Abstract:
In an example of an embodiment of the invention, a system for recording data generated by a client server and transmitted to a storage system is provided. The system comprises a storage system and a processor located remotely from the storage system and linked to the storage system via a network. The processor determines that a selected data processing operation is to be performed with respect to data stored in the storage system, and determines that a record of at least some of the data stored in the storage system is required prior to performing the selected data processing operation. The processor also generates a command comprising a request to generate a record of the at least some of the stored data, and transmits the command to the storage system to generate the record, via the network in accordance with Internet Protocol (IP). Examples of other systems and methods are also disclosed.

Description:
The present application is a continuation of U.S. application Ser. No. 11/652,412, filed on Jan. 11, 2007, now U.S. Pat. No. 7,467,259, which is incorporated by reference herein and is a division of U.S. application Ser. No. 10/612,081, filed on Jul. 2, 2003, now U.S. Pat. No. 7,165,145, both of which are is assigned to the assignee of the present application. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to a system and method for storing data, and more particularly, to a method and system for performing snapshots of data stored in a storage system. 
     BACKGROUND OF THE INVENTION 
     Generating a snapshot of data is a method commonly used to preserve and protect data stored in a storage system. A snapshot is a record of data stored in a storage system at a selected moment in time. A snapshot may be used, for example, to recover an earlier version of the data in the event a current version becomes corrupted, or may be copied to another storage device to provide a backup copy of the data. 
     In many storage systems, performing a snapshot is a relatively simple operation. However, if a cache is used in the storage system, performing the snapshot often becomes more challenging. Typically, a cache functions as a buffer, i.e., data sent by the client server to disk is recorded first in the cache, and subsequently is flushed to the disk. A cache is typically constructed of a type of memory that can be accessed more rapidly than the primary storage devices in the system. Accordingly, a storage system can increase its efficiency by temporarily storing in the cache selected items of data that are likely to be requested by a client. Many storage systems, operating on the principle that data recently sent to disk have a high probability of being accessed again, uniformly transmit data to a cache before sending it to disk. As used herein, the term “disk” has the same meaning as “storage device;” accordingly, “stored on disk” means stored on a storage device, “sent to disk” means sent to a storage device, etc. “Client” and “client server” are used herein interchangeably. 
     In many storage systems, a cache processes data sequentially to preserve data integrity. Changes made by a client server to a data file, for example, are recorded in the cache in the order they were performed and flushed to disk in the same order. Where a cache receives data from multiple clients, the cache typically processes data from a given client sequentially; however, the cache may assign a higher priority to data received from one client over data received from other clients. In such a case, the cache may not adhere to a strict first-in-first-out mode of operation. 
     To preserve data integrity, a snapshot must capture and preserve all data sent to disk by the client server up to the precise moment the snapshot is requested. However, where a cache is used, some data sent to disk by a client before the snapshot request may remain in the cache, not yet having been flushed to disk, at the moment the snapshot request is made. In such case, performing a snapshot of data stored on disk at the moment the request is made would produce an inaccurate record because it would not capture the data remaining in the cache at that moment. To produce an accurate snapshot, it is necessary to incorporate the data from the cache into the snapshot. 
     One solution to this problem requires first directing the client server to suspend transmission of data to the storage system. All data in the cache is then flushed to disk, and finally the snapshot is performed. This method may be adequate if very little data is present in the cache at the moment the snapshot is requested. Otherwise, this method is often undesirable because the client server is required to wait for the data flush to finish before it can resume normal operations. This can represent a substantial inconvenience to the client server and compromise the efficiency of the data storage provider. 
     It is often useful to generate a snapshot of data before performing a data processing task that poses a risk of corrupting data in a storage system. For example, in many systems simply maintaining a backup copy of a primary disk can occasionally pose substantial risks. If, for example, the system employs asynchronous mirroring, i.e., a cache is used to temporarily store data written to the primary disk before writing to the mirroring disk, an interruption in the communication between the cache and the mirroring disk can cause data to be lost and the backup copy to become corrupted. Generally, in such case it is necessary to synchronize the mirroring disk with the primary disk, i.e., simply copy data sector-by-sector from the primary disk to the mirroring disk. However, if the primary disk becomes corrupted before the copying procedure is completed, then there may be no uncorrupted version of the primary disk left. Moreover, in such case, the data on the mirroring disk is often corrupted by an incomplete copy procedure. It is therefore often preferable to generate a snapshot of the mirroring disk immediately before attempting to synchronize the mirroring disk with the main disk. 
     Many existing storage systems fail to determine a suitable moment for taking a snapshot. This is partly due to the fact that, in many networks, a client server (such as a data server) manages the data processing and storage functions, and storage systems merely process requests received from the client server. Therefore, the client server, rather than the storage system, determines an appropriate time for a snapshot. However, in prior art systems, the client server does not have the capability to direct the storage system to perform a snapshot at a selected moment. Instead, many storage systems are configured simply to perform snapshots at predetermined intervals, e.g., every 30 minutes. 
     SUMMARY OF THE INVENTION 
     Accordingly, there is a need for a method and system for performing snapshot copies of data stored in a data storage system in a manner that captures data transmitted to disk by a client server prior to the snapshot request without requiring the client server to cease sending data to the storage system. There is additionally a need for a system that enables a client server to direct a storage system on a network to perform a snapshot. 
     Embodiments of the present methods and systems provide for generating a snapshot that records desired data, while allowing a client server to continue sending data to the storage system. In accordance with one embodiment of the invention, a system for recording data generated by a client server and transmitted to a storage system is provided. The system comprises a storage system configured to store data, and at least one processor located remotely from the storage system and linked to the storage system via a network. The at least one processor is configured to determine that a selected data processing operation is to be performed with respect to data stored in the storage system and determine that a record of at least some of the data stored in the storage system is required prior to performing the selected data processing operation. The at least one processor is also configured to generate a command comprising a request to generate a record of the at least some of the stored data, and to transmit the command to the storage system to generate the record, via the network in accordance with Internet Protocol (IP). 
     The network may comprise a WAN, a LAN, a Fibre-Channel-based SAN, an interne, or an intranet. In one example, the at least one processor is configured to transmit the command to the storage system periodically. In another example, the at least one processor is configured to transmit the command to the storage system aperiodically. The at least one processor may be further configured to perform the selected data processing operation, after the requested record of the stored data is generated. The selected data processing operation may comprise synchronizing two storage systems. The at least one processor may be configured to generate a command comprising a request to generate a snapshot of the at least some of the stored data. 
     In accordance with another embodiment of the invention, a system is provided to store data, comprising at least one storage device configured to store data, and a cache memory configured to store data items. The system also comprises an interface configured to receive a first request to store one or more data items, and to receive a second request to perform a data processing operation. The system also comprises at least one processor configured to insert the one or more data items into the cache memory, after the first request is received, insert a second data item into the cache, after the second request is received, flush data from the cache memory to the at least one storage device, after inserting the second data item, and perform the requested data processing operation after the second data item has been flushed from the cache memory to the at least one storage device. 
     In one example, the processor is further configured to determine that the second data item has been flushed from the cache memory to the at least one storage device, and to perform the requested data processing operation after it is determined that the second data item has been flushed from the cache memory to the at least one storage device. The processor may also be configured to generate a record of data stored in the at least one storage device, after the second data item has been flushed from the cache memory to the at least one storage device. The processor may be further configured to generate a snapshot of data stored in the at least one storage device, after the second data item has been flushed from the cache memory to the at least one storage device. In another example, the processor is configured to back up data stored in the at least one storage device, after generating the snapshot of data. 
     In accordance with another embodiment of the invention, a system is provided to store data, comprising at least one storage device configured to store data, and a memory configured to maintain a queue comprising one or more data items. The system also comprises an interface configured to receive a request to generate a record of specified data stored in the at least one storage device. The system further comprises at least one processor configured to insert into the queue a marker indicating that a record of the specified data is to be generated, in response to the request, flush the one or more data items from the memory, flush the marker from the memory, and generate a record of the specified data, in response to flushing the marker from the memory. 
     In one example, the interface is configured to receive a request to generate a snapshot of specified data stored in the at least one storage device, and the processor is configured to insert into the queue a marker indicating that a snapshot of the specified data is to be generated, in response to the request, and to generate a snapshot of the specified data, in response to flushing the marker from the memory. The request may comprise a SCSI command. The request may further comprises a SCSI command to generate a snapshot of the specified data. The memory may comprises a cache memory associated with the at least one storage device. The marker may comprise a predetermined data pattern. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: 
         FIG. 1  illustrates a network for generating snapshots in accordance with one implementation of the invention; 
         FIG. 2  illustrates components of a storage system in accordance with one implementation of the invention; 
         FIG. 3A  illustrates the contents of a segment descriptor in accordance with one implementation of the invention; 
         FIG. 3B  illustrates a free segment list, a segment descriptor, and a segment map used to manage data in accordance with a dynamic disk space allocation technique; 
         FIGS. 4A-4C  show the contents of a cache at various points during the process of performing a snapshot in accordance with a prior art technique; 
         FIG. 5  is a flowchart describing a method for performing a snapshot in accordance with one implementation of the invention; 
         FIGS. 6A-6D  show the contents of a cache at various points during the process of performing a snapshot in accordance with an aspect of the invention; 
         FIG. 7  is a block diagram showing the relationship of SCSI bus phases; 
         FIG. 8  shows a system for generating a SCSI command; and 
         FIG. 9  is a flowchart showing a method for transmitting a command representing a snapshot request in accordance with one implementation of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a network for generating snapshots in accordance with an aspect of the invention. Client servers  140 - 1 ,  140 - 2  and  140 - 3  are connected over network  150  to storage system  210 . Each of client servers  140 - 1 ,  140 - 2  and  140 - 3  may be any device or software application that accesses data from storage system  210 . In one implementation, client servers  140  are server systems that provide software applications and services for users. In one implementation, each client server is a software application executing on a distinct machine, e.g., a computer. For example, according to this implementation, client server  140 - 1  is a file server application running on a workstation, client server  140 - 2  is an email server application running on a desktop PC, etc. In an alternative implementation, client servers  140 - 1 ,  140 - 2  and  140 - 3  all reside and execute on a single computer. It should be noted that, although for purposes of illustration, a file server, an email server and a remote access server are shown in  FIG. 1 , client servers  140 - 1 ,  140 - 2  and  140 - 3  may be implemented as server applications of any type. It should also be noted that, although for purposes of illustration, three client servers are shown, any number of client servers may be included. 
     Network  150  may be implemented as any one of a number of different types of networks. In accordance with one implementation, communications are conducted over network  150  by means of IP protocols. In accordance with another implementation, communications are conducted over network  150  by means of Fibre Channel protocols. Thus, network  150  may be, for example, an intranet, a local area network (LAN), a wide area network (WAN), an internet, Fibre Channel storage area network (SAN) or Ethernet. Alternatively, network  150  may be implemented as a combination of different types of networks. 
       FIG. 2  illustrates components of storage system  210  used for storing data in accordance with an aspect of the invention. Storage system  210  includes interface  218 , storage system manager  220  and storage devices  250 - 1 ,  250 - 2  and  250 - 3 . It should be noted that although three storage devices are shown in  FIG. 2 , storage system  210  may include any number of storage devices. 
     Interface  218  provides a communication gateway through which data may be transmitted between storage system  210  and network  150 . Interface  218  may be implemented using a number of different mechanisms, such as one or more SCSI cards, enterprise systems connection cards, fiber channel interfaces, modems, network interfaces, or a network hub. 
     Storage system manager  220  comprises a controller  320 , memory  324  and cache  340 . Controller  320  orchestrates the operations of storage system manager  220 , including receiving and processing data processing requests from network  150 , and sending data processing commands to storage devices  250 - 1 ,  250 - 2  and  250 - 3 . Controller  320  receives from network  150  data processing requests formatted according to IP or Fibre Channel protocols. 
     Communications between controller  320  and storage devices  250  are conducted in accordance with SCSI protocols. Accordingly, controller  320  transmits data processing commands to, and receives data from, storage devices  250 , in accordance with SCSI protocols. In one implementation, controller  320  is implemented by a software application. In an alternative implementation, controller  320  is implemented by a combination of software and digital or analog circuitry. 
     Memory  324  is used by controller  320  to manage the flow of data to and from, and the location of data on, storage devices  250 . For example, controller  320  may store in memory  324  various tables indicating the locations and types of various items of data stored in storage devices  250 . 
     Cache  340  functions as a buffer between controller  320  and storage devices  250 . Data assigned to an address on a storage device is first written in cache  340 , and subsequently flushed to disk. In one implementation, data written in cache is managed on a first-in-first-out basis, to ensure that changes made to a given data file are recorded in the correct order. Accordingly, data is written sequentially to cache  340  and, similarly, data is flushed from cache  340  to disk in sequential order. In alternative implementations, cache  340  does not maintain a strict first-in-first-out mode of operation, but instead processes data according to priorities established by controller  320 . For example, in one such implementation, cache  340  receives data from more than one client, but assigns a higher priority to data received from a specified client than to data received from other clients. Nevertheless, in this implementation, cache  340  maintains a first-in-first out mode of operation with respect to data received from any given client. Cache  340  may store data either in volatile or by non-volatile memory. 
     Each of storage devices  250  may be any type of storage device that allows block-level storage access. In one implementation, storage devices  250  are disk drives. A disk drive includes one or more disks having a medium upon which information may be written. Each disk includes a number of physical tracks, each of which, in turn, is divided into one or more physical blocks. Accordingly, in this implementation, an address identifying the location of a data block on a disk drive may specify a disk, a physical track and a physical block. Storage devices  250 - 1 ,  250 - 2  and  250 - 3  are connected to storage system manager  220 , in accordance with this implementation, by Fibre Channel interfaces, SCSI connections, or a combination thereof. 
     One implementation by which storage system manager  220  allocates disk space for storing data is described below as an example. It should be noted, however, that this implementation is for illustrative purposes only and that other techniques for allocating disk space may be used. 
     Storage system manager  220  may dynamically allocate disk space according to a technique that assigns disk space to a virtual disk drive as needed. The dynamic allocation technique functions on a drive level. In such instances, disk drives that are managed by storage system manager  220  are defined as virtual drives. The virtual drive system allows an algorithm to manage a “virtual” disk drive having assigned to it an amount of virtual storage that is larger than the amount of available physical storage. Accordingly, large disk drives can virtually exist on a system without requiring an initial investment of an entire storage subsystem. Additional storage may then be added as it is required without committing these resources prematurely. Alternatively, a virtual disk drive may have assigned to it an amount of virtual storage that is smaller than the amount of available physical storage. 
     According to this implementation, when storage system manager  220  initially defines a virtual storage device, or when additional storage is assigned to the virtual storage device, the disk space on the storage devices is divided into segments. Each segment has associated with it segment descriptors, which are stored in a free list table in memory. Generally, a segment descriptor contains information defining the segment it represents; for example, the segment descriptor may define a home storage device location, physical starting sector of the segment, sector count within the segment, and segment number.  FIG. 3A  illustrates schematically the contents of a segment descriptor  32 . Fields  32 - 1  through  32 - 5  contain data indicating, respectively, on which storage device the segment is located, the segment&#39;s starting physical address, the segment&#39;s size, a segment identifier, and other information which may be useful for identifying and processing the segment. 
     Referring to  FIG. 3B , as segments are needed to store data, the next available segment descriptor, e.g., segment descriptor  32 , is identified from the free segment list  50 , the data is stored in the segment, and the segment descriptor  32  is assigned to a new table called a segment map  66 . The segment map  66  maintains information representing how each segment defines the virtual storage device. More specifically, the segment map provides the logical sector to physical sector mapping of a virtual storage device. After the free segment descriptor  32  is moved or stored in the appropriate area of the segment map  66 , which in this example is slot  2  ( 70 ), the descriptor is no longer a free segment but is now an allocated segment. A detailed description of this method for dynamically allocating disk space can be found in U.S. Pat. No. 7,058,788, dated Jun. 6, 2006, which is incorporated herein by reference in its entirety. 
     To preserve the contents of selected data files at a selected moment in time, storage systems commonly generate snapshots of the selected data. If a cache is used, some data sent to disk by a client immediately before the snapshot request may remain in the cache, not yet having been flushed to disk, at the moment the snapshot request is made. All such data must be incorporated into the snapshot in order to produce an accurate record. 
     One technique for performing snapshots that is known in the art entails first directing the client server to suspend transmissions of data to disk. Data in the cache is then flushed to disk, and then the snapshot is performed. This prior art method is illustrated with reference to  FIGS. 4A-4C . In this example, all data items inserted in cache  340  originate from a single client server, e.g., file server  140 - 1 .  FIG. 4A  illustrates the contents of cache  340  at the moment controller  320  determines that a snapshot of data in one or more storage devices is requested. At that moment, cache  340  contains a queue made up of data items  530 - 534 . It should be noted that the direction arrow  325  represents data transmitted to cache  340  for entry into the queue. The direction arrow  245  represents data flushed from cache  340 . Referring to  FIG. 4A , data item  530  is the last data item in the queue, indicating that it was the last data item transmitted to cache  340 . After controller  320  determines that a snapshot has been requested, controller  320  stops transmitting data to cache  340  and waits until cache  340  flushes all of its contents to storage devices  250 . If the snapshot request was received from a client server, e.g., file server  140 - 1 , controller  320  directs the file server to temporarily cease transmitting data to disk.  FIG. 4B  illustrates schematically the contents of cache  340  after data items  531 - 534  have been flushed to disk, but before data item  530  has been flushed to disk.  FIG. 4C  illustrates schematically the contents of cache  340  after data item  530  has been flushed to disk. Cache  340  is now empty. At this point, controller  320  detects that cache  340  is empty and performs the snapshot. Controller  320  notifies file server  140 - 1  that it may continue transmitting data to disk. 
     It should be noted that  FIGS. 4A-4C  are figurative illustrations. The data items shown in  FIGS. 4A-4C  are presented as if they are moving from left to right to illustrate their position in the queue. However, inside a cache in a computer, there is no physical moment of data; instead, transfers of data are accomplished by electrical signals. It should further be noted that although in the example illustrated by  FIGS. 4A-4C  cache  340  receives data from only one client server, in many storage systems a cache receives data from multiple client servers. 
     This method of performing snapshots, as described with reference to  FIGS. 4A-4C , may impose considerable inconveniences on the client server as well as on the storage provider. For example, the client server is required to wait for the data flush to finish before it can resume normal operations. Such inconveniences may be overcome by generating a snapshot of data in response to a snapshot request, while ensuring that the snapshot incorporates all data sent to disk prior to the time the snapshot request is made. In addition, such inconveniences may be overcome by eliminating the need to suspend the transmission of data between the client server and storage system while a snapshot operation is being performed. 
     In accordance with one aspect of the invention, upon determining that a snapshot has been requested, controller  320  of storage system manager  220  inserts a snapshot marker into cache  340 . While the snapshot marker remains in cache  340 , storage system manager continues to flush data from cache  340  to disk. Once the snapshot marker is flushed to disk, the snapshot is performed. 
       FIG. 5  is a flowchart describing a method for performing a snapshot, in accordance with this aspect of the invention. In addition, this method is illustrated with reference to  FIGS. 6A-6D . At step  508 , controller  320  determines that a snapshot should be made. Any one of several different types of events may prompt this determination. For example, controller  320  may receive a snapshot request from a client server, e.g., file server  140 - 1 . Alternatively, controller  320  may determine that the normal operation of the storage system dictates that a snapshot should be made, e.g., controller  320  may be programmed to periodically take snapshots of selected blocks of data stored in storage devices  250 - 1 ,  250 - 2  and  250 - 3 . 
     At step  510 , controller  320  generates a snapshot marker. The snapshot marker is information that identifies the last item of data that is to be sent to disk before the snapshot is performed. According to one implementation, the snapshot marker comprises a data pattern that is statistically unlikely to occur in transmissions that are received by storage system  210 . For example, a snapshot marker may be implemented by the data string: 
     “$$$$SNAPSHOTMARKER****.” 
     At step  520 , controller  320  inserts the snapshot marker into cache  340 . Because data is written sequentially to cache  340 , the snapshot marker is inserted at the end of a queue maintained in cache  340 . The other data items in the queue, if any, were written to cache  340  before the snapshot marker was inserted. Because these data items were sent to disk before the snapshot request was made, it is necessary to incorporate them into the snapshot in order to produce an accurate record.  FIG. 6A  shows the contents of cache  340  at the moment snapshot marker  615  is inserted. Cache contains data items  1 - 5  and snapshot marker  615 . Data items  1 - 5  represent data inserted into cache  340  before snapshot marker  615  was inserted. Accordingly, data items  1 - 5  represent data sent to disk before the snapshot request was made. 
     It should be noted at this point that in the implementation illustrated by  FIGS. 6A-6D , cache  340  operates on a first-in-first-out basis, i.e., all data sent to disk are sent first to cache and subsequently flushed to disk in the same order. However, other implementations may not operate in this manner. For example, a storage system that receives data from two different data servers may cache all data that is received, but assign a higher priority to data received from one of the servers. Accordingly, the system may first flush to disk any data from the high-priority data server that is present in the cache before flushing data from the other data server. 
     It should be further noted that the frequency with which data is flushed from cache  340  may differ depending on the particular implementation, and is typically not affected by the insertion into cache  340  of a snapshot marker. In one implementation, data is flushed to disk periodically at predetermined intervals. 
     At step  530 , controller  320  flushes data from cache  340 .  FIG. 6B  illustrates schematically the contents of cache  340  after data items  1 - 3  have been flushed to disk. At this point, cache  340  contains data items  4 - 5 , snapshot marker  615  and data items  6 - 8 . Data items  6 - 8  represent data items that were inserted into cache  340  after the snapshot request was made. 
     While the snapshot marker  615  remains in cache  340 , controller  320  continues to flush data from cache  340  to disk. Referring to  FIG. 6B , snapshot marker  615  remains in cache  340 . Accordingly, controller  320  continues to flush data to disk.  FIG. 6C  shows the contents of cache  340  after data items  4 - 5  have been flushed to disk. The cache now contains snapshot marker  615  and data items  6 - 10 , i.e., items that were added to cache after the snapshot request was made. 
     Because snapshot marker  615  continues to remain in cache  340 , controller  320  continues to flush data to disk. At step  535 , snapshot marker is flushed from cache to disk.  FIG. 6D  illustrates schematically the contents of cache  340  at the moment snapshot marker  615  has been flushed to disk. In this example, cache  340  contains data items  6 - 11 . At this point, snapshot marker  615  is no longer in cache  340 . Accordingly (referring to step  540 ), controller  320  immediately performs the requested snapshot. 
     SCSI Command 
     According to another aspect of the invention, a client server transmits a command representing a snapshot request to a storage system. The command is transmitted in accordance with standard protocols. The storage system receives the command and, in response, performs a snapshot. 
     It is often useful to generate a snapshot of data before performing a data processing task that poses a risk of corrupting data in a storage system. For example, in an asynchronous mirroring arrangement, it is often preferable to generate a snapshot of the mirroring disk before attempting to synchronize the mirroring disk with the main disk. In such a case, it is useful to automatically generate a snapshot immediately before the synchronization process begins. 
     According to one implementation, a client server, e.g., file server  140 - 1 , transmits to storage system  210  a SCSI command that represents a snapshot request. The Small Computer System Interface (SCSI) is a standard for connecting computers to peripheral devices such as disk drives. The SCSI standard specifies the hardware and software interface between these devices at a level that minimizes dependencies on any specific hardware implementation. This is achieved by representing data stored on a storage device as a collection of logical blocks rather than in terms of the data&#39;s physical address. This abstraction allows the SCSI protocol to be used with a wide variety of devices. 
     The central item of hardware in a SCSI system is the SCSI bus, which must conform to the specification of the SCSI standard. A SCSI device refers to any device that is connected to the SCSI bus. Each SCSI device on the bus is assigned a SCSI ID that uniquely identifies the device during SCSI transactions. 
     The SCSI standard also establishes a set of protocols by which SCSI devices may communicate, or transact. When two SCSI devices communicate, one device acts as the initiator and the other as the target. The initiator begins a transaction by selecting a target device. The target responds to the selection and requests a command. The initiator then sends a SCSI command directing the target to perform a specified action, and the target performs the action. 
     SCSI transactions comprise up to eight distinct phases: bus free, arbitration, selection, reselection, command, data, status and message.  FIG. 7  is a block diagram showing the relationship of the SCSI bus phases. The bus free phase  815  indicates that no SCSI devices are using the SCSI bus. During arbitration phase  820 , a SCSI device may gain control of the SCSI bus. All devices requesting to use the bus assert their SCSI ID by transmitting a signal onto the bus. The device having the highest ID wins the arbitration and becomes the initiator for the next SCSI transaction. During selection phase  825 , the initiator selects a target device. The optional reselection phase (not shown) allows a peripheral that is busy performing a lengthy data processing action to disconnect from and subsequently reconnect to the SCSI bus. 
     The last four phases (command, data, status and message) are referred to collectively as the information transfer phases, and are represented in  FIG. 7  by block  840 . During these phases, data can be transferred between the initiator and target devices. During the command phase, a target device requests a command from the initiator, and in response, the initiator transfers a SCSI command to the target. A SCSI command is an instruction from an initiator to a target to perform an action, such as reading or writing a block of data. SCSI commands are contained in a data structure called a command descriptor block, which can be 6, 10 or 12 bytes in size. The first byte specifies the action requested, and the remaining bytes are parameters used for that action. The target can determine from the first command byte transmitted how many command bytes will follow. 
     During the data phase, data is transferred between the initiator and the target. To indicate that it is ready to transmit data to the initiator, the target transmits an input/output (I/O) signal indicating DATA IN. To indicate that it is ready to receive data, the target transmits a DATA OUT signal. 
     During the message phase, the target requests the transfer of a message. The SCSI standard specifies a number of possible messages that may be exchanged between SCSI devices. SCSI messages must conform to the structure specified by the SCSI standard and generally carry information about a SCSI transaction. The status phase occurs after completion of all commands and allows the target to transmit a status signal to the initiator. For example, the target may send status information indicating that an operation completed successfully, or that an error occurred. After a transaction is completed, the bus returns to the bus free phase  815 . 
     According to one aspect of the invention, a client server, in the role of initiator, sends a SCSI command to a storage system, acting as target, indicating that a snapshot should be performed.  FIG. 8  shows system  710  for generating a SCSI command, in accordance with this implementation. System  710  is a computer, e.g., a workstation or a desktop PC. Computer  710  includes a processor  730 , memory  740  and client server  720 . Computer  710  is connected to network  150  along path  745  (as shown in  FIG. 1 ). Path  745  is a connector that allows computer  710  to transmit SCSI commands via network  150  to storage system  210  (as shown in  FIG. 1 ). 
     Client server  720  executes on computer  710 . Client server  720  is, typically, a software application that provides software services and data to users. Referring to  FIG. 1 , client server  720  may be, for example, file server  140 - 1 , email server  140 - 2  or remote access server  140 - 3 . 
     In accordance with this implementation, client server  720  generates data processing requests for storage system  210  in accordance with SCSI protocols. Data processing requests are transmitted across network  150  in accordance with IP or Fibre Channel protocols, to storage system  210 . 
       FIG. 9  is a flowchart showing a method, in accordance with one implementation of the invention, for transmitting a command to a storage system indicating that a snapshot should be performed. At step  719 , client server  720  determines that a snapshot is required. A variety of different conditions may prompt client server  720  to make this determination. Client server  720  may, for example, determine that a snapshot of a mirroring disk should be generated before synchronizing a mirroring disk with a primary disk. Alternatively, client server  720  may be prompted by a human administrator to initiate a snapshot. In yet another alternative, client server  720  may be a word processing application that automatically requests a snapshot under specified conditions. 
     At step  725 , client server  720  initiates a SCSI transaction with storage system  210 . Referring to  FIG. 7 , client server  720  arbitrates to use a SCSI bus in arbitration phase  820 , and selects a target device in selection phase  825 . In accordance with one implementation, client server  720  selects storage system manager  220  as the target. Alternatively, client server  720  may select a storage device as the target. 
     At step  732 , client server  720  generates a SCSI command that represents a snapshot request. In accordance with this implementation, a customer-defined SCSI command is utilized to indicate that a snapshot be performed. Accordingly, client server  720  generates a command descriptor block (CDB) of 6, 10 or 12 bytes in size. 
     At step  742 , client server  720  transmits the SCSI command over network  150  to storage system  210 . Referring to  FIG. 7 , client server  720  transmits the SCSI command to storage system  210  during the command phase, which occurs during information transfer phases  840 . 
     At step  750 , storage system manager  220  receives the SCSI command and performs the requested snapshot. According to one implementation, storage system manager  220  receives the SCSI command and transmits a snapshot marker to cache  340  as described above. The snapshot is taken once the snapshot marker is flushed to disk. 
     In an alternative implementation, a storage system without a cache is employed. Client server  720  transmits the SCSI command to the storage system, which performs a snapshot by an alternative method.