operating system  24 . In step  46 , the intermediary program  28  waits for a read request  38  or a write request  39  from the operating system  24 . The intermediary program  28  decides (Step  48 ) whether a read request has been received, and if one has, then the intermediary program reads (Step  50 ) from the appropriate location in the persistent volatile memory  36  and returns the result to the operating system  24 . Similarly, if the intermediary program  28  decides (Step  52 ) that a write request  39  has been received, then the intermediary program  28  stores (Step  54 ) information at the appropriate location in the persistent volatile memory  36 . If neither type of request has been received, then the intermediary program  28  returns to step  46  and continues to wait for requests. Typically, read and write requests from the operating system  24  to the volatile memory  32  operate as they would have before the installation of the invention. 
     An example of the intermediary program  28  is a filter driver module. The filter driver module  28  stores certain types of I/O transactional information in the persistent volatile memory  36  so that the information does not get erased during a computer crash. Thus, following a crash of the computer  4 , the application program  18  still recognizes what the application program  18  had done just prior to the computer crash. 
     In one embodiment, the transactional information are unbuffered writes (i.e., writes requested by the application  18  and in which notification to the application  18  of the completion of the write is necessary) to the persistent mass storage  22 . Examples of an unbuffered write include, without limitation, copying a file from one directory to another directory, backing up and/or updating a file, initializing a file (e.g., writing zeros to the file), and the like. Although described above and below with transactional information, any information can be used within the scope of the invention. 
     For example, the application  18  can be a database management system (DBMS) that verifies all updates to files before that file can be made available again to the application  18 . It should be noted that a request to access the file for a read or write transaction can be from a different query from the same application or can be from a different application altogether. 
     In general and during a normal unbuffered write of transactional information, an application program executing on a computer typically writes transactional information to disk and the operating system of the computer transmits a confirmation message to the application when the write completes. If the application program does not receive a confirmation message after a predetermined time, the application often performs the previous write to disk again and subsequently waits for another confirmation message. This process is generally wasteful and decreases the performance of the application program. 
     In the invention described above and below, the application  18  generates a write to the persistent mass storage  22  (e.g., disk). The operating system  24  instead uses the filter driver module  28  and writes the information to the persistent volatile memory  36 . Following the completion of this write to the persistent volatile memory  36 , the operating system  24  transmits the confirmation to the application  18 . The application  18  receives the confirmation soon after the generation of the write, as a write to memory (e.g., persistent volatile memory  36 ) is faster than a write to the persistent mass storage  22 . Therefore, in one embodiment the application  18  receives the confirmation from the operating system  24  before the transactional information is stored in the persistent mass storage  22 . 
     The filter driver module  28  may be a passive filter or an active filter. A passive filter is a filter that monitors the information that the filter driver module  28  stores in the persistent volatile memory  36 . For example, the computer  4  configures the passive filter driver module  28  to monitor all unbuffered writes requested by a particular application  18 , such as by a DBMS. This may be used to help determine performance decreases associated with multiple unbuffered writes by a particular application  18 . 
     As an active filter, the filter driver module  28  receives an instruction to store in the persistent volatile memory  36  and performs some modification on the instruction before storing the instruction. For example, the active filter driver module  28  may receive a certain type of transactional information, such as an unbuffered write to initialize a file by writing zeros to the file. The active filter driver module  28  may alter the unbuffered write to write ones to the file if the operating system  24  determines the writing of ones to be necessary for initialization. In a further embodiment, the filter driver module  28  is created by a file systems filter driver kit (FDDK), developed by Open Systems Resources, Incorporated of Amherst, N.H. 
     Referring to  FIG. 2A , a logical flow chart depicts the operation of the computer  4  on unbuffered writes. The application  18  generates (step  205 ) an unbuffered write and the filter driver module  28  detects (step  210 ) the unbuffered write. After detecting the unbuffered writes, the filter driver module  28  updates (step  212 ) the table described above with information associated with the unbuffered write. For example, the filter driver module  28  updates the status field to a reserved state to denote that the detected unbuffered write is about to be copied to the persistent volatile memory  36 . 
     After updating the table, the filter driver module  28  stores (step  215 ) the transactional information in the persistent volatile memory  36 . In another embodiment, all writes (unbuffered writes and buffered writes) are stored in the persistent volatile memory  36 . In yet another embodiment, the filter driver module  28  stores the transactional information in volatile memory  32 , makes a copy of the transactional information stored in the volatile memory  32 , and then transfers the copy into the persistent volatile memory  36 . 
     In one embodiment and as further described below, the operating system  24  additionally starts a timer to enable future recordation of the time elapsed from the transferring of the transactional information to the persistent volatile memory  36 . In another embodiment, the operating system  24  stores the time read from a predetermined register located in the computer  4 . 
     The filter driver module  28  then updates (step  220 ) the table to denote that the transfer of the transactional information to the persistent volatile memory  36  is complete. In particular and in one embodiment, the filter driver module  28  updates the status field associated with the particular transactional information to an in-use state. Thus, if a failure of the computer  4  occurs prior to the completion of a transmittal of transactional information to the persistent volatile memory  36 , the filter driver module  28  can determine that the transmittal of particular session information did not complete prior to the computer failure (i.e., the status field associated with the transactional information will not be set to an in-use state). If the filter driver module  28  determines that the transactional information was not transmitted to the persistent volatile memory  36 , then the filter driver module  28  repeats step  215  to store the transactional information in the persistent volatile memory. 
     The operating system  24  then notifies (step  223 ) the application  18  that the transactional information has been stored in the persistent volatile memory  36 . In one embodiment, the notification occurs as a confirmation message to the application. The operating system  24  then determines (step  225 ) whether the operating system  24  should flush, or transfer, the persistent volatile memory  36  to the persistent mass storage  22 . In one embodiment, the filter driver module  28  includes a thread responsible for flushing the persistent volatile memory  36  to the persistent mass storage  22 . As described further below, the thread can flush the persistent volatile memory  36  when a particular event occurs, such as when the operating system  24  transmits a message to the filter driver module  28  instructing the filter driver module  28  to flush the persistent volatile memory  36 . In another embodiment, the thread may poll the persistent volatile memory  36  to determine whether the data stored in the persistent volatile memory should be flushed. 
       FIG. 2B  illustrates an embodiment of the steps performed by the operating system  24  to determine (step  225 ) whether the operating system  24  should flush the persistent volatile memory  36  to the persistent mass storage  22 . In one embodiment, the operating system  24  determines (step  230 ) whether the data stored in the persistent volatile memory  36  exceeds or is about to exceed some predetermined threshold (e.g., the allotted size of the persistent volatile memory  36 ). If so, the operating system  24  flushes (step  233 ) the persistent volatile memory  36  to the persistent mass storage  22 . If not, the operating system  24  does not flush (step  234 ) the persistent volatile memory  36 . The operating system  24  can also flush (step  235 ) the persistent volatile memory  36  if the operating system  24  determines (step  235 ) that a predefined amount of time since the transferring of the transactional information has elapsed. In some embodiments, the operating system  24  uses the timer described above to make this determination. In another embodiment, the operating system  24  records the current time stored in the register described above. Using this recorded time and the previously recorded time, the operating system  24  determines the amount of time that has elapsed since the transferring the transactional information to the persistent volatile memory  36 . If the elapsed time is greater than a predefined amount of time, the operating system  24  flushes (step  233 ) the persistent volatile memory  36 . 
     In another embodiment, the operating system  24  determines (step  240 ) to flush the persistent volatile memory  36  when the operating system  24  is not servicing the application  18  (i.e., the operating system is not busy). For example, the operating system  24  flushes the persistent volatile memory  36  when the application  18  is idle. 
     Alternatively, the operating system  24  determines (step  245 ) whether the application  18  requests to write the transactional information to a file that has previously been closed. In one embodiment, the application  28  transmits a message to the operating system  24  when the application  28  closes. If so, the operating system  24  flushes (step  233 ) the persistent volatile memory  36 . The operating system  24  additionally flushes (step  233 ) the persistent volatile memory  233  when the computer  4  is in the process of being (step  250 ) shut down. In one embodiment, the filter driver module  28  additionally updates (step  255 ) the table to denote that the transactional information has been stored in the persistent mass storage  22 . 
     By storing the transactional information in a persistent volatile memory  36 , the information is accessible to the computer  4  at any instant in time. Therefore, a retrieval of such information does not significantly hamper the performance of the computer  4 . Additionally, the persistent volatile memory  36  (and the persistent mass storage  22 ) enable the session information to be accessible to the computer  4  after a computer failure, which would ordinarily erase the information from a volatile memory. 
     As an example and referring again to  FIG. 2A , suppose the transaction is a catalog merchandise order phoned in by a customer and entered into the computer  4  by a customer representative. The customer enters the order into an application  18  associated with the catalog and the application  18  generates (step  205 ) the writes to a database file that should occur for the order. In particular, the order transaction involves checking an inventory database file, confirming that the item is available, placing the order and confirming that the order has been placed. Considering these steps as a single transaction, then all of the steps are to be completed before the transaction is successful and the inventory database file is actually changed to reflect the new order. 
     In greater detail, the application  18  associated with the catalog checks the inventory database file and confirms that the item is available. If the item is available, the application  18  places the order. In some prior art computer systems, the application  18  stores the transactional information in RAM  14  so that the application  18  (e.g., the DBMS) can commit the information to persistent mass storage  22  at a later time. If the computer  4  failures (e.g., crashes) at this point in time, the order would have been placed, but the information previously stored in RAM  14  is erased. Consequently, the transactional information has not been reflected in the inventory database file and, therefore, to retrieve this information to update the inventory database file, the application  18  frequently has to repeat the transaction again. 
     Unlike the above scenario, the invention uses the filter driver module  28  to determine (step  210 ) that the transaction involves an unbuffered write to the inventory database file. The filter driver module  28  then stores (step  215 ) the transactional information to the persistent volatile memory  36  so that the transactional information can survive a crash of the computer  4 . Thus, if the failure of the computer  4  occurs, the transactional information has already been stored in the persistent volatile memory  36 . The operating system  24  notifies (step  223 ) the application  18  of the completion of the write and consequently determines (step  225 ) whether to flush the persistent volatile memory  36 , as described above. 
     Having described certain embodiments of the invention, it will now become apparent to one of skill in the art that other embodiments incorporating the concepts of the invention may be used. Therefore, the invention should not be limited to certain embodiments, but rather should be limited only by the spirit and scope of the following claims.