Abstract:
The techniques introduced here provide for enabling deduplication operations for a file system without significantly affecting read performance of the file system due to fragmentation of the data sets in the file system. The techniques include determining, by a storage server that hosts the file system, a level of fragmentation that would be introduced to a data set stored in the file system as a result of performing a deduplication operation on the data set. The storage server then compares the level of fragmentation with a threshold value and determines whether to perform the deduplication operation based on a result of comparing the level of fragmentation with the threshold value. The threshold value represents an acceptable level of fragmentation in the data sets of the file system.

Description:
FIELD OF THE INVENTION 
     At least one embodiment of the present invention pertains to deduplication of data stored by a storage server, and more particularly to controlling the level of fragmentation introduced to a data set in the process of deduplication. 
     BACKGROUND 
     A storage controller is a physical processing device that is used to store and retrieve data on behalf of one or more hosts. A network storage controller can be configured (e.g., by hardware, software, firmware, or any combination thereof) to operate as a storage server that serves one or more clients on a network, to store and manage data in a set of mass storage devices, such as magnetic or optical storage-based disks, tapes, or flash memory. 
     Mass storage devices provide a series of addressable locations in which data can be stored. Some devices, such as tape drives, only permit storage locations to be accessed in sequential order, while other devices, such as hard disks or flash, permit random access. Mass storage devices may be combined to give the impression to higher layers of a single device with certain desirable characteristics. For example, a Redundant Array of Independent Disks (“RAID array”) may contain two or more hard disks with data spread among them to obtain increased transfer speed, improved fault tolerance or simply increased storage capacity. The placement of data (and calculation and storage of error detection and correction information) on various devices in a RAID array may be managed by hardware and/or software. 
     Many contemporary data processing systems consume and/or produce vast quantities of data. Mass storage devices such as hard disk drives are often used to store this data. To keep up with the amount of data consumed and produced by these processing systems, either the storage capacity of mass storage devices and/or the efficiency of the usage of space on the mass storage devices can be increased. One method for increasing the efficiency of the usage of space on a mass storage device is to perform a deduplication operation which eliminates redundant data stored on a mass storage device. 
     However, deduplication often introduces fragmentation into a data set that was previously stored as contiguous blocks on disk. Each addressable storage location can usually hold multiple data bytes; such a location is called a “block.” When the data blocks of a data set are separated and/or stored out of read order, the data set is said to be “fragmented.” A process that reads the fragmented data set might cause the storage system to perform multiple read operations to obtain the contents of the data blocks corresponding to the data set. The mechanical nature of many types of mass storage devices limits their speed to a fraction of the system&#39;s potential processing speed, particularly when a data set is fragmented and requires multiple read operations to retrieve the data set. Because fragmentation caused by deduplication can negatively impact storage system performance, many storage system users disable deduplication operations and therefore do not benefit from the space saving advantages of deduplication. 
     Therefore, a technique to balance the effects of fragmentation introduced during deduplication operations and the storage system performance desired by users is needed. 
     SUMMARY 
     The techniques introduced here enable deduplication operations for a file system without significantly affecting read performance of the file system due to fragmentation of the data sets in the file system. In one embodiment, a storage server that hosts the file system determines a level of fragmentation that would be introduced to a data set stored in the file system as a result of performing a deduplication operation on the data set. The storage server then compares the level of fragmentation with a threshold value and determines whether to perform the deduplication operation based on a result of the comparison. The threshold value represents an acceptable level of fragmentation in the data sets of the file system. 
     In one embodiment, the level of fragmentation of a data set stored in a file system is determined by performing a read-ahead to count the number of read operations to access the data set after performing a deduplication operation on the data set, and calculating a fragmentation index that is a ratio of the number of read operations after deduplication to the number of read operations to access an ideal data set. An ideal data set is one that is stored in contiguous physical data blocks and can be accessed with the lowest number of read operations. The fragmentation index is an indication of the increase in fragmentation due to the deduplication operation. 
     The storage server can calculate the number of read operations to access the data set after performing the deduplication operation by determining a list of physical volume block numbers (PVBNs) that would represent the data set after a deduplication operation has been performed, sorting the list of PVBNs, and counting the number of contiguous groups of blocks. 
     Other aspects of the techniques summarized above will be apparent from the accompanying figures and from the detailed description which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One or more embodiments of the present invention are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements. 
         FIG. 1  shows an example of a network storage system. 
         FIG. 2  is a diagram illustrating an example of a storage controller that can implement one or more network storage servers. 
         FIG. 3  schematically illustrates an example of the architecture of a storage operating system in a storage server. 
         FIG. 4A  is a block diagram representation of buffer trees for files in a file system. 
         FIG. 4B  is a block diagram representation of buffer trees for files in a file system after a deduplication process has been performed on the file system. 
         FIG. 5  is a flow diagram of a process for determining whether to perform a deduplication operation. 
         FIG. 6  is a flow diagram of a process for determining a level of fragmentation that would be introduced by performing a deduplication operation on a data set. 
         FIG. 7  is a flow diagram of a process for calculating, prior to performing a deduplication operation, the number of read operations to access the data set after deduplication. 
     
    
    
     DETAILED DESCRIPTION 
     References in this specification to “an embodiment”, “one embodiment”, or the like, mean that the particular feature, structure or characteristic being described is included in at least one embodiment of the present invention. Occurrences of such phrases in this specification do not necessarily all refer to the same embodiment. 
       FIG. 1  shows an example of a network storage system, which includes a plurality of client systems  104 , a storage server  108 , and a network  106  connecting the client servers  104  and the storage server  108 . As shown in  FIG. 1 , the storage server  108  is coupled with a number of mass storage devices  112 , such as disks, in a mass storage subsystem  105 . Alternatively, some or all of the mass storage devices  112  can be other types of storage, such as flash memory, solid-state drives (SSDs), tape storage, etc. However, to facilitate description, the storage devices  112  are assumed to be disks herein. 
     The storage server  108  can be, for example, one of the FAS-series of storage server products available from NetApp®, Inc. The client systems  104  are connected to the storage server  108  via the network  106 , which can be a packet-switched network, for example, a local area network (LAN) or wide area network (WAN). Further, the storage server  108  can be connected to the disks  112  via a switching fabric (not shown), which can be a fiber distributed data interface (FDDI) network, for example. It is noted that, within the network data storage environment, any other suitable number of storage servers and/or mass storage devices, and/or any other suitable network technologies, may be employed. 
     The storage server  108  can make some or all of the storage space on the disk(s)  112  available to the client systems  104  in a conventional manner. For example, each of the disks  112  can be implemented as an individual disk, multiple disks (e.g., a RAID group) or any other suitable mass storage device(s). Storage of information in the mass storage subsystem  105  can be implemented as one or more storage volumes that comprise a collection of physical storage disks  112  cooperating to define an overall logical arrangement of volume block number (VBN) space on the volume(s). Each volume is generally, although not necessarily, associated with its own file system. 
     The disks associated with a volume/file system are typically organized as one or more groups, wherein each group may be operated as a Redundant Array of Independent (or Inexpensive) Disks (RAID). Most RAID implementations, such as a RAID-4 level implementation, enhance the reliability/integrity of data storage through the redundant writing of data “stripes” across a given number of physical disks in the RAID group, and the appropriate storing of parity information with respect to the striped data. An illustrative example of a RAID implementation is a RAID-4 level implementation, although it should be understood that other types and levels of RAID implementations may be used according to the techniques described herein. One or more RAID groups together form an aggregate. An aggregate can contain one or more volumes. 
       FIG. 2  is a diagram illustrating an example of the hardware architecture of a storage controller that can implement one or more network storage servers, for example, storage server  108  of  FIG. 1 . The storage server is a processing system that provides storage services relating to the storage, organization, and retrieval of information on mass storage devices, such as disks  112  of the mass storage subsystem  105 . In an illustrative embodiment, the storage server  108  includes a processor subsystem  210  that includes one or more processors. The storage server  108  further includes a memory  220 , a network adapter  240 , and a storage adapter  250 , all interconnected by an interconnect  260 . 
     The storage server  108  can be embodied as a single- or multi-processor storage server executing a storage operating system  230  that preferably implements a high-level module, called a storage manager, to logically organize data in one or more file systems on the disks  112 . 
     The memory  220  illustratively comprises storage locations that are addressable by the processor(s)  210  and adapters  240  and  250  for storing software program code and data associated with the techniques introduced here. The processor  210  and adapters may, in turn, comprise processing elements and/or logic circuitry configured to execute the software code and manipulate the data structures. The storage operating system  230 , portions of which are typically resident in memory and executed by the processing elements, functionally organizes the storage server  108  by (among other things) invoking storage operations in support of the storage service provided by the storage server  108 . It will be apparent to those skilled in the art that other processing and memory implementations, including various computer readable storage media, may be used for storing and executing program instructions pertaining to the techniques introduced here. 
     The network adapter  240  includes a plurality of ports to couple the storage server  108  with one or more clients  104 , or other storage servers, over point-to-point links, wide area networks, virtual private networks implemented over a public network (Internet) or a shared local area network. The network adapter  240  thus can include the mechanical components as well as the electrical and signaling circuitry needed to connect the storage server  108  to the network  106 . Illustratively, the network  106  can be embodied as an Ethernet network or a Fibre Channel network. Each client  104  can communicate with the storage server  108  over the network  106  by exchanging packets or frames of data according to pre-defined protocols, such as Transmission Control Protocol/Internet Protocol (TCP/IP). 
     The storage adapter  250  cooperates with the storage operating system  230  to access information requested by the clients  104 . The information may be stored on any type of attached array of writable storage media, such as magnetic disk or tape, optical disk (e.g., CD-ROM or DVD), flash memory, solid-state drive (SSD), electronic random access memory (RAM), micro-electro mechanical and/or any other similar media adapted to store information, including data and parity information. However, as illustratively described herein, the information is stored on disks  112 . The storage adapter  250  includes a plurality of ports having input/output (I/O) interface circuitry that couples with the disks over an I/O interconnect arrangement, such as a conventional high-performance, Fibre Channel link topology. 
     The storage operating system  230  facilitates client access to data stored on the disks  112 . In certain embodiments, the storage operating system  230  implements a write-anywhere file system that cooperates with one or more virtualization modules to “virtualize” the storage space provided by disks  112 . In certain embodiments, a storage manager  310  ( FIG. 3 ) element of the storage operation system  230  logically organizes the information as a hierarchical structure of named directories and files on the disks  112 . Each “on-disk” file may be implemented as a set of disk blocks configured to store information. As used herein, the term “file” means any logical container of data. The virtualization module(s) may allow the storage manager  310  to further logically organize information as a hierarchical structure of blocks on the disks that are exported as named logical unit numbers (LUNs). 
       FIG. 3  schematically illustrates an example of the architecture of a storage operating system  230  for use in a storage server  108 . In one embodiment, the storage operating system  230  can be the NetApp® Data ONTAP™ operating system available from NetApp, Inc., Sunnyvale, Calif. that implements a Write Anywhere File Layout (WAFL™) file system. However, another storage operating system may alternatively be designed or enhanced for use in accordance with the techniques described herein. 
     The storage operating system  230  can be implemented as programmable circuitry programmed with software and/or firmware, or as specially designed non-programmable circuitry (i.e., hardware), or in a combination thereof. In the illustrated embodiment, the storage operating system  230  includes several modules, or layers. These layers include a storage manager  310 , which is the core functional element of the storage operating system  230 . The storage manager  310  imposes a structure (e.g., one or more file systems) on the data managed by the storage server  108  and services read and write requests from clients  104 . 
     To allow the storage server to communicate over the network  106  (e.g., with clients  104 ), the storage operating system  230  also includes a multi-protocol layer  320  and a network access layer  330 , logically under the storage manager  310 . The multi-protocol layer  320  implements various higher-level network protocols, such as Network File System (NFS), Common Internet File System (CIFS), Hypertext Transfer Protocol (HTTP), Internet small computer system interface (iSCSI), and/or backup/mirroring protocols to make data stored on the disks  112  available to users and/or application programs. The network access layer  330  includes one or more network drivers that implement one or more lower-level protocols to communicate over the network, such as Ethernet, Internet Protocol (IP), TCP/IP, Fibre Channel Protocol and/or User Datagram Protocol/Internet Protocol (UDP/IP). 
     Also, to allow the device to communicate with a storage subsystem (e.g., storage subsystem  105 ), the storage operating system  230  includes a storage access layer  340  and an associated storage driver layer  350  logically under the storage manager  310 . The storage access layer  340  implements a higher-level storage redundancy algorithm, such as RAID-4, RAID-5 or RAID DP®. The storage driver layer  350  implements a lower-level storage device access protocol, such as Fibre Channel Protocol or small computer system interface (SCSI). 
     Also shown in  FIG. 3  is the path  360  of data flow through the storage operating system  230 , associated with a read or write operation, from the client interface to the storage interface. Thus, the storage manager  310  accesses the storage subsystem  105  through the storage access layer  340  and the storage driver layer  350 . Clients  104  can interact with the storage server  108  in accordance with a client/server model of information delivery. That is, the client  104  requests the services of the storage server  108 , and the storage server may return the results of the services requested by the client, by exchanging packets over the network  106 . The clients may issue packets by using file-based access protocols, such as CIFS or NFS, over TCP/IP when accessing information in the form of files and directories. Alternatively, the clients may issue packets by using block-based access protocols, such as iSCSI and SCSI, when accessing information in the form of blocks. 
     In one embodiment, the storage manager  310  implements a message-based file system that provides logical volume management capabilities for use in access to the information stored on the storage devices, such as disks  112 . That is, in addition to providing file system semantics, the storage manager  310  provides additional storage efficiency functions such as deduplication and compression operations. The storage manager  310  can implement the WAFL file system (hereinafter generally the “write-anywhere file system”) having an on-disk format representation that is block-based using, e.g., 4 kilobyte (kB) blocks and using index nodes (“inodes”) to identify files and file attributes (such as creation time, access permissions, size and block location). The file system uses files to store metadata describing the layout of its file system; these metadata files include, among others, an inode file. 
     Operationally, a request from the client  104  is forwarded as a packet over the computer network  106  and onto the storage server  108  where it is received at the network adapter  240 . A network driver (of layer  330 ) processes the packet and, if appropriate, passes it on to a multi-protocol access layer for additional processing prior to forwarding to the storage manager  310 . Here, the storage manager generates operations to load (retrieve) the requested data from disk  112 . The storage manager then passes a message structure including the file inode to the RAID system  340 ; the file inode is mapped to a disk identifier and disk block numbers and sent to an appropriate driver (e.g., a SCSI driver) of the disk driver system  350 . The disk driver accesses the disk block numbers from the specified disk  112  and loads the requested data block(s) in memory  220  for processing by the storage server. Upon completion of the request, the storage server (and operating system) returns a reply to the client  104  over the network  106 . 
     While depicted in  FIG. 3  as a single unit, the storage operating system  230  can have a distributed architecture. For example, the multi-protocol layer  320  and network access layer  330  can be contained in an N-module (e.g., N-blade) while the storage manager  310 , storage access layer  340  and storage driver layer  350  are contained in a separate D-module (e.g., D-blade). In such cases, the N-module and D-module (not shown) communicate with each other (and, possibly, with other N- and D-modules) through some form of physical interconnect and collectively form a storage server node. Such a storage server node may be connected with one or more other storage server nodes to form a highly scalable storage server cluster. 
       FIG. 4A  is a block diagram representation of buffer trees for files in a file system. Each file in the file system is assigned an inode.  FIG. 4A , for example, includes two files represented by inodes  402 - a  and  402 - b , stored in the file system. Each inode  402  references, for example using a pointer, Level 1 (L1) indirect blocks  404 . Each indirect block  404  stores at least one physical volume block number (PVBN)  410 . Each PVBN  410  references, for example using a pointer, a physical block  406  in a mass storage device  408 . As shown in  FIG. 4A , each PVBN  410  references a unique physical block  406 . For example, PVBN  410 - b  references physical block  406 - b . Note that there may be additional levels of indirect blocks (e.g., level 2, level 3) depending upon the size and layout of the file. 
       FIG. 4B  is a block diagram representation of buffer trees for files in a file system after a deduplication process has been performed on the file system. The files represented in  FIG. 4B  correspond to those from  FIG. 4A . The deduplication operation, as briefly described above, eliminates redundant data within the file system to make space for additional data to be stored. Essentially, deduplication removes duplicate blocks, storing only unique blocks in the file system, and creates a small amount of additional metadata in the process. This additional metadata is used by the storage manager  310  to locate the data in response to a request. In one embodiment, the metadata includes a hash value (e.g., based on SHA-256) or “fingerprint” value for every block within the file system. During the deduplication process the hash value for a data block is compared against other hash values of blocks stored in the file system and if a match is found (i.e., the blocks are identical) the redundant data is eliminated by sharing one of the physical blocks between two or more indirect blocks. 
     Consider the files of  FIG. 4A  represented by inodes  402 - a  and  402 - b  for example, if the data blocks  410 - c ,  410 - d , and  410 - f  are determined to be identical after a comparison of their hash values, one block can be shared after deduplication instead of maintaining three blocks on disk. This block sharing is represented in  FIG. 4B . In the example of  FIG. 4B , PVBN  410 - d  is designated as the donor and PVBNs  410 - c  and  410 - f  are designated as the recipients. The underlying physical blocks that are no longer referenced by the recipient PVBNs (i.e., blocks  406 - c  and  406 - f ) are released and can be used by the storage manager  310  to store additional data. 
     Deduplication operations often introduce fragmentation into a file system. As shown in  FIG. 4A , the storage manager  310  allocates blocks to files in a sequential manner (i.e.,  406 - a ,  406 - b ,  406 - c ) to improve the read performance of the storage system. When the blocks of a file are contiguous, the file can be accessed by a single read operation without requiring relatively time consuming seek operations to locate file blocks. This sequential layout of blocks is broken, as shown in  FIG. 4B , when deduplication operations share a block of data that is non-contiguous with the rest of the file. Because the inode  402 - a  references blocks that are fragmented (i.e., non-contiguous blocks  406 - a ,  406 - b , and  406 - d ), multiple read operations must be performed to access the entire file. 
     There is a point where the space saving benefits of deduplication are outweighed by performance considerations of the storage system, for example, when a file becomes so fragmented from deduplication that reading the file results in a noticeable delay in retrieving the file. This poor read performance is not acceptable for storage systems that must frequently access data (e.g., primary storage systems). Thus, in order to control the fragmentation caused by deduplication without having to disable deduplication completely, the storage manager  310  can determine the degree to which read performance will be degraded prior to performing the deduplication and decide whether to perform the deduplication operation based on this determination. 
     In one embodiment, a user of the storage system (e.g., a storage administrator) can select the level of fragmentation that is acceptable for performing deduplication operations. For example, if the storage system is being used for secondary storage and the user knows that read/write access to the storage system is going to be infrequent, the user can set the deduplication operations for maximum space savings and potentially sacrifice read performance due to fragmentation. However, if the storage system is being used for primary storage and the user knows that read/write access to the storage system is going to be frequent, the user can set the deduplication operations for maximum performance and deduplication will only be performed if there is little or no impact on the read performance of the storage system due to fragmentation. Likewise, there can be intermediate settings that balance storage savings and performance at different levels. 
       FIG. 5  is a flow diagram of a process for determining whether to perform a deduplication operation. The processes described herein are organized as a sequence of operations in the flowcharts. However, it should be understood that at least some of the operations associated with these processes potentially can be reordered, supplemented, or substituted for, while still performing the same overall technique. 
     The process  500  begins at step  502  where the storage manager  310  determines a level of fragmentation that would be introduced by performing a deduplication operation a file stored in the file system. The level of fragmentation is determined without having to actually perform the deduplication operation. The fragmentation level determined at step  502  is indicative of the read performance that can be expected if the deduplication operation were to be performed. Determining the level of fragmentation is described in more detail below with reference to  FIG. 6  and  FIG. 7 . 
     At step  504 , the level of fragmentation is compared to a threshold value by the storage manager  310 . As described above, the threshold value can be determined by the user of the storage system and can depend on the intended use of the storage system. For example, the threshold for a storage system that is used as primary storage will be relatively low compared with the threshold for a storage system used for secondary or backup storage. In one embodiment, a relatively lower threshold value indicates that the storage system user is more sensitive to performance delays that may be related to fragmentation introduced by deduplication. Depending on how the fragmentation level is calculated, in some embodiments a favorable comparison may be found if the fragmentation level is below a given threshold value. 
     In the example of  FIG. 5 , if the fragmentation level of the file is determined to be above the threshold value, the process continues to step  508  where the storage manager  310  determines that the deduplication operation should not be performed. However, if the fragmentation level of the file is not above the threshold value, the process continues to step  506  where the storage manager  310  determines that the deduplication operation should be performed. The deduplication process itself is not germane to this disclosure and any known and suitable method of deduplication can be employed. The process of  FIG. 5  is repeated for each file in the file system to determine whether to perform deduplication operations. The deduplication operations can be performed continuously or periodically. 
       FIG. 6  is a flow diagram of a process for determining a level of fragmentation that would be introduced by performing a deduplication operation on a file. The process  600  is one example embodiment of step  502  of  FIG. 5 . The process begins at step  602  where the storage manager  310  calculates the number of read operations that would be needed to access the file after the deduplication operation has been performed. This process is described in more detail below with reference to  FIG. 7 . 
     At step  604 , the storage manager  310  calculates the number of read operations that are needed to access an ideal file. An ideal file is one that is stored in contiguous physical data blocks and can be accessed with the lowest number of read operations. In one embodiment, the number of read operations that are needed to access the file can be determined by performing a read-ahead operation. The read-ahead operation fetches the blocks from the physical storage and stores the blocks into memory. The storage manager can use the number of read operations needed to perform the read-ahead operation as the number of read operations that are needed to access the file. 
     In one embodiment, the storage manager  310  allocates writes to the storage system in segments of up to 64 contiguous disk blocks, and read operations can be performed on sections of a disk up to 64 contiguous blocks. Thus, for an ideal file consisting of 256 contiguous disk blocks, for example, the storage manager  310  would perform four read operations to fetch the entire file. However, after fragmentation introduced by a deduplication operation, this same read-ahead may require a higher number of read operations. 
     At step  606 , the storage manager  310  calculates the fragmentation index of the file after the deduplication operation. The fragmentation index is an indication of the fragmentation level of the file after deduplication relative to the fragmentation of the ideal file. Using the example of the ideal 256 block file from above, if the read-ahead for the ideal file takes four read operations because the file is contiguous and the read-ahead takes 16 read operations after deduplication because fragmentation has been introduced, the fragmentation index of the file would be 4. This fragmentation index can be compared to a threshold value, for example in step  504  of  FIG. 5 , and used to determine whether to perform the deduplication operation. 
       FIG. 7  is a flow diagram of a process for calculating, prior to performing a deduplication operation, the number of read operations to access the file after deduplication. The process  700  is one example embodiment of step  602  of  FIG. 6 . At step  702 , the storage manager  310  locates the PVBN for all of the donor blocks to be used in the deduplication operation. As described above, the storage system maintains a data structure with hash values or fingerprints for each block in the file system. In determining which blocks in a file can be replaced with donor blocks (typically called block “sharing”), the storage manager  310  compares the hash values for each block of the file with the stored hash values of the other files in the file system. 
     After the possible donor blocks have been located, at step  704 , the storage manager  310  replaces each PVBN of the original file with the PVBN of its corresponding donor block in a list of PVBNs that represent the file, without actually associating the logical blocks of the file with the PVBNs. The storage manager then sorts the list of PVBNs, at step  706 , to represent the layout of the blocks on the disk. From the sorted PVBN list, at step  708 , the storage manager  310  can calculate the number of read operations needed to access the file after deduplication. In one embodiment, the storage manager  310  calculates the number of read operations by counting the groups of contiguous blocks in the PVBN list. For example, if there are sixteen groups of contiguous blocks, the storage manager  310  determines that sixteen read operations would be needed to access the file after deduplication. 
     The techniques introduced above can be implemented by programmable circuitry programmed or configured by software and/or firmware, or they can be implemented entirely by special-purpose “hardwired” circuitry, or in a combination of such forms. Such special-purpose circuitry (if any) can be in the form of, for example, one or more application-specific integrated circuits (ASICs), programmable logic devices (PLDs), field-programmable gate arrays (FPGAs), etc. 
     Software or firmware for use in implementing the techniques introduced here may be stored on a machine-readable storage medium and may be executed by one or more general-purpose or special-purpose programmable microprocessors. A “machine-readable medium”, as the term is used herein, includes any mechanism that can store information in a form accessible by a machine (a machine may be, for example, a computer, network device, cellular phone, personal digital assistant (PDA), manufacturing tool, any device with one or more processors, etc.). For example, a machine-accessible medium includes recordable/non-recordable media (e.g., read-only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; etc.), etc. 
     The term “logic”, as used herein, can include, for example, special-purpose hardwired circuitry, software and/or firmware in conjunction with programmable circuitry, or a combination thereof. 
     Although the present invention has been described with reference to specific exemplary embodiments, it will be recognized that the invention is not limited to the embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than a restrictive sense.