SYSTEM AND METHOD FOR CONFIGURING A MEDIA STORAGE SYSTEM

Abstract: A system and method for tuning a storage system includes characterizing a maximum sustainable throughput for the storage system and decreasing it by a primary derate parameter to obtain a primary throughput. Sizes for buffer units may be determined at different stream rates, used to buffer a data stream between a request for and storage by generating stream simulators sufficient to consume the primary throughput and then optimizing the buffer sizes to prevent underruns. The primary throughput is decreased by a secondary derate parameter to obtain a maximum system bandwidth which sets an upper limit on admission of streams. The proportion allocated for rate-guaranteed requests may be used to determine buffer sizes that are able to meet the prefill margin. Another parameter may set the length of a seek reorder queue that orders storage requests according to their physical address. Also, the block size by which data is accessed in the storage system may be configured.

Description: TITLE: APPARATUS AND METHOD FOR CONFIGURING A MEDIA STORAGE SYSTEM BACKGROUND OF THE INVENTION 1 Field of the Invention This invention relates to computer data storage and server systems, and more particularly to digital video/audio storage and playback systems supporting multiple contmuous media streams 2 Description of the Related Art Multimedia or video server systems are used m a variety of applications for the storage and playback of video, audio or other multimedia data streams For example, multimedia servers may be used for broadcast, cable, satellite or telco solutions to distribute multimedia information to clients or consumers Professional broadcasters and associated service providers, such as networks and affiliates or cable providers, may employ digital video servers to support high bandwidth multimedia broadcast applications including multi-channel program playout, ad insertion, and digital content management Other applications for multimedia server systems may include computer-based training in which multimedia training materials or lectures may be stored on the server system accessed by students over a network or the internet Video archiving, browsing and retrieval is another multimedia server application Various movies may be stored by the server and distributed to users upon request Video-on-demand or video delivery systems may enable a plurality of users or viewers to selectively watch movies or other audio/video sequences which are stored on one or more video servers or media servers The video servers may be connected through data transfer channels, such as a broadcast cable system, satellite broadcast system or the internet, to the plurality of users or subscπbers The video servers may store a plurality of movies or other audio/video sequences, and each user can select one or more movies from the video servers for viewing Each user may include a television or other viewing device, as well as associated decoding logic, for selecting and viewmg desired movies When a user selects a movie, the selected movie may be transferred on one of the data transfer channels to the viewing device of the respective user Multimedia servers are also found in webcasting applications m which entertainment may be multicast on the internet to different subscribers Multimedia servers are found m numerous other applications as well To meet the demands of many different applications and users, it is desirable for a multimedia server system to provide flexibility and extensibility Two important requirements for a multimedia server system are storage space and filesystem bandwidth Multimedia data, such as full-motion digital video, requires a large amount of storage and data transfer bandwidth Thus, multimedia systems use vaπous types of video compression algorithms to reduce the amount of necessary storage and data transfer bandwidth In general, different video compression methods exist for still graphic images and for full-motion video Video compression methods for still graphic images or single video frames may be intraframe compression methods, and compression methods for motion video may be mterframe compression methods Examples of video data compression for still graphic images are RLE (Run-Length Encodmg) and JPEG (Joint Photographic Experts Group) compression Although JPEG compression was originally designed for the compression of still images rather than video, JPEG compression is used m some motion video applications Most video compression algorithms are designed to compress full motion video Examples of video compression techniques are MPEG (Moving Pictures Experts Group), MPEG-2, DVI (Digital Video Interactive) and Indeo, among others Even with the use of compression techniques, multimedia applications may still require extremely large amounts of storage For example, two hours of video encoded at 1Mb per second may require roughly one gigabyte (1GB) of storage A system supporting numerous different content may require up to several terabytes (TB) of storage The server system must also be able to provide enough bandwidth for the vanous users to access selected multimedia content without overloading the storage system For example, to support 100 simultaneous subscribers viewmg multimedia content encoded at 1Mb per second, a server may need to support a bandwidth m excess of 100Mb per second when allowing for overhead If enough bandwidth is not available, then some requests may have to be denied, or the play quality may suffer (video may run too slowly or may appear "jerky") To meet such storage and bandwidth needs, a multimedia server may utilize one or more RAID storage systems In a RAID system, for a given multimedia file, blocks of multimedia data may be stored across multiple hard disk units The blocks may be read out or transferred to the communication network and transmitted or broadcast to the user or users At the receiving end the blocks may be decoded for user viewing on a display device The use of hard disk drives to store information magnetically is well known A single hard disk dπve typically includes several rotating disks or platters upon whose surfaces information may be written or read by a read/write head Within a hard disk dπve the platters rotate together and all read/write heads typically are moved simultaneously to access different platter positions A platter typically is formatted into concentric tracks that define collectively, platter-to-platter, a family of concentric cylinders, and into sectors that represent a portion of a track A controller associated with the hard disk unit determines which cylmder, read/write head and sector is to be accessed for reading or writing The platters may also be considered as bemg divided mto zones Since they are physically larger, tracks in zones at the outer platter parameter contain more sectors than tracks in zones near the rotational axis of the platter Therefore, assuming the platters rotate with a constant velocity, the data bandwidth available from the outer most zones is greater than the data bandwidth available from the innermost zones Even with modern hard disk drives, there can be a 2-1 vaπation between worst case and average case disk transfer bandwidth due to sectors/track variations between outer and inner zones Many multimedia applications require contmuous media streams m which data streams must be delivered at a specified and possibly time- varying data rates and with a specified uniformity of that delivery rate To support continuous media streams, pπor video streaming systems have been based on a constant time, vaπable data storage scheduler Constant time, vaπable data schedulers deliver a stream at a particular bit rate by accessing the disk with a frequency that is common with all streams and varying the amount of data fetched at each penod to meet the required data rate of the stream The admission control problem for constant time, vaπable data disk schedulers is fairly straight forward A new stream may be admitted if the disk can sustam the required aggregate data rate over the fixed access penod Figure 1 illustrates a constant trme/vaπable data rate dependent data placement/scheduling mechanism For simplicity only two streams are illustrated Data blocks 100 are accessed at a fixed penod common to all streams The size of the data blocks 100 is vaπed between different streams accordmg to the rate required for a particular stream. As shown m Figure 1, stream one has a larger block size and thus a higher data rate than stream two. When streams are recorded to disk usmg a constant/time vaπable data schedulmg mechanism, the placement of data on the disks is rate dependent. Constant time schedulers may have some deficiencies however. These deficiencies may include significant internal fragmentation of storage. Internal fragments occur because storage is allocated m multiples of some fixed-size block. A minimum block size is the size of the device sector, but typically the blocks may be larger to minimize the size of free block lists. Internal fragmentation may occur m every block file. For example, fifty percent of storage could be lost to internal fragmentation when the desired bit rate just exceeded what was deliverable by a certain block size per penod. Constant time schedulers may also suffer from a loss of bandwidth capability due to internal fragmentation. Internal fragmentation results in a loss of storage bandwidth because the system must fetch unused bytes Constant time schedulers may also suffer from significant external fragmentation of storage. Because storage is allocated and freed in vaπable blocks, storage can be lost for use by high bit-rate files due to external fragmentation Some constant time systems avoid this problem by aggregatmg their vaπable data from multiple discontinuous storage blocks. However, in doing so they lose storage performance by requiring additional seeks. Another deficiency of constant time schedulers is that they may require a more complicated file system to accommodate vaπable block sizes. The scan requires decision-making mechanisms to choose between best fit and first fit blocks, and between blocks on a disk's inner or outer zones, to mention only two complicating factors. Other complicating factors arise when allocating memory buffers to hold the vaπable-size data blocks. Constant time schedulers result in bit-rate dependent block allocation of storage. Bit-rate dependence is inherent in constant time schedulers because the block size is chosen based on the desired stream bit rate. Thus, a file's desired streaming rate must be known prior to wπting the stream to storage. Constant time schedulers also typically lack of support for a pull model and vanable bit-rate streaming. In general, because block size and placement are based on a given anticipate stream rate, accommodatmg vanable bit streams and implementing a pull model is unpractical. While most of the above pomts are implementation or efficiency issues, lack of pull-model support is a functional issue with significant implications. It makes non-real-time migration of video files difficult. It also complicates solutions in cases where other devices are unable to slave to a server's clock or are forced to slave to another clock source, such as where encoders and decoders m a broadcast environment are slaved to a "studio clock " Non-real-time migration and providing mput and output to externally clocked devices are important requirements for the broadcast environment. Thus it is desirable to have a multimedia server and storage system that may guarantee performance on an arbitrary mix contmuous multimedia streams including reads, wntes, varying bit rates, smgle files, and multiple files A technique for controlling stream admission is desirable to ensure that current stream rates may be guaranteed. Efficient high utilization of disk bandwidth is also desirable. SUMMARY OF THE INVENTION A multimedia storage manager may operate by associatmg a ring of data buffers between the requestor of contmuous media and the disk subsystem. The number of buffers m the ring may be a function of a contracted guarantee rate of the associated media stream, configurable parameters of the disk schedulmg system, and service charactenstics of the disk system. This ring of N buffers may be used to hold the next N blocks of the contmuous media stream to be accessed by the requester. Once a buffer in the ring has its data consumed by the requester, the now empty buffer along with a deadlme time is queued with the disk scheduler. The deadlme time mdicates the latest time when the buffer can be filled and meet the guaranteed rate requirement of the stream. It is calculated simply as currentTime + (N - 1) * bufTime, where bufTime is the minimum time m which the requestor can consume a buffer without exceeding the contracted rate guarantee. Simultaneously with guaranteed rate requests bemg queued with the disk scheduler, pnoπtized, but non- guaranteed rate requests are also queued. Non-guaranteed rate requests do not carry deadlines, but do carry priorities. In a prefeπed embodiment, the disk scheduler issues the queued requests to disk in an order which meets the deadlines associated with the request, obtams a high proportion of the disk system bandwidth, and allocates residual disk bandwidth after guaranteed requests to non-guaranteed requests in a manner consistent with their associated priorities. The storage manager places requests from contmuous media requestors mto a earliest deadline ordered queued (the "deadlme queue"), and it places pnoπtized non-guaranteed rate requests mto a separate highest pnoπty ordered queue (the "pnoπty queue"). The disk scheduler may also maintams a third list of disk requests. This list has requests ordered by the position of the requested data block on the disk device. This list is referred to later as the "seek reorder queue" The seek reorder queue may have fixed maximum size that is a configurable parameter of the system. Requests from the two queues described above are migrated to this list. Requests are migrated from the deadlme and pπonty queues to the seek reorder queue m a "cycle". A cycle is of a fixed number of requests and within the cycle "slots" are assigned to either the deadlme queue or the priority queue in proportion equal to the desired allocation of disk bandwidth between guaranteed and non- guaranteed access. The slots allocated to each use are evenly distributed through the cycle Requests from the deadline and pπonty queues are migrated to the seek reorder queue whenever the seek reorder queue is not filled to its maximum size. Each migration is done from the queue mdicated by the current slot of the cycle and then the cycle advances to the next slot. If the queue indicated by the slot is empty, then an entry from the alternate queue is chosen if it is non-empty. The migrated entry is ordered m the seek reorder queue such that all requests to one side of it refer to data blocks with disk addresses greater than or equal to its and all blocks on the other side in the list have data blocks with disk address less than or equal to its. The seek reorder queue is concuπently traversed by a thread that moves continuously in one direction (l e m mcreasmg or decreasmg disk addresses) until no further entπes exist on the list, it then reverses direction and resumes. This thread at each step issues the disk request to the disk subsystem and advances to the next request when the previously issued request has been completed by the disk system. In a preferred embodiment, the storage manager ensures that bandwidth is allocated as desired between guaranteed and non-guaranteed rate requestors, allows for pπontized non-guaranteed access, and reduces average seek distance as disk load mcreases. The system also addresses admission control (e.g. determining if an additional guaranteed rate requestor can be accommodated without impacting service to existing guaranteed rate requestors) and determination of the appropπate number of buffers m a ring for a desired guaranteed stream access rate. Admission control may be addressed by characterizing the disk system performance with a synthetic load that reflects the characteristics of a typical load. Construction of a representative load may be simplified by constraining the file system to allocate sequential blocks m a "zoned random" manner: the disk block address range is divided into two halves, sequential file block allocations are chosen from random positions within a zone and then alternating between the two zones In a preferred embodiment, this process is followed because disk systems have performance vaπations with respect to block address. Allocatmg from alternate "zones" ensures that disk performance is approximately equal both across different files and across positions within a smgle file The synthetic load is now a single zoned allocation sequence which is sufficiently representative of the true load (and aggregation of multiple of zoned allocations). Disk performance may be characterized with this load and derated to provide margin. The derated bandwidth value may then be multiplied by the fraction of total bandwidth allocated in the "cycle" process to guaranteed rate requestors. This results in the maximum admission bandwidth Guaranteed rate requestors may now be admitted until they have consumed the entire admission bandwidth. Determination of the appropnate number of buffers m the buffer nng may be done empincally. This may be done by running a collection of guaranteed rate streams, all contracted for the same rate. The rate is chosen so that the collection of streams exactly consumes the entire admission bandwidth. All streams are initiated agamst the scheduler simultaneously. Ring buffers are then sized to assure that all deadlines are met with a stated margm The margin may be a configurable parameter Generally speakmg, one embodiment includes a system for scheduling storage accesses of multiple contmuous media streams. The system may mclude a plurality of media stream clients. Associated with each media stream client is one of a plurality of media stream managers. Each media stream manager mamtams a nng of buffers configured to buffer media stream data between its associated media stream client and one or more storage systems. A different deadlme queue may be associated with each one of the storage systems. Each deadline queue may be configured to queue buffer requests from the media stream managers. Each buffer request may mclude a deadlme by which the buffer request must be fulfilled by the corresponding storage system. Each media stream manager may be configured so that once one of its buffers is consumed by the associated media stream client, the media stream manager submits a buffer request and deadlme for that buffer to the appropnate deadlme queue Buffer requests may be ordered in each deadlme queue from earliest to latest deadlme. Each media stream manager may be configured to provide a guaranteed maximum media stream rate to its associated media stream client. The system may also mclude an admission controller configured to negotiate a maximum media stream rate with each media stream client. The admission controller may allow additional media stream clients only if negotiated maximum media stream rates can be mamtamed for all current media stream clients. For each nng of buffers, each buffer is consumed by its associated media stream client one after another in a circular order. Each buffer may be equally sized to hold one block of data, where data is accessed m the storage systems accordmg to a block Each nng of buffers may contain a number of buffers, where the number is chosen accordmg to a maximum stream rate requested by the associated media stream client. The number is chosen to provide just enough buffers to avoid underrun at the maximum stream rate. The system may also mclude a bandwidth allocator configured to allocate a set portion of storage system bandwidth to request from the deadlme queue for a storage system and the remainmg storage system bandwidth to non-rate-guaranteed requests. One embodiment may include a method for schedulmg storage accesses of multiple continuous media streams. The method may mclude establishing a guaranteed maximum stream rate for each of a plurality of multimedia streams. The method may further mclude establishing a nng of buffers for each multimedia stream with each multimedia stream fulfilling multimedia data requests from its associated nng of buffers. As each buffer in one of the ring of buffers is consumes, the method mcludes queumg a request for that buffer m a deadlme queue associated with the storage system from which the request is to be satisfied For each deadlme queue the method includes issuing requests from the deadline queue to the associated storage system The request with the earliest deadlme is issued first. Queuing the request may mclude calculating a deadlme time mdicatmg the latest time by which the buffer request can be fulfilled and still meet the correspondmg guaranteed maximum stream rate. An embodiment may include a method for managing a multimedia data stream. The method may include receiving a request from a stream requestor to establish a multimedia data stream to access a multimedia file stored on a storage system The method may include linking to the multimedia file and creating a rmg of buffers The method may further include initially filling the rmg of buffers with multimedia data for the stream and then streaming multimedia data between the multimedia file on the storage system and the multimedia stream requestor. Streaming multimedia data may include consuming one of the buffers m the rmg of buffers and then generatmg a request and a deadlme for the just consumed buffer. The deadlme may indicate the latest time the request must be fulfilled in order to prevent buffer underrun. The streammg then mcludes issumg the request and deadlme for the just consumed buffer to a deadline queue. The request may be ordered m the deadlme queue accordmg to the deadlme. The next buffer in the ring of buffers may then be consumed and the process repeated. A system for managmg storage accesses for rate guaranteed continuous multimedia data streams and non- rate-guaranteed storage requests may include a plurality of rate guaranteed requestors for multimedia streams and one or more non-rate guaranteed requestors A disk scheduler may also be mcluded The disk scheduler may have a guaranteed rate queue for queuing storage requests from the rate guaranteed requestors and a non-rate-guaranteed queue for queuing requests from the non-rate-guaranteed requestors The disk scheduler may also mclude a bandwidth allocator coupled to the guaranteed rate queue and the non-rate-guaranteed queue and further coupled to a storage system. The bandwidth allocator may be configured to allocate bandwidth of the storage system between the guaranteed rate queue and the non- rate-guaranteed queue accordmg to a predetermmed ratio. The rate guaranteed requestors may be limited so that an aggregate of maximum stream rates guaranteed for each rate guaranteed requestor does not exceed the portion of the bandwidth allocated to the guaranteed rate queue. Requests may be ordered in the guaranteed rate queue accordmg to a deadlme where the deadlme for each request mdicates a latest time by which that request must be fulfilled to meet a guaranteed maximum stream rate for the correspondmg multimedia data stream Requests may be ordered in the non-rate guaranteed queue accordmg to a pnonty where the bandwidth allocator is configured to migrate the request with the highest pnoπty from the non- rate-guaranteed queue. The system may also mclude a buffer rmg for each multimedia data stream from one of the rate guaranteed requestors The buffer nng may buffer multimedia stream data between an associated rate guaranteed requestors and the disk scheduler. The buffer nng may contain a number of buffers chosen to prevent buffer undemin at a guaranteed maximum stream rate. The buffers of each buffer rmg may be consumed by the associated rate guaranteed requestor one after another in a circular order. As each buffer is consumed a request and a deadlme for the just consumed buffer may be issued to the guaranteed rate queue. A method for managmg storage accesses for rate guaranteed continuous multimedia data streams and non- rate-guaranteed storage requests may include allocatmg a portion of the storage systems bandwidth to stream requests from rate guaranteed requestors and allocating the remaining portion of the storage systems bandwidth to request from non-rate-guaranteed requestors. Storage requests from rate guaranteed requestors may be queued m a guaranteed rate queue and storage requests from non-guaranteed rate requestors may be queued m a non-rate guaranteed queue The method further mcludes selectmg either the guaranteed rate queue or the non-rate- guaranteed queue according to the bandwidth allocation m order to migrate one of the requests to the storage system. Each guaranteed rate requestor may request access for a multimedia stream havmg a maximum guaranteed rate The method mcludes limiting the number of multimedia streams so that the aggregate of the maximum guaranteed rate for each stream does not exceed the bandwidth allocated to stream requests from rate guaranteed requestors Unused bandwidth for guaranteed rate requests may be dynamically reallocated to non-rate-guaranteed requests and vice versa A system and method may be provided for configuring or tuning a media storage system. The method may include characterizing a maximum sustamable throughput for the media storage system The maximum sustainable throughput may be decreased by a pnmary derate parameter to obtam a primary throughput. The method may further include determining sizes for buffer units at different media stream rates where during operation each buffer unit is used for buffenng a media data stream between a media stream requestor and the storage system The buffer unit sizes may be determined by generatmg stream simulators sufficient to consume the pnmary throughput and then optimizing the buffer sizes to prevent underruns for the stream simulators. This may be repeated at different stream rates in order to determme a table of buffer sizes versus stream rates The method may also include decreasmg the pnmary throughput to obtam a maximum system bandwidth which, during operation of the system, sets an upper limit on admission of streams. The amount by which the pnmary throughput is decreased for this purpose may be determined by a configurable secondary derate parameter When buffer sizes are determined, a prefill margm parameter may be set by which request deadlmes must be met. Also, an available rate parameter may be set to proportion available bandwidth between rate guaranteed streams and non-rate guaranteed requests. The proportion allocated for rate guaranteed requests may be used to determme buffer sizes that are able to meet the configured prefill margm. Another parameter may be configured to set the length of a seek reorder queue that orders storage system requests accordmg to their physical storage address This parameter allows a tradeoff between seek efficiency and vanabihty m service time A higher vaπabihty in service time may require larger buffer nng sizes Also, the block size by which data is accessed m the storage system may be configured BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanymg drawings m which: Figure 1 illustrates a constant trme/vaπable data rate dependent data placement/scheduling mechanism; Figure 2 is an illustration of a video server and storage system; Figure 3 is an illustration of a distnbuted multimedia file system employmg a number of video servers and files systems; Figure 4 is a detailed diagram of a video storage manager; Figure 5 illustrates one example of a constant data, vanable tune rate-independent placement mechanism of the video storage manager for two simultaneous continuous media streams; Figure 6 is a flow chart illustrating a constant data, vanable tune access mechanism employing buffer rmgs and deadlme queues; Figure 7 illustrates a system which provides for both guaranteed rate streams and non-rate-guaranteed available rate accesses; Figure 8 illustrates an example of a cycle by which requests are migrated from the deadlme and pnonty queues to the storage system; Figure 9 is a flow chart illustrating a method for providing storage access for multiple contmuous media streams with a rate guarantee and storage access for non-rate guaranteed requests; Figure 10 illustrates a video storage manager combmmg mechanisms illustrated in Figures 4 and 7, Figure 1 1 is a flow chart illustrating operation of the seek reorder shown in Figure 10, Figure 12 is a flowchart illustrating storage characterization for admission control; and Figure 13 is a flow chart illustrating determination of the optimum number of buffers for a buffer nng for a vaπety of stream rates. While the invention is susceptible to vanous modifications and alternative forms, specific embodiments thereof are shown by way of example m the drawings and will herem be descπbed in detail. It should be understood, however, that the drawings and detailed description thereto are not mtended to limit the invention to the particular form disclosed, but on the contrary, the mtention is to cover all modifications, equivalents and alternatives falling within the spint and scope of the present invention as defined by the appended claims. DETAILED DESCRIPTION OF THE INVENTION Referring now to Figure 2, a video server and storage system 200 is illustrated. System 200 mcludes server 202 and storage systems 204. The storage systems 204 may be connected to the server 202 by one or more buses 205. The server may include one or more processors (not shown) which may communicate with the storage systems 204 via a penpheral bus, such as one or more PCI buses and one or more SCSI mterfaces. The server 202 may also mclude a number of codecs for encoding and decodmg multimedia data streams. The codecs may also be coupled to one or more PCI buses. Each storage system 204 may mclude one or more RAID systems as shown. In order to support multiple contmuous media streams m which data streams are delivered at a specified and possibly time-varying data rate, the server 202 mcludes a video storage manager 206. The video storage manager controls the storage and access of multimedia streams on the storage systems 204. In a preferred embodiment, multimedia files are stored via the video storage manager 206 m high quality MPEG-2 format, although other suitable compression formats may be used. Clients or requestors for a multimedia stream contract with the video storage manager 206 for access to a file at a desired bit rate. The video storage manager 206 assesses available storage bandwidth and available buffer memory to determme whether or not the request can be met Once the video storage manager has established that the request can be accommodated, the client is given access to the file at any bit rate up to the contracted rate. If the request exceeds available storage bandwidth and/or buffer memory is exhausted, the video storage manager must reject the request and the client is free to adjust and/or resubmit the request at a later time. By providmg a guaranteed stream rate the video storage manager fully supports vaπable bit rate accesses m addition to constant bit rate accesses. A client may arbitranly vary the rate of access to a file from zero bits per second to any pomt up to the contract rate. This flexibility supports a number of features including frame accurate initiation and jog/shuttle functionality. Multiple different clients may request different streams at different bit rates from the video storage manager These streams may be an arbitrary mix of reads, wntes, stream rates and files accessed. Each stream may have a different contract rate and an individual stream may arbitranly range m rate up to the contract rate wherem the total aggregate for all stream rates does not exceed the total aggregate streammg capacity of the server system There is no requirement that all streams be of the same bit rate, or that the bit rate of a stream be chosen from a set of discrete allowable rates. The video storage manager also permits clients to access the same files, different files, or any combination in-between As will be descnbed below, the video storage manager provides this flexibility without impacting on server aggregate bandwidth Turning now to Figure 3, a distributed multimedia file system 300 is illustrated employing a number of video servers 202 and files systems 204 In this embodiment the files systems 204 communicate with video servers 202 via fibre channel Each storage system 204 may include a number of RAID systems linked on a fibre channel arbitrated loop (FC-AL). Each video server 202 may also connect to its own local file system or tape library, for example In addition, other storage systems, such as a tape library, may be accessible to the system on the fibre channel. Clients may request multimedia streams to be sent on transmission network 208. Transmission network 208 may be a computer network, the internet, a broadcast system or any other suitable transmission medium for multimedia streams. A video storage manager executing on one or more of the video servers controls the initiation and addition of multimedia streams for accessmg files on storage systems 204 The video storage manager manages multiple contmuous media streams to be delivered through a wide range of hardware mterfaces, such as MPEG encoders and decoders, DVB multiplexors, ATM, SONET, and ethernet, to transmission network 208 The video storage manager as employed m systems such as illustrated in Figures 2 and 3, addresses how to schedule disk or storage accesses for multiple continuous sequential media streams m a manner that guarantees data for all contmuous media streams and provides an accurate mechanism for determining whether a new request for guaranteed rate access can be accommodated Turning now to Figure 4, a detailed diagram of a video storage manager 206 is shown. The video storage manager 206 mcludes a request processor 402 which mterfaces client requests to stream managers 404 Each stream manager 404 maintam a buffer nng 405 A separate stream manager 404 corresponds to each contmuous multimedia stream A file system 406 is provided for mappmg stream accesses to the storage systems 204. Disks schedulers 408 are provided for each storage system 204 to manage to flow of storage accesses to each storage system. Each disk scheduler may include a deadlme queue for 410, as descπbed m more detail below. The video storage manager, file system, and disk scheduler place stream data on the storage systems m a manner that is completely mdependent of the inherent bit rate of that mateπal. This feature provides for additional flexibility in that clients may transfer content on and off the video storage manager file system with guaranteed rate service at data rates many times higher (or lower) than the inherent rate of the stream data. The video storage manager, file system, and data placement mechanism is a fixed block size mechanism. For example, data is transfeπed to or from the storage systems m a constant block size. In a preferred embodiment a block size of 256 kilobytes may be chosen The video stream manager may provide for configuration of the block size duπng system initiation or configuration. The fixed block size mechanism ensures that no external fragmentation of storage occurs and that mtemal fragmentation occurs only at the last block of the file (smce a file is unlikely to end exactly at a block boundary) Unlike rate-dependent, vaπable block size mechanisms, which suffer from both external fragmentation and varying levels of per block internal fragmentation that results in great vanations and storage requirements for a particular file dependmg on stream rate and current file system contents, the video storage manager's rate independent fixed block size mechanism ensures predictable storage requirements for any file regardless of rate or cuπent file system contents. Turning bnefly to Figure 5, one example of the constant data, vaπable time rate-mdependent placement mechanism of the video storage manager is illustrated for two simultaneous contmuous media streams. As shown, the data block size is fixed for all media streams, but the time at which a data block is accessed vanes for each stream accordmg to the desired bit rate. One problem that arises from a constant data (fixed block), vanable time access schedulmg mechanism is that multiple streams, each with its own frequency and phase of storage accesses, make requests to the storage system and the interaction of these access patterns results m peaks and lulls in the storage activity. The different frequency and phases of storage accesses by the different streams results m times in which numerous accesses may be pending at once and other times in which very few accesses may be pendmg. One solution to this problem is to simply require the storage systems to support the peak rate of activity, however, this solution is clearly not cost effective. Referπng back to Figure 4, the virtual storage manager of the present invention addresses the above-noted problem by leveling storage activity by introducing a rmg of buffers between each client and the file system. Each media stream is associated with a different buffer nng 405 managed by a stream manager 404 Thus, the stream manager 404 associates a ring of data buffers between the requester of continuous media and the disk subsystems. The number of buffers in a nng is determined accordmg to the contracted guarantee rate of the associated media stream and characteristics of the storage system so that the guaranteed rate is always met. The buffer πngs 405 exploit the fact that video streammg is inherently sequential and lets the file system pre-queue storage requests This approach allows fumre requests to be satisfied duπng lulls, shifting the load from peaks to valleys and smoothing storage activity over tune. Each nng 405 of N buffers is used to hold the next N blocks of the contmuous media stream to be accessed by the requester. Once a buffer m the nng has its data consumed by the requester, an access to fill the now empty buffer is queued to the appropnate disk scheduler 408 m order to fill the empty buffer with the next block for the media stream. Requests to fill (or empty) buffers of buffer πngs 405 are mapped by file system 406 to the appropπate disk scheduler 408. File system 406 maps logical blocks to physical blocks m the storage systems 204 The file system 406 may maintam a map of logical to physical block locations (e.g. an mode). Because requests for multiple streams may be queued m each disk scheduler 408, the system must ensure that future request from one stream are not fulfilled before more urgent requests from another stream so that the guaranteed rate may be mamtamed for each stream To accomplish this goal deadlmes are associated with each request submitted to the storage a system. The system calculates the deadlme to comcide with the time a buffer will be needed by noting the stream rate block size and the number of existing unconsumed buffers When a request for an empty buffer is queued, a deadlme time is queued with the request m the appropπate deadlme queue 410 m the disk scheduler 408 The deadlme tune mdicates the latest time when the buffer can be filled and still meet the guaranteed rate requirement of the particular stream. The deadlme time is calculated as: current time + (N-l) * buff_πme, where N is the number of buffers in the buffer rmg 405 and buff time is the minimum time m which a requestor can consume a buffer without exceeding the contracted rate guarantee. The disk scheduler 408 must now issue the queue request to the particular storage system 204 m an order which meets the deadlmes associated with the requests. The disk scheduler places requests from contmuous media requesters mto each deadlme queue 410 and maintams an order of earliest to latest so that requests with the earliest deadlme are satisfied first. In order for the system to meet a stream's deadlme it must set up a sufficiently large buffer nng to ensure that any request can be queued with the storage system far enough in advance of its deadlme so that the worst possible service tune for the request will not exceed the deadlme. Because worst case service time is a function of the aggregate load on the system, and the aggregate load is a direct result of the aggregate stream rate (independent of the actual stream rate mix), buffer rmg size for a particular stream on a given storage system is a function of that particular stream's rate and is independent of the stream rates of other streams in the mix. Given this independence, appropπate ring sizes for various stream rates may be generated at storage characterization time as detailed further below Turning now to Figure 6 a flow chart is provided illustrating the constant data, vaπable time access mechanism employmg buffer πngs 405 and deadlme queues 410. When a new stream is initiated, the stream manager for the new stream determines the guaranteed stream rate and the block size for the stream as mdicated at 602. The stream is attached to the requested file through the file system 406 and the stream manager 404 creates the buffer ring 405 for the new stream Requests for blocks from the associated file are then issued to the appropπate storage systems to fill the buffer nng Each buffer may be sized for one block. After the buffer nng is filled (606) streammg may begm as mdicated at 608 As each buffer is consumed by the stream requester, a block request is issued along with a deadlme time to fill the now consumed buffer, as indicated at 610. The block request and deadlme time are queued m the deadlme queue 410 for the appropπate storage system accordmg to where the requested block is located. The requests are ordered in the deadlme queue from earliest to latest deadlme time. Requests are issued from the deadlme queue according to the earliest deadlme as mdicated at 612. During streammg the buffers of the buffer nng are accessed one after another m a circular manner The deadlme tune assures that each buffer is filled before it is needed by the stream requester accordmg to the guaranteed rate. The buffer nng and associated deadlme times take advantage of the inherently sequential nature of multimedia streammg to pre-queue storage requests. This allows future requests to be satisfied duπng lulls of storage activity thus shiftmg the load from peaks to valleys and smoothmg storage activity over tune Note that while Figure 6 has been descnbed m terms of stream read requests, the same mechanism may be employed for wπte stream requests. As each buffer is filled with a block of stream data a request and deadlme may be queued m a deadlme queue to wπte the block mto the storage system The video storage manager 206 supports a plurality of different media stream clients at different rate guarantees. A different media stream manager 404 and nng buffer 405 may be provided for each stream. A separate disk scheduler 408 and deadlme queue 410 are provided for each storage system 204. Thus, each deadlme queue 410 may mclude requests correspondmg to several different media streams. The deadlme times for each request m the deadlme queues 410 are all calculated relative to a common current time so that the earliest deadlme from any requester stored m a particular deadlme queue is issued first. The rime between requests bemg satisfied for any particular stream varies depending upon the number of other pendmg requests, however, the associated deadlme time assures that the rate guarantee will be met. In addition to providing for rate guaranteed contmuous media streams, it may be desirable for a multimedia server to provide access to data stored m the storage systems m a pπontized but non-late guaranteed manner. Such accesses should not impact the guarantees made for the contmuous rate-guaranteed media streams. For example, an NFS or FTP requester may wish to access a file. Typically such accesses are non-real-time and no rate guarantee is required Such accesses may be satisfied usmg residual disk bandwidth available after all guaranteed rate accesses are satisfied. Any storage bandwidth that remams after all guaranteed rate requests have been met is allocated to a general pool. Available bandwidth clients may access this bandwidth on a first come, fist served basis. The video storage manager dynamically determines the amount of available bandwidth. Any bandwidth from an unused guaranteed rate contract may become part of the pool of available bandwidth. Turning now to Figure 7 a system is illustrated which provides for both guaranteed rate streams and non- rate-guaranteed available rate accesses. As shown in Figure 7 the video storage manager 206 may accept requests from both guaranteed rate clients and available rate clients. A stream buffer 712 may be associated with each guaranteed rate client. In a preferred embodiment, each stream buffer 712 is a buffer nng as descnbed m regard to Figures 4 and 6. Guaranteed rate requests are mapped by file system 406 to an appropnate disk scheduler 408 and queued in a guaranteed rate queue 706 In a preferred embodiment the guaranteed rate queue is a deadline queue as described in regard to Figures 4 and 6. Available rate requests that are non-rate guaranteed are also mapped by file system 406 to the appropriate disk scheduler for the storage system m which the requested data is located. A data pool 704 may be provided as a shared buffer for the available rate requests. Available rate requests are queued m a pπoπty queue 708 associated with each storage system. Another source of file requests may be the file system 406 itself. These requests may include requests for metadata required to support the vaπous data streams (e.g. blocks that holds lists of blocks to stream, such as indirect blocks). These type of metadata requests may be tune cπtical m that streammg will stop if a stream pomter block (indirect block) pomtmg to the next data block to the stream is unavailable. Thus, request for tune cntical metadata also carry deadlmes and may be scheduled directly along with streaming data requests m the guaranteed rate or deadlme queue 706. The file system constantly monitors its progress by means of the cuπent mdirect block. At an appropπate threshold it calculates a deadlme and schedules the fetch of the next indirect block from the storage system. Other metadata requests may be non-cπtical such as other types of file management and read and wnte operations unrelated to streammg (e.g. listing files m the file system) These non-time-critical metadata requests are queued m the pπoπty queues 708. A metadata pool 702 may be associated with file system 406 from which the metadata requests are issued. Although other metadata requests and available bandwidth requests do not have stnct service tune requirements, they may have a pnonty relationship. For example, metadata wntes may be considered the highest pπonty because their completion may be essential for closmg a particular stream episode. Metadata reads may be next m pnoπty to ensure timely processmg of file lists, file creations, etc. Available I O requests may have the lowest pnonty and may be filled when resources are available. Requests m the pπoπty queues are ordered from highest to lowest pnonty The disk schedulmg mechanism issues the queued requests to the storage system m an order which meets the deadlmes associated with the requests and also allocates residual bandwidth after guaranteed requests to non- guaranteed requests m a manner consistent with their associated pnonties. A bandwidth allocator 710 may be employed to allocate a certain portion of storage bandwidth to guaranteed rate requests and the remaining bandwidth portion to non-guaranteed pπoπty requests. At storage characterization time a configurable percentage of a storage system's bandwidth is reserved for honoring the non-guaranteed pnonty requests. For example, 90 percent of the bandwidth may be reserved for the guaranteed rate requests from guaranteed rate queue 706 and the remaining 10 percent allocated to non-rate guaranteed requests from pnonty queue 708. Based on the percentages reserved for guaranteed and non-guaranteed requests, the disk scheduler chooses a request from one or the other queue to hand off to the operating system to be satisfied from the storage system. When the chosen request queue is empty, the scheduler attempts to de-queue a request from the other queue thus allowing both non-guaranteed and guaranteed requests to absorb unused storage bandwidth. In a preferred embodiment requests are migrated from the deadlme and pnoπty queues to the storage system accordmg to a cycle. An example of a cycle is shown in Figure 8. A cycle is of a fixed number of slots with each slot assigned to either the deadlme queue or pπonty queue m proportion equal to the desired allocation of disk bandwidth between guaranteed and non-guaranteed accesses In Figure 8, slots marked with a D pomt to the deadlme queue and slots marked with a P pomt to the pπoπty queue. The slot is repeatedly traversed and a request is chosen from one of the queues according to the current slot. In the example of Figure 8, the bandwidth is proportioned so that the disk scheduler will first look to the deadlme queue for 13 out of every 16 storage accesses and first look to the priority queue for the remainmg three out of every 16 accesses. This allocation is merely one example and in a preferred embodiment the allocation may be nme out of ten slots pomtmg to the deadlme queue and one out of every ten slots pointing to the pnoπty queue. In a preferred embodiment the slots allocated to each use are as evenly distributed as possible throughout the cycle. In a preferred embodiment requests from the deadlme and pπonty queues are migrated to the storage system accordmg to the current slots and the cycle then advances to the next slot. If the queue mdicated by current slot is empty then an entry from the other queue is chosen if it is not empty Therefore, non-rate guaranteed requests may actually achieve more when their allocated bandwidth if the full rate guarantee bandwidth through the deadlme queue is not bemg utilized. Turning now to Figure 9 a flow chart is provided illustrating a method for providing storage access for multiple continuous media streams with a rate guarantee and storage access for non-rate guaranteed requests. A portion of the storage bandwidth is allocated to rate guaranteed requests and the residual bandwidth is allocated to non-rate guaranteed requests, as indicated at 902 Rate guaranteed requests are queued m a guaranteed rate queue and non-rate guarantee requests are queued m a pnoπty queue, as mdicated at 904. The rate guaranteed requests are entered mto and issued from the rate guaranteed queue m a manner to ensure that they are satisfied in a timely fashion to meet the particular rate guaranteed for each stream. The non-rate-guaranteed requests may be ordered m the pnonty queue so that higher pnoπty requests are satisfied before lower pπonty requests The system then selects a queue to issue a request to the storage system accordmg to a current slot from a cycle that proportions the storage bandwidth accordmg to the bandwidth allocation, as mdicated at 906. If the selected queue contains an entry, then that request is issued from the selected queue, as mdicated at 908, 910 and 912. If the selected queue is empty, then the system looks to the other queue for a request to issue, as mdicated at 908 and 914. If the other queue is not empty, then an entry is removed and issued, as mdicated at 916 and 912 The system then traverses the cycle to the next slot, as mdicated at 918, and repeats the queue selection process. If the other queue is empty at 914, the process is repeated until a queue is found containing an entry. In one embodiment, the slot is not advanced if both queues are empty Alternatively, the slot may be advanced if both queues are empty. Tummg now to Figure 10 a video storage manager is illustrated combmmg the mechanisms as discussed m regard to Figures 4 and 7. The storage manager of Figure 10 supports multiple continuous media streams m which clients contract for access to a file at a guaranteed bit rate. Each stream client is allowed to vary the rate of its access to its file from any rate up to the guaranteed rate. In addition, the storage manager of Figure 10 support available bandwidth clients A certain portion of the storage bandwidth is allocated to available bandwidth or non- rate guaranteed clients, such as available rate client 752. In addition, any bandwidth not used by the guaranteed rate clients may be available for the available rate clients. Thus, the video storage manager of Figure 10 may support any mix of guaranteed rate clients while delivering the same aggregate bandwidth and also support available rate clients at a non-guaranteed rate. As discussed in regard to Figure 4, each guaranteed rate client communicates with an associated stream manager 404 which mamtains a buffer nng 405 for the particular stream. The buffer rmg is used to hold the next N blocks of the continuous media stream to be accessed by the requester where N is the number of buffers m the buffer nng Each buffer may be sized equally for one block of data per buffer. Once a buffer m the nng has its data consumed by the requester, a request for the now empty buffer along with a deadlme time its queued with the appropriate disk scheduler 408 as determined by file system 406. The deadlme tune mdicates the latest time when the buffer request can be satisfied and still meet the guaranteed rate requirement of the stream The deadlme time may be calculated as deadline time = current_trme + (N - 1) * buff time where N is the number of buffers in the nng and buff_tιme is a mmimum time m which the requester can consume a buffer without exceedmg it contracted rate guarantee. Simultaneously with guaranteed rate request bemg queued with the appropnate disk scheduler 408, pnoπtized but non-guaranteed rate request are also queued. Non- guaranteed rate request do not carry deadlmes but do carry pnonties. The disk schedulers issue the queued requests to the storage systems in an order which meets the deadlmes associated with the requests while obtaining a high proportion of the disk system bandwidth and allocating residual disk bandwidth after guaranteed requests to non- guaranteed requests m a manner consistent with their pnonties. Guaranteed requests from contmuous stream requesters are placed mto an earliest deadlme ordered queue 410 m the appropnate disk scheduler. Non-guaranteed rate request are placed mto a separate highest pnoπty ordered queue 708 In addition to request from available rate clients 752 and guaranteed rate clients 754, requests may also come from the file system itself. Some requests from the file system may be time cπtical such as request for blocks that contam pointers to future stream blocks. Deadlmes are associated with these requests and they are inserted m the appropπate deadlme queue 410. Other requests, such as non-time cπtical file management requests, are assigned a pnonty and inserted in the appropnate pπonty queue 708. The file system requests may be buffered in a meta pool 702. Available rate client request may be buffered m a data pool 704. Requests are migrated from the deadlme and pπoπty queues by a bandwidth allocator 710, accordmg to a cycle which allocates bandwidth according to a configurable allocation. For example, 90 percent of a particular storage system's bandwidth may be assigned to the deadlme queue and thus guaranteed rate stream clients, and 10 percent assigned to the pnonty queue for available rate clients. The bandwidth allocator 710 may migrate requests from the deadlme and pnonty queues to a seek reorder queue 750 Request may be reordered m the seek reorder queue accordmg to the position of the requested data block on the storage device. The seek reorder queue may have a configurable maximum size. Requests from the deadlme and pπoπty queues are migrated to the seek reorder queue according to the cuπent cycle slot whenever the seek reorder queue is not filled to its maximum size. Each migration is done from the queue indicated by the cuπent slot of the cycle and then the cycle advances to the next slot If the queue mdicated by the slot is empty, then an entry from the alternate queue is chosen if it is non- empty The migrated entry is reordered in the seek reorder queue such that all requests to one side of the entry refer to data blocks with storage addresses greater than or equal to its address and all entπes on the other side of the queue request data blocks with disk addresses less than or equal to its address. Each seek reorder queue 750 is concurrently traversed continuously in one direction (i.e., in mcreasmg or decreasmg disk addresses) until no further entπes exist m the queue in that direction and it then reverses direction and resumes. Thus, the disk scheduler issues requests from the seek reorder queue to the storage system in order of disk addresses and advances to the next request when the previously issued request has been completed by the disk system Because the deadlme and pnonty queues contam requests from many different streams and clients, the sequence of blocks resulting from these queues is essentially random. If these requests where serviced accordmg to their order in the deadlme and pnonty queues, excessive disk seek overhead would result from the random pattern of requests. The seek reorder queue 750 improves seek time by reordering request out of the deadlme and pnonty queues accordmg to their disk position. Turning now to Figure 11, a flow chart is provided illustrating operation of the seek reorder queue 750. As indicated at 1102, when the seek reorder queue is not full, a request is migrated from either the deadlme or pnoπty queue accordmg to the cuπent cycle slot. If the mdicated queue is empty, the request is taken from the alternate queue if that queue is non-empty as indicated at 1104. The migrated request is inserted mto the seek reorder queue accordmg to the disk address of the requested block so that requests m the seek reorder queue are ordered by mcreasmg or decreasing disk addresses. Simultaneously, the seek reorder queue is traversed m one direction and the next request is issued to the disk system as mdicated at 1108. If the end of the seek reorder queue has been reached then the direction of queue traversal is reversed as mdicated at 1110 and 1114. If the end of the seek reorder queue has not been reached, then the cuπent traversal direction is maintained as mdicated at 1110 and 1112. Once the cuπent request has been satisfied by the disk system, the next request m the seek order queue is issued to the disk system as indicated at 1116 and 1108. As noted earlier, block requests as viewed by the storage system are inherently random because the storage system is presented with requests from many streams. Given this randomness it would be inefficient to sequentially allocate blocks for a particular file. Because I/O cylmders of a disk often have different transfer rates, block allocation within a particular file bounces back and forth between I/O zones of the disk. Thus for any particular stream file, block storage request are assigned disk addresses so that the blocks will be located m alternating I/O zones of the disk. This ensures that all files see an average storage throughput and that no file bemg streamed could end up coming entirely from a low performance zone of the disk. As mentioned above, the video storage manager must control admission of new continuous streams to ensure that the aggregate of the guaranteed stream rates does not exceed the aggregate storage bandwidth allocated for continuous media streams. Before any streammg is begun the storage systems are characterized to determme their performance or bandwidth Once a storage system bandwidth has been determined, then when streammg begins, as each new stream is requested the video storage manager determines whether or not the requested bit rate would exceed the remammg available bandwidth allocated for contmuous streams. If so, the request is demed and the requester is free to resubmit the request at a later tune or with a lower bit rate request. If sufficient bandwidth exits the request is granted and a stream manager creates an associated buffer ring as discussed above Because a sequence of requests presented to the storage system while streammg is essentially random, modeling the stream load to characterize storage bandwidth may be simplified. This performance may be characterized with a synthetic load that reflects the charactenstics of a typical load. The synthetic load may vary from a purely random sequence of blocks to take mto account the fact that blocks for any given file may be placed in alternating I/O disk zones. Thus a representative load may be constructed by constrammg the file system to allocate sequential blocks in a zoned random manner. The disk block address range may be divided mto two halves and sequential file block allocations may be chosen from random positions within a zone alternating between the two zones. Disk performance may be characterized usmg this synthetic load and then de-rated to provide margm. The amount of de-rate may be refeπed to as the pnmary de-rate parameter. The de-rated bandwidth value is then multiplied by the fraction of the total bandwidth allocated in the cycle process for guaranteed rate requesters. The resultmg guaranteed rate bandwidth may be de-rated again by a secondary de-rate parameter to allow for additional deadlme safety margin The result is the maximum admission bandwidth for the aggregate of all guaranteed rate requests. Guaranteed rate requesters can then be admitted until they have consumed the entire guaranteed rate admission bandwidth Storage characterization for admission control is summarized m Figure 12. A synthetic load is created by allocating blocks in a zoned random manner so that sequential file block allocations are chosen from random positions within a zone alternating between an I/O disk zone as mdicated 1202. Storage system bandwidth is determined usmg this synthetic load as indicated at 1204. The determined bandwidth is de-rated by a primary derate parameter to provide a certain margin as mdicated at 1206. The de-rated bandwidth is reduced accordmg to the portion of the bandwidth allocated for guaranteed rate request as mdicated at 1208. This portioned bandwidth may then agam be de-rated by a secondary de-rate parameter to provide extra deadlme margm as mdicated at 1210. The resultant bandwidth may then be used as a maximum aggregate admission bandwidth for guaranteed rate streams as mdicated at 1212. The characterization process may also mclude determining appropπate buffer nng sizes for vanous stream rates across the storage system's desired operational range. The optimum number of buffers for a buffer nng may be determined for a vaπety of stream rates as follows. Referring to Figure 13, for each particular stream rate, the characterization routine creates enough stream simulators to consume the entire aggregate throughput of the storage system as mdicated at 1302. For each stream simulator, a nng buffer is modeled as mdicated at 1304. Each stream simulator then generates block requests alternating between random blocks between zones as mdicated at 1306 The simulated streams are then run until a completion of a test tune or until any one of the streams suffers an underrun. An undemin occur when a buffer request is not completed before the request deadlme. In a prefeπed embodiment, a prefill margin parameter may be set so that an underrun occurs if a buffer request is not completed withm the prefill margm time before the request deadlme. The number of nng buffers m the model may be adjusted and the simulation repeated as indicated at 1308 and 1310 until the coπect nng buffer size is obtamed. The entire simulation may then be repeated for a different stream rate as mdicated at 1312. Thus a table of appropriate ring buffer sizes may be constructed during characterization for a vanety of stream rates up to the maximum streams rates supported by the system. Duπng operation whenever a new stream is admitted, an appropπately sized ring buffer may be created for the new stream by accessmg this table. The performance of the video storage manager may be tuned by adjusting a number of parameters as discussed above These parameters are summarized m the following table. Table 1 System Characterization Parameters Parameter Comments pnmaryDeτate Adjusts operational load level of storage systems relative to the maximum throughput. That is, adjusts service tune (queue lengths) for storage system load at which buffer rings are sized. available I/O rate Specifies storage bandwidth reserved for metadata and available I O. secondar Deτate Reduces streaming bandwidth to allow for additional deadlme safety margin prefill margin Specifies deadlme safety margm. Note - secondaryDerate obtams underrun protection at the cost of potential streammg bandwidth; prefill margin obtams underrun protection at the cost of additional buffer memory. wOverlap Specifies the target number of I O requests kept queued with the operatmg system m the see reorder buffer. loOverlap trades off seek efficiency agamst service time variability. (Higher service tune vanabihty requires more memory for buffers.) blockSize Specifies block size. blockSize trades off seek amortization agamst buffer fragmentation at lower stream rates. These parameters may be used to configure and adjust the performance of a media storage system such as the system described above. The maximum sustainable throughput of the storage system may be characterized as descnbed above, such as by usmg a synthetic load. In order to adjust the operation load level of the storage system relative to the maximum throughput, the characterized maximum sustainable throughput may be derated by the pnmary derate parameter The pnmary derate parameter is configurable and may be set duπng system configuration. Queues, such as the deadline queues descπbed above, may be sized based on the derated maximum throughput as derated by the pnmary derate factor The resultant throughput may be called the pnmary throughput. This pnmary throughput may be used for sizing the buffer rings as descnbed above. The pnmary derate parameter provides a safety margin for the operational load level of the storage system at the expense of lowenng the available maximum throughput. By setting the pnmary derate parameter during system configuration, the user may adjust this trade off as needed for any particular application of the storage system. The available I O rate parameter specifies the storage bandwidth reserved for non-rate guaranteed requests, as discussed above in regard to the bandwidth allocator. The amount of bandwidth reserved for non-guaranteed- rate requests versus guaranteed rate requests may be configured usmg this parameter. Depending upon a system's needs, the user may adjust the proportioning between non-guaranteed and guaranteed rate requests by adjusting this available rate parameter The secondary derate parameter reduces bandwidth available for rate guaranteed streams The primary throughput is proportioned according to the available rate parameter and the proportion allocated for rate guaranteed streams is further reduced by the secondary derate parameter to provide additional deadlme safety margm Durmg operation additional streams may be admitted up to the pomt that the aggregate of all stream rates entirely consumes the portion of the primary throughput allocated to guaranteed rate streams as derated by the secondary derate parameter The prefill margin parameter specifies a deadline safety margm used durmg the calculation of buffer ring sizes. Durmg system configuration buffer rmg sizes may be calculated for vanous stream rates, such as descnbed in regard to Figure 13 The prefill margm parameter specifies a margm by which the deadlmes must be met durmg this buffer nng size calculation process, e g., the prefill margm provides a margm by which buffer underrun must be avoided when the buffer nng sizes are bemg determined Note that the prefill margm parameter obtams additional underrun protection at the cost of additional memory used for larger nng buffers. A larger prefill margin will result in larger ring buffer sizes since, for certain stream rates, additional buffers will be required in the buffer ring to avoid missing the requests' deadlmes by the specified prefill margm In contrast, the secondary derate parameter obtains additional underrun protection at the cost of potential bandwidth for rate guaranteed streams Thus, the secondary derate parameter and prefill margm parameter provide a user of the storage system with the capability to adjust the system performance by makmg several different tradeoffs as is optimum for a particular application. For example, if plenty of memory is available, but additional bandwidth is needed, then the secondary derate may be lowered and the prefill margm mcreased. However, if memory is at a premium, the prefill margm may be decreased and the secondary derate parameter mcreased. The I/O overlap parameter (also refeπed to as the seek reorder buffer length parameter) specifies the number of storage requests queued with the operating system for a storage unit. For example, m the system descπbed above, a seek reorder queue is used to queue requests to the storage units m an order accordmg to the physical disk address of the storage requests. The length of such a queue may be configured by the I/O overlap parameter. This parameter trades off seek efficiency agamst service time vanabihty. For example, the larger the seek reorder queue is made, the more requests may be presented to the storage unit m a linear order thus mcreasmg drive seek efficiency However, smce the requests are reordered from their deadlme and pπonty ordeπngs, a longer seek reorder queue length will increase the vaπabihty m meeting request deadlmes. This parameter may be taken into account when sizing the buffer rings such that larger seek reorder queue sizes may result m larger buffer rmg sizes to account for the variability in satisfymg request deadlmes Therefore, the I O overlap parameter may allow the user to trade off memory that must be made available for buffers versus higher dπve seek efficiency In one embodiment the block size by which media data is accessed on the storage units may be configured accordmg to a block size parameter Configurmg the block size may allow for tradmg off seek amortization agamst buffer fragmentation at lower stream rates. A larger block size may allow for greater seek efficiency, however, a larger block size may also result in more fragmentation and less efficient use of storage capacity for certain file sizes Vanous modifications and changes may be made as would be obvious to a person skilled in the art havmg the benefit of this disclosure It is intended that the following claims be interpreted to embrace all such modifications and changes and, accordmgly, the specifications and drawmgs are to be regarded m an illustrative rather than a restrictive sense

Claims: WHAT IS CLAIMED IS: 1 A method for configurmg a multimedia storage system, the method compπsmg: characterizing a maximum sustamable throughput for a multimedia storage system; decreasmg the maximum sustamable throughput by a pnmary derate parameter to obtam a pnmary throughput, determining sizes for buffer units at different multimedia stream rates, wherein durmg operation each buffer unit is for buffenng a multimedia data stream between a multimedia stream requester and the storage system, and wherem said determining is performed accordmg to the pnmary throughput, decreasmg the primary throughput to obtam a maximum system bandwidth, wherein during operation the maximum system bandwidth sets an upper limit on admission of multimedia stream requesters to the storage system. 2 The method as recited in claim 1, wherein during operation additional multimedia stream requesters are not accepted if their requested maximum stream rate causes the aggregate of maximum stream rates for all streams to exceed the maximum system bandwidth. 3 The method as recited in claim 1, wherein said characterizing a maximum sustainable throughput comprises creating a synthetic load of data blocks in the storage system, wherem said creatmg a synthetic load compnses allocating blocks randomly m inner and outer storage disk zones, wherem said allocatmg blocks randomly alternates between the inner and outer zones; and determining said maximum sustamable throughput usmg said synthetic load 4 The method as recited in claim 1 , wherem durmg operation, requests to read or wnte multimedia data from or to the storage system are placed m one or more queues, wherem the length of the queues is determmed accordmg to the pnmary throughput 5 The method as recited m claim 4, wherem the queues are deadlme queues, wherein requests are satisfied from the deadlme queues accordmg to an earliest deadlme, wherem deadlmes are calculated for each request to prevent buffer underrun 6 The method as recited m claim 1, wherem said determining sizes for buffer units compnses generating a number of stream simulators, each at the same stream rate, sufficient to consume the entire pnmary throughput, each stream simulator generatmg stream requests from a model buffer unit; adjustmg the size each model buffer units up or down dependmg on whether or not an underrun condition occurs; and repeatmg said generating stream requests and said adjustmg until an optimum buffer unit size is found that just avoids an underrun condition 7 The method as recited m claim 6, wherem said determining sizes for buffer units further compnses creatmg a table of buffer unit sizes for different stream rates by repeatmg said generating a number of stream simulators, said generating stream requests, said adjustmg, and said repeating said generatmg stream requests, until an optimum buffer unit size is found for each of a plurality of stream rates 8 The method as recited in claim 6, wherem said model buffer unit compnses a nng of buffers consumed by an associated multimedia stream requester one after another m a circular order, said buffer unit size compnses the number of buffers in the rmg of buffers 9 The method as recited m claim 8, wherem each buffer in each nng of buffers is equally sized to hold one block size of data, wherein data is accessed in the storage system according to said block size, and wherem said block size is configurable 10 The method as recited in claim 6, wherein said underrun condition occurs when one of said stream requests is not completed to its model buffer unit withm a prefill deadlme, wherem said prefill deadlme is dependent upon the buffer unit size and the stream rate 11 The method as recited in claim 10, wherem said prefill deadlme compnses a prefill margm and a buffer deadline, wherein said prefill margin is configurable 12 The method as recited in claim 11, wherein each model buffer unit comprises a nng of N buffers, wherem the buffer deadlme is calculated as current time + (N - 1) * buf time, wherein cuπent_tιme is a time reference for the system and buf_tιme is the minimum time m which a multimedia stream requester can consume one of the N buffers without exceedmg the stream rate 13 The method as recited m claim 1, further compπsmg configuring a block size, wherem multimedia data is accessed in the storage system according to the configured block size 14 The method as recited m claim 13, wherem said determining sizes for buffer units is performed accordmg to the configured block size, wherem each buffer unit compnses a nng of buffers accessed m a circular order, wherem each buffer is configured to store one block size of data 15 The method as recited m claim 1 , wherem said determining sizes for buffer units is performed accordmg to a prefill margm, wherem said prefill margm is configurable and specifies a margm to prevent buffer underrun 16. The method as recited m claim 1 , further compnsmg allocatmg a portion of the primary throughput to rate- guaranteed storage accesses the remaining portion to non-rate-guaranteed storage accesses. 17 The method as recited m claim 16, wherem said allocatmg is performed accordmg to a configurable available rate parameter. 18 The method as recited in claim 16, wherein said decreasing the pnmary throughput compnses decreasmg the pnmary throughput accordmg to the portion allocated to rate-guaranteed storage accesses further decreased by a secondary derate parameter. 19 The method as recited in claim 1, wherem said decreasmg the pnmary throughput compnses decreasmg the primary throughput according to a secondary derate parameter. 20 The method as recited in claim 1, further compπsmg specifying a seek reorder queue length, wherein during operation all storage requests are ordered in said seek reorder queue in order of the requested physical address 21 The method as recited in claim 20, wherem said determmmg sizes for buffer units is performed according to the specified seek reorder queue length 22. A method for adjusting performance of a storage system, wherem the storage system mcludes a buffer nng for each data stream, wherein the buffer ring buffers data between a requester for the stream and one or more storage units, the method comprising. specifymg a primary derate parameter; determmmg buffer ring sizes for a plurality of different stream rates, wherem said determmmg comprises adjusting buffer rmg sizes to prevent underrun at a simulated load at a maximum throughput of the storage system derated by the primary derate parameter; and specifying an available rate parameter for allocatmg a portion of the storage system maximum throughput derated by the pnmary derate parameter to rate-guaranteed stream and a remaining portion to non-rate-guaranteed streams. 23. The method as recited in claim 22, further compnsmg specifymg a secondary derate parameter, wherem the system throughput portion specified by said available rate parameter is further derated by said secondary derate parameter to obtam a maximum aggregate bandwidth for all rate-guaranteed streams 24. The method as recited in claim 22, further compnsmg specifymg a prefill margm wherem said determmmg buffer nng sizes is performed accordmg to said prefill margm, wherem said prefill margm specifies a margm by which buffer underrun must be avoided duπng determmmg buffer nng sizes. 25 The method as recited in claim 22, further compnsmg specifying a seek reorder queue length, wherem durmg operation all storage requests are ordered m said seek reorder queue m order of the requested physical address. 26 The method as recited m claim 25 wherein said determmmg buffer nng sizes is performed accordmg to the specified seek reorder queue length 27 The method as recited in claim 22, further compπsmg specifymg a block size, wherem data is accessed m the storage units by said block size, and wherem each buffer in each buffer nng is sized to hold one block of data. 28 The method as recited in claim 27, wherem said determmmg buffer rmg sizes is performed using said block size 29 A storage system comprising a plurality of data stream clients, wherem a maximum stream rate is set for each of said plurality of data stream clients, a plurality of buffer rings, wherein each buffer nng is associated with a different one of said data stream clients and buffers data between its associated data stream client and one or more storage units, wherein each buffer nng is sized accordmg to a predetermmed set of buffer nng sizes for a plurality of different maximum stream rates; a bandwidth allocator configured allocate a portion of storage system bandwidth to rate-guaranteed streams from said plurality of data stream requesters and the remammg portion to non-rate- guaranteed requests, wherein said portion is determmed by a configurable available rate parameter; and wherein the storage system is configured to accept data stream requesters up to a maximum aggregate stream rate 30 The system as recited in claim 29, wherem each buffer nng size m said predetermmed set of buffer rmg sizes is calculated to prevent buffer underrun at one of said plurality of different maximum stream rates operatmg under a maximum storage system bandwidth derated by a configurable pnmary derate parameter. 31. The system as recited m claim 29, wherem said maximum aggregate stream rate is set accordmg to configurable parameters compnsmg- a configurable pnmary derate factor for deratmg a maximum storage system bandwidth; and said available rate parameter, wherein said maximum aggregate stream rate coπesponds to the maximum storage system bandwidth derated by said pnmary derate parameter and proportioned by said available rate parameter 32 The system as recited m claim 29, wherem said maximum aggregate stream rate is set as a maximum storage system bandwidth derated by a configurable pnmary derate parameter, proportioned accordmg to said available rate parameter, and further derated by a configurable secondary derate parameter 33 The system as recited in claim 32, wherem the system is configured to determine said maximum storage system bandwidth by creatmg a synthetic load of data blocks m the one or more storage units, wherem said creating a synthetic load compnses allocatmg blocks randomly m inner and outer storage disk zones, wherem said allocating blocks randomly alternates between the inner and outer zones, and determmmg said maximum storage bandwidth usmg said synthetic load 34 The system as recited in claim 29, wherem each buffer rmg size m said predetermmed set of buffer nng sizes is calculated to prevent buffer underrun by a configurable prefill margin at one of said plurality of different maximum stream rates 35 The system as recited in claim 29, wherem data is accessed in the storage system accordmg to a configurable block size 36 The system as recited in claim 29, wherem each buffer nng size m said predetermmed set of buffer nng sizes is calculated to prevent buffer underrun at one of said plurality of different maximum stream rates, wherein data is transfeπed between the buffer rings and said one or more storage units according to a configurable block 37 The system as recited in claim 29, further compnsmg a seek reorder queue for each said one or more storage units, wherein each seek reorder queue is configured to queue data requests one of the storage units in order of the physical address of the request in the storage unit, wherem the length of the seek reorder queue is determmed by a configurable seek reorder queue length parameter

Inventors: DEMONEY MICHAEL A, DEMONEY, MICHAEL, A.
Applicants: SUN MICROSYSTEMS INC, SUN MICROSYSTEMS, INC.
Application year: 2000
Main classification: H04N5
IPC codes: H04N5