Abstract:
When recording packetized real-time streaming data, e.g. multimedia data, it is in general not possible to know in advance the size of the data stream, and thus the required storage area. Therefore the storage device may be full before the data stream is completely stored. The disclosed method for storing and retrieving the remaining part of the data stream on another storage device uses metadata tags ( 10,11 ) and data buffers (IB 1, IB 2 ) to split a data stream seamlessly into chunks while recording it, and distribute the chunks in real-time to different connected storage devices (D 1, D 2 ), so that the chunks can be seamlessly concatenated again in real-time for replaying the stream. The metadata tags ( 10,11 ) contain identifiers for the successive storage node (N 2 ) and/or the preceding storage node (N 1 ) and for the last stored application packet.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to a method for distributed storage and retrieval of data streams. In particular it relates to seamless real-time splitting and concatenating of data streams, e.g. multimedia data streams.  
       BACKGROUND  
       [0002]     Multimedia home networks are often used to process data streams containing audio, video and other data. This includes recording and replaying large amounts of data. Usually a data stream that contains a unit, like a video film or an audio track, is handled as a single file, and stored as such. Internally, these streams are often structured into packets, like e.g. defined for Digital Video (DV), or in the MPEG-2 standard. Depending on the standard, these packets may have different lengths, e.g. 188 bytes for MPEG-2.  
         [0003]     While data amounts as well as available storage capacity increase drastically, organizing the storage area is an annoying task for the user. Various automation approaches exist, like e.g. storage-area networks that are used to separate storage devices from other devices, thus offering storage capacity to all connected data generating or data consuming devices. Such techniques are called distributed storage, since they may automatically distribute a file that shall be stored to any of the connected storage devices, and find and retrieve it later. In such systems the user usually does not need to know in detail where certain content is stored. To be able to handle various types of application packets, distributed storage systems for multimedia networks often put the application packets into packet containers for storage, like e.g. defined for the DVD Stream Recording standard “DVD Specifications for Rewritable/Re-recordable Discs, Part 5, Stream Recording, May 2000”. To enable proper real-time playback, timestamps (ATS) are assigned to the application packets.  
         [0004]     Distributed storage methods may also be applied for home networks, which tend to become convenient automatic storekeepers for various kinds of data. The system may look without any user intervention autonomously for a storage device that may provide enough storage capacity for recording. For the application, e.g. video recording, this behavior of the storage system is usually transparent.  
         [0005]     However, when real-time streaming data shall be recorded, it is in general not possible to know in advance the size of the data stream, and thus the required storage area. Therefore storage area management is difficult, so that recording of real-time data is usually terminated when the selected storage device is full.  
         [0006]     The architecture of distributed storage systems, like networks in general, can be based on either client-server or peer-to-peer (P2P) principles. P2P networks require no central network organization, since the nodes, or peers, usually communicate directly with each other. Services and resources, e.g. storage area, may be shared, thus offering numerous enhanced functions to the user.  
       SUMMARY OF THE INVENTION  
       [0007]     A distributed storage system for multimedia home networks should be able to support real-time recording of streaming data. Since due to the nature of real-time recording the length of a recorded file, or data stream, is not known at recording time, there is the risk of the selected storage device being full before the data stream is completely stored. In that case the remaining part of the data stream may be stored on another storage device. Anyway, selecting and activating the other storage device takes time, which hampers real-time behavior.  
         [0008]     The problem to be solved by the invention is to split a data stream seamlessly into chunks while recording it, and distribute the chunks in real-time to different connected storage devices, so that the chunks can be seamlessly concatenated again in real-time for replaying the stream.  
         [0009]     A method to solve this problem is disclosed in claim  1 . An apparatus that utilizes the method for storing is disclosed in claim  9 . A method for retrieving the portions of a data stream from different storage media is disclosed in claim  2 . An apparatus that utilizes the method for data retrieval is disclosed in claim  10 .  
         [0010]     According to the invention, metadata tags and temporary buffers are used to enable seamless real-time splitting and concatenating of data in a distributed storage environment. When the maximum capacity of a storage device is reached while a data stream is being recorded on it, the inventive method makes it possible to split the stream and store the remainder on another storage device. The storage capacity that is available on the recording storage node and on the other storage nodes of a network may be monitored for this purpose.  
         [0011]     While a data stream is recorded on a first storage device, and the available storage capacity on this device comes to a limit, another storage device in the network is selected that is suitable for continuing the recording later on, after the currently active storage device is full. When the remaining free storage area of the first device is below a threshold, the other storage device gets a notification and starts buffering the streaming data in a temporary buffer, in parallel to the first storage device. When the first storage device is full, i.e. it may store no more packets, it sends to the second device a notification containing an identifier that defines the last packet stored by the first device. Upon reception of this notification, the second device identifies the first packet that it has to store, and transfers this start packet and the successive data packets from its temporary buffer to its storage area. Thus it can continue recording the data stream.  
         [0012]     Advantageously the method prevents data loss, and thus prevents noticeable interruption at playback of the stream.  
         [0013]     The process of selecting another storage device to continue the recording may be based on any parameters, like e.g. free storage capacity, possible recording speed or type of content.  
         [0014]     Similarly, for replaying the complete data stream the different chunks are concatenated. The first chunk, containing the beginning of the stream, is marked as such by a metadata tag. This mark may be explicit or implicit by specifying no predecessor. The metadata tag also contains an identifier, e.g. a timestamp value or UUID, of the last application packet contained in the related data chunk. It may also contain an identifier of the storage device that holds the next chunk, and an identifier of the next or the previous chunk or both, e.g. PlaycellUUID. While the first chunk is read from the storage device and buffered for replaying, its metadata tag is evaluated and a message is sent at least to the next storage device, identifying the last packet of the first chunk, and identifying the next chunk. After the second storage device detected that it has the required chunk, e.g. with the PlaycellUUID, it monitors the data packets replayed by the first device on the common data connection, and when it detects the last packet, it starts replaying the packets of the second chunk. Depending on the length of the single data chunks, the second device may receive the message early enough to read the first packets of its data chunk in advance and buffer them in a temporary buffer, so that replaying can start without delay created by data retrieval from the second storage medium. Alternatively, the message sent to the second device may contain the identifier of the currently replayed chunk, e.g. its PlaycellUUID, and the metadata tag of the second chunk stored on the second device contains the identifier of its preceding chunk. When the identifiers are equal, the second chunk is the next chunk to be replayed, after the last packet of the first chunk, detected by its ATS.  
         [0015]     Thus, a real-time data stream may be broken into data chunks that may be stored on different devices, and that can be concatenated seamlessly for playback. To handle such concatenated streams in a distributed storage system, the inventive method comprises describing the complete sequence of data chunks by metadata descriptors that are generated automatically by the decentralized distributed system and then associated with the data chunks as navigation data, thus generating a linked list of data chunks. According to the invention, the metadata descriptors contain for each chunk at least all the information that is required for reconstruction of the original stream, e.g. for replaying the data. The metadata descriptors contain at least the following information: an identifier for the data stream to which the data chunk belongs, an identifier for the preceding data chunk if one exists, an identifier for the successive data chunk if one exists, and an identifier for the last packet. Further, it may appear useful to store the following information in the metadata descriptors: identifier for the storage device that is expected to hold the preceding/successive chunk, date and time of recording, title of data stream, and timestamps of the first and/or the last packet, e.g. ATS. These metadata descriptors may contain hierarchy, e.g. when the chunk related metadata descriptors refer to another, stream related metadata descriptor.  
         [0016]     Advantageous embodiments of the invention are disclosed in the dependent claims, the following description and the figures. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     Exemplary embodiments of the invention are described with reference to the accompanying drawings, which show in  
         [0018]      FIG. 1  distributed stream objects being linked by metadata descriptors;  
         [0019]      FIG. 2 a  distributed storage system model in the case of recording;  
         [0020]      FIG. 3 a  messaging and buffering scenario when seamlessly switching from a storage device to another while recording;  
         [0021]      FIG. 4 a  distributed storage system model in the case of playback;  
         [0022]      FIG. 5 a  messaging and buffering scenario when seamlessly switching from a storage device to another during playback;  
         [0023]      FIG. 6 a  distributed storage system model in the case of playback, routing all chunks via one node; and  
         [0024]      FIG. 7 a  distributed storage system model in the case of playback, with an input buffer at a non-storage node. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     The following description of exemplary embodiments of the inventive method is based on the DVD Stream Recording standard, but the method can as well be used for data according to any other standard for stream recording.  FIG. 1  shows an exemplary embodiment of the invention, being a distributed storage system that stores distributed stream objects linked by metadata descriptors. In this case, two nodes N 1 ,N 2  are involved in the storage of a data stream that is composed of multiple units SOBU# 1 , . . . , SOBU#j, and that is split into two data chunks  18 , 19 . The DVD Stream  
         [0026]     Recording standard defines Stream Objects (SOB) consisting of Stream Object Units (SOBU) of 64 kB length, each as a container for variable length application packets. In  FIG. 1 , a first storage device N 1  records a stream  18  containing SOBUs SOBU# 1 , . . . , SOBU#i- 1 , as long as it has storage area available. While recording, it also generates and stores an information file  12  referring to the stream object  18  and containing the playlists and playcells labeled with unique labels PlaycellUUID 1 , PlaycellUUID 2 , and other navigation information  14 , e.g. for the program. The information file  12  also contains a Mapping List (MAPL)  16  for navigation purposes. In the MAPL, the arrival time intervals for each SOBU are captured. It is used to convert these times into the address of the corresponding SOBU, which contains the application packet with the specified packet arrival time. Playlists contain start and end time of one or more playcells. Further, a metadata descriptor  10  is generated and attached to the information file  12  or the stream object  18  respectively. Initially, the metadata descriptor  10  contains the playcell label PlaycellUUID 1 .  
         [0027]     While the first storage device N 1  is recording, other connected storage devices may be monitored for available storage capacity. Alternatively, e.g. a broadcast message may be sent to all connected storage devices, requesting storage area. One of these storage devices N 2  is selected as potential successor of the currently active device N 1 . Then, if the storage capacity limit of the first storage device N 1  is reached while the stream continues, the other storage device N 2  is activated to continue recording of the stream. This device N 2  gets a notification about the ATS of the last application packet that was stored in the first storage device N 1 , being contained in the last stored unit SOBU#i- 1 . The second storage device N 2  continues storage of the stream in another stream object  19 , being composed of the units SOBU#i, . . . , SOBU#j following the last unit SOBU#i- 1  stored by the first device N 1 , wherein this stream object  19  contains the subsequent application packets. The second storage device generates for this stream object  19  an information file  13  containing a MAPL  17  and identifiers  15 , and a metadata descriptor  11 . The metadata descriptor  11  contains not only the playcell labels PlaycellUUID 1 , PlaycellUUID 2  but also a pointer to the metadata descriptor  10  of the other storage device N 1  storing the preceding data chunk, i.e. stream object  18 . Likewise, the metadata descriptor  10  in the first storage device N 1  is updated such that it contains a pointer to the second storage device N 2 .  
         [0028]     In one embodiment of the invention the metadata descriptors  10 , 11  may also contain detailed information about where the preceding and/or successive SOB is stored, e.g. address, file name or path name.  
         [0029]     One requirement for the distributed storage system is the automatic self-organizing of file splitting when the storage capacity limit of a node is reached. In P2P networks, a self-controlled exchange of messages between the nodes is used for this task.  FIG. 2  depicts a simplified distributed storage system model with two storage nodes N 1 ,N 2  and a separate node N 3  for the input service IS, in the case of recording. Exemplarily, the nodes are assumed to be peers in a P2P network, so that the bus connections between nodes mentioned in the following are virtual connections, using the same physical connection. Data are received e.g. via a radio frequency receiver RFR. An input service IS, e.g. a decoder, in the receiver node N 3  extracts the application packets, captures timestamps and distributes the packets on an application packet bus APB to the storage nodes N 1 ,N 2 . The storage nodes N 1 ,N 2  are equipped with interfaces to the application packet bus APB, input buffers IB 1 ,IB 2  and stream recording units SRU 1 ,SRU 2  that generate navigation information of the streaming data, and pack the application packets into streaming packets according to the utilized stream recording standard, e.g. SOBUs for the DVD Stream Recording standard. The stream recording units SRU 1 ,SRU 2  in each storage node may access storage media D 1 ,D 2 , e.g. optical discs or hard-disks, where they may store the streaming packets. The nodes N 1  . . . N 3  have message controllers MC 1  . . . MC 3  that are connected to a common control message bus CMB, so that the nodes can exchange control messages and thus organize recording and file splitting if necessary.  
         [0030]     According to the DVD Stream Recording standard, time information has to be added to every application packet to enable proper real-time playback of stored transport packets. This capturing of application timestamps (ATS) can be performed e.g. by the input service IS that receives and decodes a transport stream from the receiver RFR. The input buffers IB 1 ,IB 2  of the storage nodes N 1 ,N 2  are used to bridge the timeframe for switching from one node to another when file splitting becomes necessary. Though not depicted in  FIG. 2 , it may also be advantageous to use any transparent network protocol for the application packet bus APB, adding error correction data or the like.  
         [0031]     Further, control messages between the two storage nodes N 1 ,N 2  are exchanged, containing the identifiers, or UUIDs, of the generated playcells. Those UUIDs are used to describe the splitting of the stream, and are written into the metadata descriptor of the stream object at the related nodes. The metadata descriptor is in this example stored on the same medium as the stream object. If the DVD Stream Recording standard mentioned above is used, the Application Private Data Field can be used as container for the metadata descriptors.  
         [0032]     A buffer scenario shown in  FIG. 3  explains the switching from one node to another in the case of recording a stream. In this exemplary scenario the buffering of a data stream in an input buffer has been initially started at a first node Node_A, at a time t 0 . The input buffer filling V in -V Out  increases until at time t 1  the recording of buffered data on a mass storage medium starts. Emptying the buffer and recording the buffered data is usually faster than receiving and buffering new data, so that the buffer filling will oscillate around an average value, but for this simplified figure it is assumed that the buffer filling remains constant. The storage node Node_A monitors its own remaining free storage capacity. When it notices at time t 2  that its capacity will be reached soon, the node Node_A sends a message with a “call for free capacity” to some or all other connected nodes. A second node Node_B is chosen as a successor. In P2P networks this may be done in a negotiation process. In other cases another unit, e.g. a main server, may select a successor node. When the second node Node_B is selected and notified at time t 3 , it starts to buffer the input stream in its input buffer, as a stand-by node in parallel to the active storage node Node_A. Since the buffered data in the stand-by node Node_B are not yet being recorded, the buffer filling increases. When the buffer is almost full at time t 4 , before a “full” message from the active node Node_A is received, the oldest buffered data may be overwritten, like e.g. in a ring buffer. During this time the buffer filling is constant since the buffer remains full, or almost full.  
         [0033]     When the first node Node_A has no free storage capacity left, it stops recording and sends a “full” message to the stand-by node Node_B at time t 5 . This message includes the application timestamp (ATS) value of the last recorded application packet. After some time, at the time t 6 , the stand-by node Node_B has received this message and finished its storage configuration. From now on it is considered the active node. It evaluates the ATS included in the message and starts recording buffered application packets that have a higher ATS than the ATS contained in the message. Other application packets are dropped.  
         [0034]     The recording process includes reading the application packets from the input buffer, packing them in streaming packets according to the employed streaming standard and writing the streaming packets to the storage medium, e.g. disk. Thus, the buffer filling decreases until at a time t 7  it has reached an average level, as described above. This level may e.g. be programmed or hardware-defined. The size of the input buffer B max  has to be sufficient for bridging the time between the first node Node_A sending the “full” message at t 5  and the second node Node_B beginning the recording at t 6 . This time includes also the mentioned messaging and the preparation and configuration of the recording process at the second node Node_B. The buffer must be able to hold all application packets received during this timeframe.  
         [0035]     For continuous playback of the streaming data stored in a distributed storage system, output buffers are necessary to bridge the time slot when switching between nodes.  FIG. 4  demonstrates a simplified distributed storage system model in the playback case, with two storage nodes N 1 ,N 2  and one node N 3  for the display service DS, e.g. a monitor. The storage nodes N 1 ,N 2  are equipped with interfaces for the application packet bus APB, output buffers OB 1 ,OB 2 , buffer control units BCU 1 ,BCU 2  and playback units PU 1 ,PU 2  being connected to storage media D 1 ,D 2 . The playback units PU 1 ,PU 2  convert the streaming packets that are read from the storage media D 1 ,D 2  back to application packets. Furthermore, all nodes contain message controllers MC 1  . . . MC 3  that may exchange control messages on a control message bus CMB. The output buffers OB 1 ,OB 2  have two functions: first to continuously provide application packet data while switching between nodes, and second to match the data rate supplied by the playback units PU 1 ,PU 2  with the required application packet data rate. If e.g. data read from a storage medium D 1  on a first node N 1  are played back, then a metadata tag associated with the respective playlist may indicate that the stream is not completely stored on this medium D 1 , but a successive chunk of the same stream is stored on another node N 2 . The metadata tag also includes the ATS of the last stored application packet, and an identifier of the other playcell. It may also include more detailed information about where the data are stored.  
         [0036]     While the first node N 1  plays back its data chunk on a common connection, e.g. application packet bus APB, the message controller MC 1  sends a message to the other nodes message controller MC 2 , requesting preparation for playback of the respective successive data stream. Upon reception of this message, the other node N 2  may start reading the data from its storage medium D 2  and buffer the application packets in its output buffer OB 2  until the message controller MC 2  receives an “empty” message from the active node N 1 . When the active playback unit PU 1  has written its last application packet to its output buffer OB 1 , the active node N 1  sends an “empty” message to the other node N 2 . Upon reception of this message, the other node N 2  becomes the active node. Then the second node N 2  monitors the ATSs of the application packets on the bus APB, using its buffer control unit BCU 2 . When the last packet is detected, the buffer control unit BCU 2  enables the output buffer OB 2  to output its contents on the bus APB, and also enables the playback unit PU 2  to read more data from the storage medium D 2  into the output buffer OB 2 .  
         [0037]     A buffer scenario shown in  FIG. 5  explains for distributed stream recording the switching from one node to another, in case of playback operation.  
         [0038]     The playback of a stream has been initially started at a first node Node_A at time t 0 . Depending on the used buffer filling strategy, a fast disc reading process can lead to a bursty data behavior, indicated as variable buffer filling, before the buffer reaches a quasi-stable state at time t 1 . Due to the mentioned data bursts the buffer filling always varies around this quasi-stable state, as mentioned above. From the metadata descriptor that characterizes the stream splitting, the first node Node_A knows, or may find out by a request, which other node Node_B is the successor node that holds the next part of the streaming data. At time t 2  the last packet was read and buffered at the active node Node_A, and it sends a “call for the successor node” message to the other node Node_B, including the timestamp value of the last application packet. The successor node Node_B receives the message, prepares the playback process and starts to buffer application packets at time t 3 . The filling of its output buffer OB 2  increases from now on. To achieve seamless playback when switching between nodes, the successor node Node_B reads from the application packet bus APB the application packets sent by the active node Node_A and evaluates the timestamps. The buffer control unit BCU 2  compares these ATSs with the ATS from the “call for successor node” message. When these timestamps are equal, meaning that the output buffer OB 1  of the first node Node_A is empty at time t 4 , the output buffer OB 2  starts to transmit its packets to the bus APB after the end of the current packet, so that the display service DS in the display node N 3  receives the application packets seamlessly from the two storage nodes N 1 ,N 2 . The size of the output buffers OB 1 ,OB 2  is calculated such that the time for the messaging and the configuration of the playback process in the successor node may be bridged.  
         [0039]     In the following, two other embodiments of the invention, being solutions for seamless playback of distributed chunks of a stream, are described.  
         [0040]      FIG. 6  shows a simplified distributed storage system model for playback with two storage nodes N 1 ,N 2  and one node N 3  for the display service. Correspondingly this model may also be used for recording. In this model all parts of the stream are routed via one node N 1 , and only one buffer control unit BCU 1  is active. Only this node controls the seamless playback of the stream, while the other node N 2  sends its application packets not to the application packet bus APB, but to the active buffer control unit BCU 1 . As mentioned above, in the case of P2P networks this virtual extension bus XB is the same physical connection.  
         [0041]     Similarly,  FIG. 7  shows another simplified distributed storage system model for playback with two storage nodes N 1 ,N 2  and one node N 3  for the display service. In this model the buffering is performed at the display service node N 3 , which controls the seamless playback of the stream. With this solution it is necessary to buffer data at the non-storage node. A message controller MC 3  at the display service node N 3  receives a message from the active storage node N 1 , indicating the ATS of its last application packet. A buffer control unit BCU 3  monitors the timestamps of the buffered application packets, and when reception of the last packet from the active storage node N 1  was detected, a message is sent to the message controller MC 2  of the successor node N 2 , which in turn begins to retrieve application packets from its storage medium D 2  and send it to the input buffer IB 3  of the display service node N 3 . The input buffer IB 3  must be able to hold enough application packets to bridge the required messaging and data retrieval time.  
         [0042]     The inventive method may also be used in systems where a monotonous series of numbers, e.g. sequence numbers, are used instead of time stamps for the identification of application data packets. In this case the numbers must be long enough to prevent misinterpretation after wrap-around, e.g. 32 bit like for TCP/IP packets.  
         [0043]     In one embodiment of the invention the different storage devices are located within the same device, e.g. multiple hard-disks or other magnetic discs within one multimedia recorder.  
         [0044]     In one embodiment of the invention the recording may be done such that the second storage device gets no message about when to start buffering data. Instead it buffers the input data continuously. In this case it is sufficient to receive a message that identifies the first buffered application packet to be stored.  
         [0045]     In one embodiment of the invention the metadata descriptor contains an identifier for the storage device that is expected to hold the preceding and/or successive chunk.  
         [0046]     In one embodiment of the invention the metadata descriptor may also contain detailed information about where the preceding and/or successive chunk is stored, e.g. address, file name or path name.  
         [0047]     In one embodiment of the invention the data stream may be split not only when the first storage medium is full, but also due to other reasons, e.g. recording problems in the first storage device.  
         [0048]     The inventive method is suitable for storing and retrieving any data stream that is composed of identifiable units, like packets with identifiers, on a plurality of storage devices. Further, the data retrieval may serve any purpose, e.g. copying, encoding, transmitting or replaying.