Abstract:
One embodiment of the present invention provides a system for elevating a secondary file server to act as a new primary file server in a highly available file system. Upon determining that a primary file server in the highly available file system has failed, the system promotes the secondary file server to become the new primary file server. During this process, the new primary file server scans file objects to look for a file lock indication. Upon finding a file lock indication, the new primary file server converts an associated file identifier into a virtual node. Otherwise, conversion of file identifiers into virtual nodes is delayed until the first time a file is subsequently accessed by the new primary server, thereby speeding up the failover process.

Description:
BACKGROUND 
   1. Field of the Invention 
   The present invention relates to highly available file systems. More specifically, the present invention relates to a method and apparatus for facilitating a failover to a secondary file server upon failure of a primary file server in a highly available file system. 
   2. Related Art 
   Highly available file systems allow a client to continue accessing files from a storage device when a primary file server for the storage device fails or otherwise becomes inaccessible. Highly available file systems typically make use of multiple file servers that are coupled to one or more multi-ported storage devices. During operation, one file server is designated as the primary file server. This primary file server services all requests to access files on the storage device. The other file servers act as backup secondary file servers, which do not normally access the storage device. The primary file server periodically sends checkpoint information to the secondary file servers. This allows the secondary file servers to maintain copies of state information from the primary file server. 
   When the primary file server subsequently fails or otherwise becomes inaccessible, a secondary file server is promoted to act as a new primary file server. This process of promoting a secondary file server to act as a new primary file server is referred to as a “failover.” Promoting the secondary file server allows operations in progress to complete or to be automatically retried. 
   In some operating systems, a primary file server maintains file objects, which contain a pointer to a virtual node (vnode). A vnode holds state information that is used to access the file on the underlying file system. The secondary file server also maintains file objects. However, instead of including pointers to vnodes, these file objects include pointers to file identifiers (FIDs). FIDs can be used to identify a file on the storage device. However, an FID must first be converted to a vnode before the file can be accessed. 
   Hence, during a failover operation, the new primary file server must convert FIDs to vnodes. This can be an extremely slow process because a large number of FIDs may have to be converted to vnodes, and furthermore, each conversion may require an access to the storage device. During this conversion process, the new primary file server cannot accept new file requests. This can cause long waits for client applications that need to access the file system, which result in timeouts or other failures. 
   What is needed is a method and an apparatus that facilitates failover to a secondary file server in a highly available file system without the problems described above. 
   SUMMARY 
   One embodiment of the present invention provides a system for elevating a secondary file server to act as a new primary file server in a highly available file system. Upon determining that a primary file server in the highly available file system has failed, the system promotes the secondary file server to become the new primary file server. During this process, the new primary file server scans file objects to look for a file lock indication. Upon finding a file lock indication, the new primary file server converts an associated file identifier into a virtual node. Otherwise, conversion of file identifiers into virtual nodes is delayed until the first time a file is subsequently accessed by the new primary server, thereby speeding up the failover process. 
   In one embodiment of the present invention, the file lock indication is stored within a file object referenced by a list of file objects. 
   In one embodiment of the present invention, the system converts a file identifier into a corresponding virtual node during a subsequent access to the file object by the new primary file server. 
   In one embodiment of the present invention, converting the associated file identifier into the virtual node involves locating a file referenced by the associated file identifier and reading the file. 
   In one embodiment of the present invention, the system periodically sends a checkpoint from the primary file server to the secondary file server. 
   In one embodiment of the present invention, the checkpoint includes the file identifier associated with the virtual node. 
   In one embodiment of the present invention, a checkpoint is sent for each state change at the primary file server. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1A  illustrates highly available file system  100  in accordance with an embodiment of the present invention. 
       FIG. 1B  illustrates failover mechanism  110  in accordance with an embodiment of the present invention. 
       FIG. 2A  illustrates the process of checkpointing in a highly available file system in accordance with an embodiment of the present invention. 
       FIG. 2B  illustrates a failed primary file server in a highly available file system in accordance with an embodiment of the present invention. 
       FIG. 3  illustrates a number of data structures within a highly available file system in accordance with an embodiment of the present invention. 
       FIG. 4  illustrates file object  310  in accordance with an embodiment of the present invention. 
       FIG. 5  is a flowchart illustrating the process of promoting a secondary file server to be a new primary file server in accordance with an embodiment of the present invention. 
       FIG. 6  is a flowchart illustrating the process of converting a file identifier into a vnode in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
   The data structures and code described in this detailed description are typically stored on a computer readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. This includes, but is not limited to, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs) and DVDs (digital versatile discs or digital video discs), and computer instruction signals embodied in a transmission medium (with or without a carrier wave upon which the signals are modulated). For example, the transmission medium may include a communications network, such as the Internet. 
   Highly Available File System 
     FIG. 1A  illustrates highly available file system  100  in accordance with an embodiment of the present invention. Highly available file system  100  includes primary server  104  and secondary server  106 . Primary server  104  and secondary server  106  can generally include any nodes on a computer network including a mechanism for servicing requests from a client for computational and/or data storage resources. Note that highly available file system  100  may include more than one secondary server. 
   Primary server  104  and secondary server  106  are coupled to disk  112 . Disk  112  can include any type of system for storing data in non-volatile storage. This includes, but is not limited to, systems based upon magnetic, optical, and magneto-optical storage devices, as well as storage devices based on flash memory and/or battery-backed up memory. Disk  112  can include a redundant array of inexpensive (or independent) disks (RAID) system. Moreover, disk  112  includes multiple ports for communicating with file servers. Note that disk  112  may include more than the two ports shown in FIG.  1 A. 
   Primary server  104  and secondary server  106  include failover mechanisms  108  and  110 , respectively. Failover mechanisms  108  and  110  form a distributed management system for disk  112 , which operates as described below. 
   Primary server  104  and secondary server  106  are also coupled to network  102 . Network  102  can generally include any type of wire or wireless communication channel capable of coupling together computing nodes. This includes, but is not limited to, a local area network, a wide area network, or a combination of networks. In one embodiment of the present invention, network  102  includes the Internet. 
   Failover Mechanisms 
     FIG. 1B  illustrates the structure of failover mechanism  110  in accordance with an embodiment of the present invention. Failover mechanism  110  includes failure detector  114 , file identifier converter  120 , and checkpoint mechanism  122 . 
   Failure detector  114  operates in conjunction with the corresponding failure detectors in other servers to detect when primary server  104  fails. Techniques for detecting failure of primary file server  104  are know in the art and will not be discussed further herein. Upon detecting failure of primary server  104 , failover mechanism  110  promotes secondary server  106  to be the new primary server. 
   After secondary server  106  has been promoted to be the new primary server, failover mechanism  110  scans a file object list as described below in conjunction with  FIGS. 3 and 5  to determine if the associated file object includes a lock indication. A file objects that includes a lock indication has its file identifier immediately converted into a virtual node (vnode). In contrast, a file object without a lock indication delays having its file indicator converted into a vnode until the associated file is subsequently accessed. 
   Checkpoint mechanism  122  within primary server  104  sends checkpoints to a corresponding checkpoint mechanism  122  within secondary server  106  so that secondary server  106  is aware of file operations in progress within primary server  104 . These checkpoints include a file identifier (FID) for each file in use by primary server  104 . Note that primary server  104  maintains a vnode for each file rather than an FID. 
   Checkpointing 
     FIG. 2A  illustrates the process of checkpointing in a highly available file system in accordance with an embodiment of the present invention. During operation, primary server  104  receives a file access request from network  102  and accesses disk  112  through I/O channel  204  to respond to this request. 
   Primary server  104  periodically sends checkpoints  202  to secondary server  106  so that secondary server  106  can maintain data related to file operations in progress on primary server  104 . If primary server  104  fails, secondary server  106  makes use of the data provided in checkpoints  202  to become a new primary server. 
   Failure of Primary Server 
   In  FIG. 2B , primary server  104  has failed and secondary server  106  has been promoted to be a new primary. Upon being promoted to be a new primary server, secondary server  106  converts FIDs associated with locks into vnodes and then retries uncompleted file operations if necessary. FIDs not associated with locks are not converted into vnodes until a subsequent access to the file occurs. Note that secondary server  106  communicates with disk  112  across I/O channel  210 . 
   Data Structures 
     FIG. 3  illustrates a number of data structures within a highly available file system in accordance with an embodiment of the present invention. Note that  FIG. 3  includes portions of file system client  302 , primary server  104 , and secondary server  106 . File system client  302  includes virtual node data  308  related to a file within the highly available file system. File system client  302  receives data from the associated file and updates the associated file through interface  318 . Interface  318  accesses file object  310  within primary server  104 . File object  310 , in turn, accesses file system vnode  314  using backpointer  322  within file object  310 . Note that file object list  304  includes a pointer to every file object on primary server  104 , including file object  310 . 
   Primary server  104  sends checkpoints  202  to secondary server  106 . Secondary server  106  uses checkpoints  202  to maintain file object  312 . Note that file object  312  is substantially the same as file object  310  except that backpointer  324  within file object  312  points to file identifier  316  rather than a vnode. File object list  306  includes a pointer to each file object within secondary server  106 . 
   File Object 
     FIG. 4  illustrates file object  310  in accordance with an embodiment of the present invention. File object  310  includes has_locks  402  and back_object  404 . Has_locks  402  is a Boolean variable indicating whether the related file has been locked by an application. Typically, open files are locked only when an update operation is in progress, therefore, has_locks  402  is typically false. Back_object  404  is a pointer pointing to the file system vnode  314 . The respective back_object pointer in file object  312  points to file identifier  316 . Note that the has_locks Boolean variable within file object  312  has the same state as the has_locks Boolean variable  402  within file object  310 . 
   Converting a Secondary Server 
     FIG. 5  is a flowchart illustrating the process of promoting a secondary file server to be a new primary file server in accordance with an embodiment of the present invention. The system starts when secondary server  106  determines that primary server  104  has failed (step  502 ). In response to this determination, secondary server  106  is promoted to be a new primary server (step  504 ). During this process, the new primary server stops accepting new I/O requests (step  506 ). 
   After stopping the new I/O requests, the new primary server inspects the file objects in a loop to determine which files have their has_locks Boolean variables set to true. The first action in the loop is to get a file pointer from the file object list (step  508 ). Next, the new primary server determines if the has_locks Boolean variable is true (step  510 ). If so, the file identifier converter converts the file identifier to a vnode (step  512 ). Note that converting the file identifier involves accessing the disk to read the file. After this conversion, or if the has_locks Boolean is false, the new primary server determines if there are more pointers in the file object list (step  514 ). If so, the process returns to step  508  to retrieve the next file object pointer. Note that the new file server converts only files with has_locks Boolean variables set to true, which can save a considerable amount of time during the failover process. 
   After scanning the entire file object list, the new primary file server retries any outstanding I/O requests (step  516 ). Finally, the new primary file server resumes accepting new I/O requests (step  518 ). 
   Converting Unconverted FIDs 
     FIG. 6  is a flowchart illustrating the process of converting a file identifier into a vnode in accordance with an embodiment of the present invention. This process takes place the first time a file is accessed by a new primary file server after a failover. During this first access, the primary file server retrieves a back pointer from a file object associated with the file (step  602 ). Next, the primary file server determines if the back pointer points to a vnode or to an FID (step  604 ). If the back pointer points to an FID, the primary file server converts the FID into a vnode (step  606 ). After the vnode is created, the system resumes operation (step  608 ). 
   The foregoing descriptions of embodiments of the present invention have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention. The scope of the present invention is defined by the appended claims.