Abstract:
Integrating content into a storage system with substantially immediate access to that content. Providing high reliability and relatively easy operation with a storage system using redundant information for error correction. Having the storage system perform a “virtual write,” including substantially all steps associated with writing to the media to be integrated, except for the step of actually writing data to that media, including rewriting information relating to used disk blocks, and including rewriting any redundant information maintained by the storage system. Integrating the new physical media into the storage system, including accessing content already present on that media, free space already present on that media, and reading and writing that media. Recovering from errors during integration.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of copending application U.S. patent application Ser. No. 11/890,280, filed on Aug. 3, 2007, which is a continuation of U.S. Pat. No. 7,257,732, filed on Feb. 13, 2004. Each of the applications is hereby incorporated by reference as if fully set forth herein. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The invention relates to integrating content-laden media with a storage system, such as for example integrating a disk drive already including digital content with a RAID storage subsystem. 
     2. Related Art 
     Distribution of digital content representing media streams, such as for example movies, is subject to several problems. One problem is that digital content representing an individual movie, or a set of movies, can include a relatively large amount of data. In known systems, receiving that digital content involves at least two significant delays: (1) a first delay between when the digital content is requested and when it is actually received, and (2) a second delay between when the digital content is received and when it is actually available for use. The second delay poses a particular problem in that, while users might be used to waiting for delivery of a product, it is more annoying to have to wait, again, to use that product once it has been delivered and received. It would be advantageous to be able to distribute digital content, particularly digital content representing media streams, without substantial time delay in receiving that content, and especially without substantial time delay in being able to use that content once it is received. 
     One method of distributing digital content is to physically send a copy of that content on physical media, such as for example a DVD or magnetic disk. While this method generally achieves the goal of sending a relatively large amount of content in a relatively convenient package, it is subject to the drawback that it involves either (1) substantial complexity and possible awkwardness if that content is not integrated into the file system, or (2) integrating the content into a file system or other storage system when the content is to be used. For a first example, if that content is maintained on the original DVD or magnetic disk on which it was received, it would involve the storage system tracking what content is on what media, rather than advantageously leaving that task to a set of lower level operations for reading and writing physical media. For a second example, if that content is maintained off-line, the user might be involved in physically adding that content to what is accessible by the storage system. For a third example, even if that content is maintained on-line, it might still take substantial time to access it, such as for example when using a DVD carousel. 
     If that content is not integrated into the file system, as described in option 1 above, that content remains on its own separate physical media. Systems using this technique are subject to several drawbacks: (A) The storage system is not generally able to avail itself of redundant information techniques to protect that content against error or loss. (B) It would either involve permanently maintaining multiple namespaces for content, such as for example one for each such disk, or involve the storage system tracking what content is on what media, rather than advantageously leaving that task to a set of lower level operations for maintaining portions of the storage namespace on multiple physical media. (C) It might pose a problem if there were more such disks than the storage system was able to physically accommodate. For example, a storage system might involve significant extra effort if more than 12 disk drives or more than several hundred DVD&#39;s were used. 
     If that content is integrated into the file system, as described in option 2 above, one method of integrating that content is to copy the data from the physical media used to transport it to a different set of physical media used to maintain a file system. While this method is effective in providing the content for use with the file system, it is subject to several drawbacks: (A) It can take a relatively long time to copy the data. For example, a 350 Gigabyte disk might take several hours to copy using known disk drives. (B) Also, it might involve providing at least one empty disk drive slot from which to copy the data. 
     Accordingly, it would be advantageous to provide a technique for integrating content that is not subject to drawbacks of the known art. 
     SUMMARY OF THE INVENTION 
     The invention provides a method and system capable of integrating content into a storage system with substantially immediate access to that content. The invention also has the advantages of providing high reliability, and of providing relatively easy operation with a storage system using redundant information for error correction. 
     In one embodiment, the method includes having the storage system perform substantially all steps associated with writing to the media to be integrated, except for the step of actually writing data to that media, herein called a “virtual write” to that media. A virtual write can include rewriting information relating to used disk blocks, with the effect of designating the newly virtually-written block as in use, and rewriting any redundant information maintained by the storage system, with the effect of being able to recover the newly virtually-written block from that redundant information. 
     The method also includes integrating the new physical media into the storage system, with the effect that the storage system can perform its operations with regard to that physical media. For some examples, not intended to be limiting in any way, the storage system can access content already present on that media, the storage system can read and write that media without the possibility of creating storage system inconsistencies, and the file system can perform file system operations on that media, such as for example creating or deleting files, extending or truncating files, creating or deleting directories, adding or removing directory entries, allocating free space already present on that media, combinations or conjunctions of the above, and the like. 
     In one embodiment, the method includes recovering from errors during integration, with the effect that the storage system can concurrently access that media during integration, without the possibility of creating storage system inconsistencies. 
     In various embodiments, the invention contemplates using storage systems in which the redundant information includes parity information, either maintained on a designated parity disk, such as for example in a RAID-4 system, or distributed across more than one disk. For a first example, not intended to be limiting in any way, parity might be distributed across more than one original data disk  110  (with the effect that the original data disks  110  include parity information otherwise described herein to be on the parity disk  120 ), while one or more new disks  130  include only the new content included in disk blocks  131  for the new disks  130 . For a second example, parity might be written to one or more new disks  130  as the storage system  100  proceeds with its normal operation after integrating each new disk  130  into the storage system  100 . 
     The invention also contemplates using storage systems in which the redundant information includes more than one bit of error-correcting information. 
     After reading this application, those skilled in the art would recognize that the techniques described herein provide an enabling technology, with the effect that heretofore advantageous features can be provided that heretofore were substantially infeasible. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a block diagram of a storage system and a new disk to be integrated into the storage system. 
         FIG. 2  shows a process flow diagram of a method of integrating a new disk into a storage system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Preferred embodiments of the invention are described herein, including preferred device coupling, device functionality, and process steps. After reading this application, those skilled in the art would realize that embodiments of the invention might be implemented using a variety of other techniques not specifically described herein, without undue experimentation or further invention, and that such other techniques would be within the scope and spirit of the invention. 
     DEFINITIONS 
     The general meaning of each of these terms is intended to be illustrative and in no way limiting. 
     The phrase “media stream” describes information intended for presentation in a sequence, such as motion pictures including a sequence of frames or fields, or such as audio including a sequence of sounds. As used herein, the phrase “media stream” has a broader meaning than the standard meaning for “streaming media,” (of sound and pictures that are transmitted continuously using packets and that start to play before all of the content arrives). Rather, as described herein, there is no particular requirement that media streams must be delivered continuously. Also as described herein, media streams can refer to other information for presentation, such as for example animation or sound, as well as to still media, such as for example pictures or illustrations, and also to databases and other collections of information. 
     The phrase “digital content” describes data in a digital format, intended to represent media streams or other information for presentation to an end viewer. “Digital content” is distinguished from packaging information, such as for example message header information. For the two phrases “digital content” and “media stream,” the former describes a selected encoding of the latter, while the latter describes a result of presenting any encoding thereof. 
     The phrases “error correcting information,” “error detecting information,” “parity information,” and “redundant information” generally describe any information used in conjunction with data to detect cases where that data has been lost, in part or wholly, and cases where that data can be recovered, in part or wholly. “Digital content” is distinguished from redundant information, such as for example parity information. The former might be sent with only a CRC or secure hash to determine that the digital content has not been altered or lost, while the latter is generally for use in conjunction with the digital content to dynamically determine whether any of the digital content has been lost, and if so, to possibly recover it. For one example, not intended to be limiting in any way, when used in combination with the disk&#39;s own error detection techniques, parity is a specific example of an error correcting code. 
     The phrase “storage system” generally describes any system or object within a system (where an “object” is meant in a generic design sense) that includes a set of blocks on physical media. In most storage systems, storage blocks are each uniquely identifiable, with the effect that the storage system can specify which storage block is to be read or written. In preferred embodiments, there is more than one device providing the physical media, such as for example a set of (magnetic or optical) disk drives. 
     The phrase “extent list” generally describes an object within a storage system, each extent list being associated with a disk, that includes a description of which 
     blocks have been written to an individual disk, that is, are “in use” as described herein. In one embodiment, such as in storage systems provided by Kaleidescape, each disk has an associated extent list. In such embodiments, each disk also has an associated holding bin, capable of maintaining a second extent list, such as for example maintained when integrating that disk into the storage system. In such embodiments, consistency of the extent list with the actual disk blocks is maintained persistently across system errors and other failures. For one example, not intended to be limiting in any way, in systems where write operations are performed episodically, with a recovery technique to account for those write operations that only partially complete, the extent list can be maintained consistently and persistently. 
     The phrase “redundant storage system” generally describes any storage system in which redundant information is maintained in addition to actual data. In one embodiment, such as in storage systems provided by Kaleidescape, storage blocks might include actual data or parity information. Although in such embodiments, parity information is often maintained on only a single disk, this is not required in the context of the invention. In such embodiments, consistency of the redundant information with the actual data is maintained persistently across system errors and other failures. For one example, not intended to be limiting in any way, in systems where write operations are performed episodically, with a recovery technique to account for those write operations that only partially complete, the redundant information can be maintained consistently and persistently. 
     The phrase “file system” generally describes any system or object within a system (such as for example making use of the storage system) including a set of separately identifiable data units, called “files,” each possibly more than one data block in size. In one embodiment, such as in systems provided by Kaleidescape, the file system includes a hierarchical structure of directories and files, but this is not required in the context of the invention. In such embodiments, the file system provides operations for creating, deleting, reading from, and writing to, directories and files. In such embodiments, the file system also includes a set of free storage blocks, sometimes herein called a “free list,” which is also maintained consistently with the files and persistently across system errors and other failures. 
     The phrase “content disk” generally describes a single disk including one or more file systems having actual data maintained therein, although in a preferred embodiment, in practice there would be only one such file system. In alternative embodiments, there might a set of such disks which collectively include content to be added contained in a storage system that spans several disks, although in a preferred embodiment, in practice there would be only one such contact disk. 
     The scope and spirit of the invention is not limited to any of these definitions, or to specific examples mentioned therein, but is intended to include the most general concepts embodied by these and other terms. 
     System Elements 
       FIG. 1  shows a block diagram of a storage system and a new disk to be integrated into the storage system. 
     Storage System and Disks 
     A storage system  100  includes a set of data disks no (already integrated into the storage system  100 ), at least one parity disk  120  including redundant information from which data can be reconstructed after an error, and a new disk  130  capable of being integrated into the storage system  100 . 
     As further described below, each data disk  110  is logically divided into a set of disk blocks in, for each of which the storage system  100  is capable of reading and writing information. Similarly, the parity disk  120  (or each of them, if there is more than one) is logically divided into a set of parity disk blocks  121 , for each of which the storage system  100  is capable of reading and writing information. Similarly, the new disk  130  is logically divided into a set of new disk blocks  131 , for each of which the storage system  100  is capable of reading and writing information. 
     In alternative embodiments, the storage system  100  may include parity disk blocks  121  distributed across more than one parity disk  120 , and may include data disk blocks in distributed with those parity disk blocks  121  on both data disks  110  and parity disks  121 . In such alternative embodiments, data disks  110  and parity disks  120  might be collectively referred to as old disks  110 , and one or more of those old disks  110  might each include both data disk blocks in and parity disk blocks  121 . 
     For example, not intended to be limiting in any way, the old disks no might include a RAID-5 system including N old disks  110 , with the effect that each RAID stripe includes one parity disk block  121 , and with the effect that those parity disk blocks  121  are substantially equally distributed across the old disks no. One such example, where N=4, is shown in Table 1 below: 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 RAID-5 System with N = 4 Old Disks 
               
             
          
           
               
                 RAID Stripe 
                 Disk 0 
                 Disk 1 
                 Disk 2 
                 Disk 3 
               
               
                   
               
               
                 0 
                 B0 
                 B1 
                 B2 
                 Parity(B0:B2) 
               
               
                 1 
                 B3 
                 B4 
                 Parity(B3:B5) 
                 B5 
               
               
                 2 
                 B6 
                 Parity(B6:B8) 
                 B7 
                 B8 
               
               
                 3 
                 Parity(B9:B11) 
                 B9 
                 B10 
                 B11 
               
               
                 4 
                 B12 
                 B13 
                 B14 
                 Parity(B12:B14) 
               
               
                   
               
             
          
         
       
     
     In this table, Bn is the nth data block in in the RAID array and Parity(Bi:Bj) is the parity block  121  computed for data blocks Bi through Bj. As described above, since there are N=4 disks  110 , each 4th disk block in is a parity disk block  121 . 
     In such alternative embodiments, when a new disk  13   o  is added to the storage system  100 , that new disk  130  does not (initially) include any parity disk blocks  121 , but the parity disk blocks  121  on the old disks  110  are adjusted to account for the new disk blocks  131 . One such example, with N=4 and one new disk  130 , is shown in Table 2 below: 
     
       
         
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Quasi-RAID-5 System with N = 4 Old Disks and 1 New Disk 
               
             
          
           
               
                 RAID 
                   
                   
                   
                   
                 New 
               
               
                 stripe 
                 Disk 0 
                 Disk 1 
                 Disk 2 
                 Disk 3 
                 Disk 
               
               
                   
               
               
                 0 
                 B0 
                 B1 
                 B2 
                 Parity(B0: 
                 N0 
               
               
                   
                   
                   
                   
                 B2, N0) 
               
               
                 1 
                 B3 
                 B4 
                 Parity(B3: 
                 B5 
                 N1 
               
               
                   
                   
                   
                 B5, N1) 
               
               
                 2 
                 B6 
                 Parity(B6: 
                 B7 
                 B8 
                 N2 
               
               
                   
                   
                 B8, N2) 
               
               
                 3 
                 Parity(B9: 
                 B9 
                 B10 
                 B11 
                 N3 
               
               
                   
                 B11, N3) 
               
               
                 4 
                 B12 
                 B13 
                 B14 
                 Parity(B12: 
                 N4 
               
               
                   
                   
                   
                   
                 B14, N4) 
               
               
                   
               
             
          
         
       
     
     In this table, Nn is the nth new block  131  and Parity(Bi:Bj,Nk) is the parity block  121  computed for old blocks  111  Bi through Bj and new block  131  Nk. As described above, since there are N=4 disks  110 , each 4th old block  111  is a parity disk block  121  (on the old disks no). This is referred to herein as a “quasi-RAID-5 system.” It is not a true RAID-5 system because there is no parity on the new disk. 
     As also further described below, each data disk  110  is associated with an extent list  112 , describing which ones of the disk blocks  111  on that disk are “in use.” In this context, the phrase “in use,” used with regard to a particular disk block  111 , indicates that the particular disk block  111  has been written to since the data disk  100  was initialized (or otherwise made empty of data). In this context, a particular disk block  111  might be said to be “in use” even if that particular disk block  111  is not in fact allocated to a particular file (or other storage system structure, such as possibly a directory or inode). For example, this might occur if the block had been allocated to a file, then the file (and, in particular, that block) was written and then the file deleted. 
     The extent list  112  includes one or more data structures maintained in association with the disk  110 , indicating a distinction between those disk blocks in which are “in use” and those which are not “in use” on that disk  110 . This has the effect that use of the extent list  112  involves updating the extent list  112  as disk blocks  111  are written on that disk no, while preserving at least the following properties even in the event of a failstop, particularly a power failure while writing to a disk:
         The extent list  112  is substantially always readable.   The extent list  112  is substantially always internally consistent (that is, operations which update the extent list  112  appear substantially atomic to the storage system  100 ).   The extent list  112  is substantially always consistent with the set of disk blocks in it represents (that is, the extent list  112  is substantially always accurate regarding which disk blocks  111  are “in use” and which disk blocks are not “in use.”   The extent list  122  associated with the parity disk  120  is substantially always consistent with the extent lists  112  associated with the data disks  110  and the extent list  132  associated with the new disk  130 .       

     In one embodiment, the extent list  112  includes a set of sequences of disk blocks  111  which are in use on that disk, such as for example blocks “1-12, 15, and 17-25.” The parity disk  120  is associated with its own extent list  122 , similar in nature to the extent list  112 . The new disk  130  is also associated with its own extent list  132 , also similar in nature to the extent list  112 . 
     After reading this application, those skilled in the art would recognize that the extent list  112  is substantially equivalent to a block map of those disk blocks  111  in use on its associated data disk  110 , and is alternatively substantially equivalent to a compressed form of such a block map, such as for example a Liv-Zempel encoding of such a block map. In alternative embodiments, an uncompressed block map, or a block map compressed or otherwise represented using another technique, may be used in conjunction with or instead of the extent list  112 . However, in such alternative embodiments that the properties of the extent list  112  described above, it might occur that implementation of the extent list  112  involves more effort than in preferred embodiments. 
     Similarly, after reading this application, those skilled in the art would recognize that the extent list  122  is equivalent to a block map of those disk blocks  121  in use on the associated parity disk  120 , and is alternatively equivalent to a compressed form of such a block map, such as for one example, not intended to be limiting in any way, a Liv-Zempel encoding or other compression encoding of such a block map. In alternative embodiments, an uncompressed block map, or a block map compressed or otherwise represented using another technique, may be used in conjunction with or instead of the extent list  122 . 
     Similarly, after reading this application, those skilled in the art would recognize that the extent list  132  is equivalent to a block map of those disk blocks  131  in use on the new disk  130 , and is alternatively equivalent to a compressed form of such a block map, such as for example a Liv-Zempel encoding of such a block map. In alternative embodiments, an uncompressed block map, or a block map compressed or otherwise represented using another technique, may be used in conjunction with or instead of the extent list  132 . 
     Moreover, after reading this application, those skilled in the art would recognize that there is no particular requirement that the extent lists  112 , the extent list  122 , or the extent list  132 , involve the same or similar data structures (although in a preferred embodiment, all extent lists  132  involve substantially the same data structures). In alternative embodiments, these data structures might be completely different. 
     One feature of the data structures used for the extent lists  112 , the extent list  122 , and the extent list  132 , is that it might occur that the degree of compression from a uncompressed block map is not sufficient to fit one of those data structures into a relatively smaller allocated section of memory or mass storage. For example, a data structure for one of the extent lists  112  might represent disk blocks  111  numbered “2-3, 5, 7, 11, 13, 17, etc.,” and thus take up a substantial amount of space for that data structure. In one embodiment, the data structure for that one of the extent lists  112  might be modified to further compress it. For one example, not intended to be limiting in any way, that data structure might be modified (along with zeroing the data for those selected disk blocks) to indicate selected disk blocks  111  as being “in use” (as that phrase is described herein), with the effect of combining multiple extents listed in the data structure into a fewer number of extents, and with the effect of involving less memory or mass storage for recording or maintaining the extent list  112 . 
     In one embodiment, those disk blocks in added to the extent list  112  as being “in use” are written with substantially all zero values, with the effect that associated parity disk blocks  121  need not be substantially changed. 
     In one embodiment, the storage system  100  includes a file system  101 , preferably including a hierarchical structure of directories and files, with the effect that each file represented by the storage system  100  is associated with a directory, and each directory is associated with a parent directory, up to a root directory  102  for the hierarchical structure. The storage system  100  also includes a free list of blocks not in use by any directory or file in the file system  101 , the free list being designated by a free list pointer  103 . 
     In one embodiment, the new disk  130  includes a file system  133 , similar to the file system  101 , and preferably including a root directory  134 . The file system  133  also includes a free list of blocks not in use by any directory or file, the free list being designated by a free list pointer  135 . 
     In one embodiment, the storage system  100  includes an extent holding bin  104 , capable of maintaining a copy of the extent list  132  associated with the new disk  130 . This has the effect that both the extent holding bin  104  and a new extent list  132  for the new disk  130  are associated with the new disk  130 . 
     Alternative Storage Systems 
     After reading this application, those skilled in the art would recognize that the file system  101  need not be restricted to an explicit tree structure, but for one example, not intended to be limiting in any way, might include a lattice. For one example, not intended to be limiting in any way, if Unix-style links are included, the file system  101  would have the effect that a directory or file might be associated with more than one containing directory. 
     Similarly, after reading this application, those skilled in the art would recognize that the storage system  100  need not be restricted to an explicit file system  101 , but for one example, not intended to be limiting in any way, might include a database or database system (possibly including either a set of database objects or a set of relational tables), a set of lisp objects, or some other technique for maintaining data. 
     Similarly, after reading this application, those skilled in the art would recognize that the storage system  100  need not be restricted to a single technique for maintaining data, but for example, not intended to be limiting in any way, might include a plurality of distinct techniques, each appropriate to a subset of information to be maintained thereon (although in a preferred embodiment, the storage system  100  uses a substantially uniform set of data structures for each disk integrated therein). 
     After reading this application, those skilled in the art would recognize that the new disk  130  need not be restricted to a storage system, or to any other technique for maintaining data, that is the same as or similar to techniques included in the storage system  100 . In alternative embodiments, at least in theory, the new disk  130  might include techniques substantially different from the storage system  100 , or might include multiple heterogeneous techniques, only some of which are similar to techniques included in the storage system  100  (although in practice, a substantially uniform set of data structures would be preferred). 
     Method of Integrating New Disk 
       FIG. 2  shows a process flow diagram of a method of integrating a new disk into a storage system. 
     Although described serially, the flow points and method steps of the method  200  can be performed by separate elements in conjunction or in parallel, whether asynchronously or synchronously, in a pipelined manner, or otherwise. In the context of the invention, there is no particular requirement that the method must be performed in the same order in which this description lists flow points or method steps, except where explicitly so stated. 
     Integrating Parity Information 
     At a flow point  210 A, the storage system  100  is ready to generate parity information integrating the new disk  130  into the storage system  100 . 
     At a step  211 , the storage system  100  copies the extent list  132  for the new disk  130  into the extent holding bin  104  (located, in one embodiment, on the new disk  130  itself), and clears the extent list  132  for the new disk  130 . This has the effect that the extent list  132  and the extent holding bin  104  for the new disk  130  collectively describe the blocks that have been written (and which can therefore be read). 
     At a step  212 , the storage system  100  designates a disk block  131  associated with the new disk  130  for which a virtual write operation is to be performed. In one embodiment, the designated disk block  131  is selected from the list of disk blocks  131  identified by the extent holding bin  104  but which is not contained in the extent list  132 . 
     At a step  213 , the storage system  100  performs a virtual write operation for the designated disk block  131  on the new disk  130 . The virtual write operation includes the following sub-steps:
         At a sub-step  213 ( a ), the storage system  100  reads the parity block corresponding to the designated disk block.   At a sub-step  213 ( b ), the storage system  100  computes new parity information for the corresponding disk block  121  of the parity disk  120 . In one embodiment, the storage system  100  computes the new parity information using parity addition, that is, it computes (new parity)=(old parity) XOR (new disk data), where “XOR” is a logical exclusive- or function.   At a sub-step  213 ( c ), the storage system  100  marks the designated disk block  131  as in use by the storage system  100  in the extent list  132  for the new disk  130 .   At a sub-step  213 ( d ), the storage system  100  atomically writes both (A) the new parity information to the corresponding disk block  121  of the parity disk  120 , and (B) the new extent list  132 .       

     This step therefore has the same effect as if the actual data of the new disk block  131  was written to the new disk  130 , if the new disk  130  had been already included in the storage system  100 . 
     At a step  214 , the storage system  100  determines if the extent list  132  for the new disk  130  is equal to the extent holding bin  104 . If not, the method  200  proceeds with the step  212  (again), and the steps following. If so, the method  200  continues with the flow point  210 B. 
     At a flow point  210 B, the system  100  has generated parity information integrating the new disk  130  into the storage system  100 , and is ready to proceed. 
     Integrating File Systems 
     At a flow point  220 A, the system  100  is ready to integrate the file system  133  included on the new disk  130  with the file system  101  included in the storage system  100 . 
     In this flow sequence, the storage system  100  performs an atomic operation of unifying the file system  133  on the new disk  130  (including its root directory  134  and its free list pointer  135 ) with the file system  101  (including its root directory  102  and its free list pointer  103 ). In one embodiment, this atomic operation is “atomic” (that is, in the sense that there is no detectable state in which this atomic operation is only partially performed) both (a) with regard to other operations performed by the storage system  100  and (b) with regard to writing the indicated changes to any of the disks in the storage system  100 . To perform this atomic operation, the storage system  100  performs the following steps. 
     At a step  221 , the storage system  100  designates the beginning of an atomic operation that must not be interrupted. Performing atomic operations is known in the art of operating systems. For one example, not intended to be limiting in any way, designating the beginning of an atomic operation might include setting a lock. 
     At a step  222 , the storage system  100  creates a new directory entry in the root directory  102  of the file system  101 . The storage system  100  merges the file system  101  with the file system  133  by copying the pointer contained in the root directory  134  of the file system  133  on the new disk  130  into a directory of the file system  101 . This has the effect of merging the directory and file entries for the file system  133  into the file system  101 . 
     At a step  223 , the storage system  100  writes, at the end of the free list for the file system  101  (pointed to by the free list pointer  103 ), the free list for the file system  133 . This has the effect of merging the free list for the file system  133  into the file system  101 . 
     At a step  224 , the storage system  100  designates the end of the atomic operation. For one example, not intended to be limiting in any way, designating the end of an atomic operation might include releasing a lock. 
     This flow sequence therefore has the effect of merging the file system  133  from the new disk  130  into the file system  101  already in use by the storage system  100 . 
     At a flow point  220 B, the system  100  has integrated the file system  133  included on the new disk  130  with the file system  101  included in the storage system  100 , and is ready to proceed. 
     Error Recovery 
     In one embodiment, the method  200  includes recovering from errors during 
     1. Reading Ordinary Disks 
     At a flow point  230 A, the storage system  100  encounters an error in reading one of the ordinary disks no. An error in reading one of the ordinary disks  110  implies that it was not possible to recover that data using ordinary RAID operations. 
     In one embodiment, the storage system  100  is more aggressive than would otherwise occur with known RAID systems performing read operations from the disk. In known disk drive systems, the disk itself, as part of a RAID system, would re-attempt the read operation several times, each time waiting for a failure timeout, and then itself attempt to recover the data using ordinary RAID operations. In one embodiment, the storage system  100  recognizes the original error in the read operation, and proceeds with its own attempt to recover the data. The inventors have found this to be quicker and more efficient than known disk drive systems. 
     At a step  231 , the storage system  100  recognizes an error in attempting to read an ordinary disk no, while in the process of integrating the new disk  130  (that is, during the steps between the flow point  210 A and the flow point  210 B, or during the steps between the flow point  220 A and the flow point  220 B). 
     At a step  232 , the storage system  100  determines if the error involved one of the disks in a RAID stripe for which the storage system  100  has virtually written a corresponding disk block  131  on the new disk  130 . If not, that is, the error involved a disk in a RAID stripe for which the storage system  100  has not yet virtually written a corresponding disk block  131  to the new disk  130 , the method  200  proceeds with the next step. If so, that is, the error involved a RAID stripe for which the storage system  100  has already virtually written the corresponding disk block  131  to the new disk  130 , the method  200  proceeds with the step  234 . 
     At a step  233  (that is, at which the error involved a RAID stripe for which the storage system  100  has not yet virtually written a corresponding disk block  111  to the new disk  130 ), the storage system  100  uses known RAID techniques, with the effect of performing error recovery, but excludes the new disk  130 . In one embodiment, this step involves the following sub-steps:
         At a sub-step  233 ( a ), the storage system  100  reads data from the rest of the ordinary disks  110  and from the parity disk  120 . Where one or more of those rest of the ordinary disks  110  are shown by their extent lists  112  to involve a disk block  111  not yet written to, those individual ordinary disks  110  are ignored. (This description is simplified to not take into account the possibility that blocks might be cached in RAM or some other faster storage medium than disk. After reading this application, it would be clear to those skilled in the art that such alternatives are workable, are within the scope and spirit of the invention, and would not involve undue experimentation or further invention.)   At a sub-step  233 ( b ), the storage system  100  computes the correct data for the corresponding disk block in of the ordinary disk no for which the read operation failed. In one embodiment, not intended to be limiting in any way, the storage system  100  performs a bitwise logical XOR operation on the data read in the previous sub-step, with the effect of generating information for a disk block  111  to be written to the ordinary disk no for which the read operation failed.   At a sub-step  233 ( c ), the storage system  100  writes the computed correct data to the corresponding disk block in of the ordinary disk no for which the read operation failed.       

     This step has the effect that the correct data, for the corresponding disk block in of the ordinary disk no for which the read operation failed, has been recovered from other information in the storage system  100 . The error having been recovered from, the method  200  proceeds with the flow point  230 B. 
     At a step  234  (that is, at which the error involved a RAID stripe for which the storage system  10   o  has already virtually written a corresponding disk block  111  to the new disk  130 ), the storage system  100  uses known RAID techniques, with the effect of performing error recovery, but includes the new disk  130 . In one embodiment, this step involves the following sub-steps:
         At a sub-step  234 ( a ), the storage system  100  reads data from the rest of the ordinary disks  110 , from the parity disk  120 , and from the new disk  130  at the disk block  111  in the corresponding stripe. Where one or more of those rest of the ordinary disks  110  are shown by their extent lists  112  to involve a disk block  111  not yet written to, those individual ordinary disks  110  are ignored.   At a sub-step  234 ( b ), the storage system  100  computes the correct data for the corresponding disk block in of the ordinary disk no for which the read operation failed. In one embodiment, not intended to be limiting in any way, the storage system  100  performs a bitwise logical XOR operation on the data read in the previous sub-step, with the effect of generating information for a disk block  111  to be written to the ordinary disk  110  for which the read operation failed.   At a sub-step  234 ( c ), the storage system  100  writes the computed correct data to the corresponding disk block in of the ordinary disk no for which the read operation failed.       

     This step has the effect that the correct data, for the corresponding disk block  111  of the ordinary disk  110  for which the read operation failed, has been recovered from other information in the storage system  100 . The error having been recovered from, the method  200  proceeds with the flow point  230 B. 
     At a flow point  230 B, the storage system  100  has encountered and handled an error in reading or writing one of the ordinary disks  110 , and is ready to proceed. 
     2. Reading Parity Disk(s) 
     At a flow point  240 A, the storage system  100  encounters an error in reading the parity disk  120 . 
     At a step  241 , the storage system  100  recognizes an error in attempting to read the parity disk  120 , while in the process of integrating the new disk  130  (that is, during the steps between the flow point  210 A and the flow point  210 B, or during the steps between the flow point  220 A and the flow point  220 B). 
     At a step  242 , the storage system  100  uses known RAID techniques, with the effect of performing error recovery for the parity disk  120 . In one embodiment, this step involves the following sub-steps:
         At a sub-step  242 ( a ), the storage system  100  reads data from the rest of the ordinary disks  110  in each corresponding stripe. Where one or more of those rest of the ordinary disks  110  are shown by their extent lists  112  to involve a disk block  111  not yet written to, those individual ordinary disks no are ignored.   At a sub-step  242 ( b ), the storage system  100  computes the correct data for the corresponding disk block  111  of the parity disk  120 . In one embodiment, not intended to be limiting in any way, the storage system  100  performs a bitwise logical XOR operation on the data read in the previous sub-step, with the effect of generating information for a disk block  111  to be written to the parity disk  120 .   At a sub-step  242 ( c ), the storage system  100  writes the computed correct data to the corresponding disk block in of the parity disk  120 .       

     At a flow point  240 B, the storage system  100  has encountered and handled an error in reading the parity disk  120 , and is ready to proceed. In one embodiment, the method  200  proceeds at the flow point  210 A, with the effect of continuing the operation of integrating the new disk  130 . 
     3. Reading New Disk(s) 
     At a flow point  250 A, the storage system  100  encounters an error in reading the new disk  130 . 
     At a step  251 , the storage system  100  recognizes an error in attempting to read the new disk  130 , while in the process of integrating the new disk  130  (that is, during the steps between the flow point  210 A and the flow point  210 B, or during the steps between the flow point  220 A and the flow point  220 B). 
     At a step  252 , similar to the step  243 , the storage system  100  uses known RAID techniques, with the effect of recovering from the error detected when reading from the new disk  130 . If the block on the new disk whose read failed has been integrated into parity, it is be recomputed as described in this step. If the block on the new disk whose read failed has not yet been integrated into parity, a read error is returned. In one embodiment, this step involves the following sub-steps:
         At a sub-step  252 ( a ), the storage system  100  reads data from the rest of the ordinary disks  110  in each corresponding stripe. Where one or more of those rest of the ordinary disks  110  are shown by their extent lists  112  to involve a disk block  111  not yet written to, those individual ordinary disks no are ignored.   At a sub-step  252 ( b ), the storage system  100  computes the correct data for the corresponding disk block in of the new disk  13   o . In one embodiment, not intended to be limiting in any way, the storage system  100  performs a bitwise logical XOR operation on the data read in the previous sub-step, with the effect of generating information for a disk block  111  to be written to the new disk  130 .   At a sub-step  252 ( c ), the storage system  100  writes the computed correct data to the corresponding disk block in of the new disk  130 .       

     At a flow point  250 B, the storage system  100  has encountered and handled an error in reading the new disk  130 , and is ready to proceed. In one embodiment, the method  200  proceeds at the flow point  210 A, with the effect of continuing the operation of integrating the new disk  130 . 
     4. Writing to Disks 
     At a flow point  260 A, the storage system  100  encounters an error in writing one of the disks (either an ordinary disk  110  or parity disk  120 ). 
     In one embodiment, read errors are treated differently from write errors. When a read error occurs, the block being read can be reconstructed, as described above. When a write error occurs, however, there is not a convenient and general way to tell what was actually written to the disk. 
     At a step  261 , the storage system  100  retries the write operation several times before reporting an error. In one embodiment, this step is performed by a set of disk driver code associated with writing to the disk. 
     At a step  262 , the storage system  100  remaps the sector of the disk where the write error occurred to another sector. This step includes the following sub-steps:
         At a sub-step  262 ( a ), the sector where the write error occurred is marked as bad, for example, by the disk firmware.   At a sub-step  262 ( b ), the sector where the write error occurred is remapped to another sector, for example, by the disk firmware.   At a sub-step  262 ( c ) the write operation is retried from the beginning, for example by the disk driver software. If successful, the method  200  proceeds at the flow point  260 A. If unsuccessful, the method  200  determines there is something seriously wrong with the disk, and continues with the next step.       

     At a step  263 , the storage system  100  marks the entire disk where the write error occurred as bad, and removes that disk from its pool of usable disks. As part of this step, the storage system  100  informs an operator or user of this result. 
     The method  200  then continues at the flow point  210 A. 
     At a flow point  260 B, the method  200  has handled an error in writing one of the disks (either an ordinary disk  110  or parity disk  120 ), and is ready to proceed. 
     Alternative Embodiments 
     Although preferred embodiments are disclosed herein, many variations are possible which remain within the concept, scope, and spirit of the invention. These variations would become clear to those skilled in the art after perusal of this application. 
     The invention is not restricted to digital content for media streams, but is generally applicable to storage systems of any kind. 
     The invention is not restricted to storage systems in which the redundant information includes only parity information, but also contemplates storage systems in which the redundant information other than parity information, or in which the redundant information includes more than one bit of error-correcting information. 
     In one embodiment, the storage system  100  writes data blocks into storage blocks on disk episodically, with each write episode having two stages. In a first stage, the storage system  100  writes data blocks into storage blocks in a separate region of the disk (herein sometimes called a “committal region”), concurrently for all disks, both data disks  110  and parity disks  120 . The storage system  100  treats the collective act of writing to the committal regions of all disks as an atomic operation, in the sense that synchronization is used to assure that the write operation either completely succeeds or is considered to have failed, with the effect that if a system error or other failure occurs during the first stage, the storage system  100  can roll back its status to just before the first stage. In a second stage, the storage system  100  writes blocks to their respective target locations on disk. Since the write operation to the committal region is atomic, the storage system  100  can safely repeat this second stage if a system error or other failure occurs during the second stage. 
     The invention is not restricted to storage systems in which the redundant information is maintained on only a single designated parity disk, such as for example in a RAID-4 system, but also includes storage systems in which the redundant information is distributed across more than one disk, such as for example in a RAID-5 system. 
     The invention is not restricted to unitary systems in which the redundant information is maintained under the control of a single storage system, such as for example in a RAID system, but also includes distributed systems in which the redundant information is maintained by a plurality of substantially independent devices, such as for example a multi-device system including independent actors and cross-checking of persistent storable objects. For one example, not intended to be limiting in any way, if data can be transmitted among the plurality of substantially independent devices substantially as rapidly as it can be written to conventional disks, and if the probability of failure of one of the devices or of its being partitioned from the rest is about the same as the probability of a single disk failure, the invention might be applicable to such systems. 
     After reading this application, those skilled in the art will recognize that these alternative embodiments and variations are illustrative and are intended to be in no way limiting. 
     After reading this application, those skilled in the art would recognize that the techniques described herein provide an enabling technology, with the effect that heretofore advantageous features can be provided that heretofore were substantially infeasible.