Source: http://www.google.com/patents/US20050198454?ie=ISO-8859-1
Timestamp: 2014-07-24 13:08:25
Document Index: 97564210

Matched Legal Cases: ['art 140', 'art 140', 'art 200', 'art 270', 'art 300', 'art 300', 'art 350', 'art 440', 'art 440', 'art 500', 'art 540', 'art 540', 'art 540', 'art 560', 'art 560', 'art 600', 'art 300', 'art 650', 'art 560', 'art 700', 'art 700', 'art 720', 'art 350', 'art 720', 'art 350', 'art 720', 'art 350', 'art 720', 'art 740', 'art 140', 'art 740', 'art 800']

Patent US20050198454 - Switching between virtual ordered writes mode and synchronous or semi ... - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign in<nobr>Advanced Patent Search</nobr>PatentsTransitioning from a first data transfer mode to a second data transfer mode that is different from the first data transfer mode includes a primary storage device receiving a plurality of data writes while being in the first data transfer mode, the primary storage device associating data writes begun...http://www.google.com/patents/US20050198454?utm_source=gb-gplus-sharePatent US20050198454 - Switching between virtual ordered writes mode and synchronous or semi-synchronous RDF transfer modeAdvanced Patent SearchPublication numberUS20050198454 A1Publication typeApplicationApplication numberUS 10/795,686Publication dateSep 8, 2005Filing dateMar 8, 2004Priority dateMar 8, 2004Also published asUS7228396Publication number10795686, 795686, US 2005/0198454 A1, US 2005/198454 A1, US 20050198454 A1, US 20050198454A1, US 2005198454 A1, US 2005198454A1, US-A1-20050198454, US-A1-2005198454, US2005/0198454A1, US2005/198454A1, US20050198454 A1, US20050198454A1, US2005198454 A1, US2005198454A1InventorsBenjamin Yoder, David Meiri, Vadim LonginovOriginal AssigneeEmc CorporationExport CitationBiBTeX, EndNote, RefManReferenced by (25), Classifications (8), Legal Events (2) External Links: USPTO, USPTO Assignment, EspacenetSwitching between virtual ordered writes mode and synchronous or semi-synchronous RDF transfer modeUS 20050198454 A1Abstract Transitioning from a first data transfer mode to a second data transfer mode that is different from the first data transfer mode includes a primary storage device receiving a plurality of data writes while being in the first data transfer mode, the primary storage device associating data writes begun after a first time and before a second time with a first chunk of data, the primary storage device associating data writes begun after the second time with a second chunk of data different from the first chunk of data, and the primary storage device transitioning to the second data transfer mode after the second time by first transferring writes associated with the first chunk of data to a secondary storage device after completion of all writes associated with the first chunk of data and then, after all writes associated with the first chunk of data have been transferred to the secondary storage device, transferring writes associated with the second chunk of data to the secondary storage device using the first data transfer mode, where writes begun after initiating transitioning are provided to the secondary storage device using the second data transfer mode. Transitioning from a first data transfer mode to a second data transfer mode that is different from the first data transfer mode may also include, following the primary storage device transferring all writes associated with the first chunk of data to the secondary storage device, the primary storage device sending a message to the secondary storage device, and, in response to receiving the message from the primary storage device, the secondary storage device storing the data writes associated with the first chunk of data. Images(21) Claims(36)
DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS Referring to FIG. 1, a diagram 20 shows a relationship between a host 22, a local storage device 24 and a remote storage device 26. The host 22 reads and writes data from and to the local storage device 24 via a host adapter (HA) 28, which facilitates the interface between the host 22 and the local storage device 24. Although the diagram 20 only shows one host 22 and one HA 28, it will be appreciated by one of ordinary skill in the art that multiple HA's may be used and that one or more HA's may have one or more hosts coupled thereto. Data from the local storage device 24 is copied to the remote storage device 26 via an RDF link 29 to cause the data on the remote storage device 26 to be identical to the data on the local storage device 24. Although only the one link 29 is shown, it is possible to have additional links between the storage devices 24, 26 and to have links between one or both of the storage devices 24, 26 and other storage devices (not shown). Note that there may be a time delay between the transfer of data from the local storage device 24 to the remote storage device 26, so that the remote storage device 26 may, at certain points in time, contain data that is not identical to the data on the local storage device 24. Communication using RDF is described, for example, in U.S. Pat. No. 5,742,792, which is incorporated by reference herein. The local storage device 24 includes a first plurality of RDF adapter units (RA's) 30 a, 30 b, 30 c and the remote storage device 26 includes a second plurality of RA's 32 a-32 c. The RA's 30 a-30 c, 32 a-32 c are coupled to the RDF link 29 and are similar to the host adapter 28, but are used to transfer data between the storage devices 24, 26. The software used in connection with the RA's 30 a-30 c, 32 a-32 c is discussed in more detail hereinafter. The storage devices 24, 26 may include one or more disks, each containing a different portion of data stored on each of the storage devices 24, 26. FIG. 1 shows the storage device 24 including a plurality of disks 33 a, 33 b, 33 c and the storage device 26 including a plurality of disks 34 a, 34 b, 34 c. The RDF functionality described herein may be applied so that the data for at least a portion of the disks 33 a-33 c of the local storage device 24 is copied, using RDF, to at least a portion of the disks 34 a-34 c of the remote storage device 26. It is possible that other data of the storage devices 24, 26 is not copied between the storage devices 24, 26, and thus is not identical. Each of the disks 33 a-33 c is coupled to a corresponding disk adapter unit (DA) 35 a, 35 b, 35 c that provides data to a corresponding one of the disks 33 a-33 c and receives data from a corresponding one of the disks 33 a-33 c. Similarly, a plurality of DA's 36 a, 36 b, 36 c of the remote storage device 26 are used to provide data to corresponding ones of the disks 34 a-34 c and receive data from corresponding ones of the disks 34 a-34 c. An internal data path exists between the DA's 35 a-35 c, the HA 28 and the RA's 30 a-30 c of the local storage device 24. Similarly, an internal data path exists between the DA's 36 a-36 c and the RA's 32 a-32 c of the remote storage device 26. Note that, in other embodiments, it is possible for more than one disk to be serviced by a DA and that it is possible for more than one DA to service a disk. The local storage device 24 also includes a global memory 37 that may be used to facilitate data transferred between the DA's 35 a-35 c, the HA 28 and the RA's 30 a-30 c. The memory 37 may contain tasks that are to be performed by one or more of the DA's 35 a-35 c, the HA 28 and the RA's 30 a-30 c, and a cache for data fetched from one or more of the disks 33 a-33 c. Similarly, the remote storage device 26 includes a global memory 38 that may contain tasks that are to be performed by one or more of the DA's 36 a-36 c and the RA's 32 a-32 c, and a cache for data fetched from one or more of the disks 34 a-34 c. Use of the memories 37, 38 is described in more detail hereinafter. The storage space in the local storage device 24 that corresponds to the disks 33 a-33 c may be subdivided into a plurality of volumes or logical devices. The logical devices may or may not correspond to the physical storage space of the disks 33 a-33 c. Thus, for example, the disk 33 a may contain a plurality of logical devices or, alternatively, a single logical device could span both of the disks 33 a, 33 b. Similarly, the storage space for the remote storage device 26 that comprises the disks 34 a-34 c may be subdivided into a plurality of volumes or logical devices, where each of the logical devices may or may not correspond to one or more of the disks 34 a-34 c. Providing an RDF mapping between portions of the local storage device 24 and the remote storage device 26 involves setting up a logical device on the remote storage device 26 that is a remote mirror for a logical device on the local storage device 24. The host 22 reads and writes data from and to the logical device on the local storage device 24 and the RDF mapping causes modified data to be transferred from the local storage device 24 to the remote storage device 26 using the RA's, 30 a-30 c, 32 a-32 c and the RDF link 29. In steady state operation, the logical device on the remote storage device 26 contains data that is identical to the data of the logical device on the local storage device 24. The logical device on the local storage device 24 that is accessed by the host 22 is referred to as the �R1 volume� (or just �R1�) while the logical device on the remote storage device 26 that contains a copy of the data on the R1 volume is called the �R2 volume� (or just �R2�). Thus, the host reads and writes data from and to the R1 volume and RDF handles automatic copying and updating of the data from the R1 volume to the R2 volume. Referring to FIG. 2, a path of data is illustrated from the host 22 to the local storage device 24 and the remote storage device 26. Data written from the host 22 to the local storage device 24 is stored locally, as illustrated by the data element 51 of the local storage device 24. The data that is written by the host 22 to the local storage device 24 is also maintained by the local storage device 24 in connection with being sent by the local storage device 24 to the remote storage device 26 via the link 29. In the system described herein, each data write by the host 22 (of, for example a record, a plurality of records, a track, etc.) is assigned a sequence number. The sequence number may be provided in an appropriate data field associated with the write. In FIG. 2, the writes by the host 22 are shown as being assigned sequence number N. All of the writes performed by the host 22 that are assigned sequence number N are collected in a single chunk of data 52. The chunk 52 represents a plurality of separate writes by the host 22 that occur at approximately the same time. Generally, the local storage device 24 accumulates chunks of one sequence number while transmitting a previously accumulated chunk (having the previous sequence number) to the remote storage device 26. Thus, while the local storage device 24 is accumulating writes from the host 22 that are assigned sequence number N, the writes that occurred for the previous sequence number (N−1) are transmitted by the local storage device 24 to the remote storage device 26 via the link 29. A chunk 54 represents writes from the host 22 that were assigned the sequence number N−1 that have not been transmitted yet to the remote storage device 26. The remote storage device 26 receives the data from the chunk 54 corresponding to writes assigned a sequence number N−1 and constructs a new chunk 56 of host writes having sequence number N−1. The data may be transmitted using appropriate RDF protocol that acknowledges data sent across the link 29. When the remote storage device 26 has received all of the data from the chunk 54, the local storage device 24 sends a commit message to the remote storage device 26 to commit all the data assigned the N−1 sequence number corresponding to the chunk 56. Generally, once a chunk corresponding to a particular sequence number is committed, that chunk may be written to the logical storage device. This is illustrated in FIG. 2 with a chunk 58 corresponding to writes assigned sequence number N−2 (i.e., two before the current sequence number being used in connection with writes by the host 22 to the local storage device 26). In FIG. 2, the chunk 58 is shown as being written to a data element 62 representing disk storage for the remote storage device 26. Thus, the remote storage device 26 is receiving and accumulating the chunk 56 corresponding to sequence number N−1 while the chunk 58 corresponding to the previous sequence number (N−2) is being written to disk storage of the remote storage device 26 illustrated by the data element 62. In some embodiments, the data for the chunk 58 is marked for write (but not necessarily written immediately), while the data for the chunk 56 is not. Thus, in operation, the host 22 writes data to the local storage device 24 that is stored locally in the data element 51 and is accumulated in the chunk 52. Once all of the data for a particular sequence number has been accumulated (described elsewhere herein), the local storage device 24 increments the sequence number. Data from the chunk 54 corresponding to one less than the current sequence number is transferred from the local storage device 24 to the remote storage device 26 via the link 29. The chunk 58 corresponds to data for a sequence number that was committed by the local storage device 24 sending a message to the remote storage device 26. Data from the chunk 58 is written to disk storage of the remote storage device 26. Note that the writes within a particular one of the chunks 52, 54, 56, 58 are not necessarily ordered. However, as described in more detail elsewhere herein, every write for the chunk 58 corresponding to sequence number N−2 was begun prior to beginning any of the writes for the chunks 54, 56 corresponding to sequence number N−1. In addition, every write for the chunks 54, 56 corresponding to sequence number N−1 was begun prior to beginning any of the writes for the chunk 52 corresponding to sequence number N. Thus, in the event of a communication failure between the local storage device 24 and the remote storage device 26, the remote storage device 26 may simply finish writing the last committed chunk of data (the chunk 58 in the example of FIG. 2) and can be assured that the state of the data at the remote storage device 26 is ordered in the sense that the data element 62 contains all of the writes that were begun prior to a certain point in time and contains no writes that were begun after that point in time. Thus, R2 always contains a point in time copy of R1 and it is possible to reestablish a consistent image from the R2 device. Referring to FIG. 3, a diagram 70 illustrates items used to construct and maintain the chunks 52, 54. A standard logical device 72 contains data written by the host 22 and corresponds to the data element 51 of FIG. 2 and the disks 33 a-33 c of FIG. 1. The standard logical device 72 contains data written by the host 22 to the local storage device 24. Two linked lists of pointers 74, 76 are used in connection with the standard logical device 72. The linked lists 74, 76 correspond to data that may be stored, for example, in the memory 37 of the local storage device 24. The linked list 74 contains a plurality of pointers 81-85, each of which points to a slot of a cache 88 used in connection with the local storage device 24. Similarly, the linked list 76 contains a plurality of pointers 91-95, each of which points to a slot of the cache 88. In some embodiments, the cache 88 may be provided in the memory 37 of the local storage device 24. The cache 88 contains a plurality of cache slots 102-104 that may be used in connection to writes to the standard logical device 72 and, at the same time, used in connection with the linked lists 74, 76. Each of the linked lists 74, 76 may be used for one of the chunks of data 52, 54 so that, for example, the linked list 74 may correspond to the chunk of data 52 for sequence number N while the linked list 76 may correspond to the chunk of data 54 for sequence number N−1. Thus, when data is written by the host 22 to the local storage device 24, the data is provided to the cache 88 and, in some cases (described elsewhere herein), an appropriate pointer of the linked list 74 is created. Note that the data will not be removed from the cache 88 until the data is destaged to the standard logical device 72 and the data is also no longer pointed to by one of the pointers 81-85 of the linked list 74, as described elsewhere herein. In an embodiment herein, one of the linked lists 74, 76 is deemed �active� while the other is deemed �inactive�. Thus, for example, when the sequence number N is even, the linked list 74 may be active while the linked list 76 is inactive. The active one of the linked lists 74, 76 handles writes from the host 22 while the inactive one of the linked lists 74, 76 corresponds to the data that is being transmitted from the local storage device 24 to the remote storage device 26. While the data that is written by the host 22 is accumulated using the active one of the linked lists 74, 76 (for the sequence number N), the data corresponding to the inactive one of the linked lists 74, 76 (for previous sequence number N−1) is transmitted from the local storage device 24 to the remote storage device 26. The RA's 30 a-30 c use the linked lists 74, 76 to determine the data to transmit from the local storage device 24 to the remote storage device 26. Once data corresponding to a particular one of the pointers in one of the linked lists 74, 76 has been transmitted to the remote storage device 26, the particular one of the pointers may be removed from the appropriate one of the linked lists 74, 76. In addition, the data may also be marked for removal from the cache 88 (i.e., the slot may be returned to a pool of slots for later, unrelated, use) provided that the data in the slot is not otherwise needed for another purpose (e.g., to be destaged to the standard logical device 72). A mechanism may be used to ensure that data is not removed from the cache 88 until all devices are no longer using the data. Such a mechanism is described, for example, in U.S. Pat. No. 5,537,568 issued on Jul. 16, 1996 and in U.S. patent application Ser. No. 09/850,551 filed on Jul. 7, 2001, both of which are incorporated by reference herein. Referring to FIG. 4, a slot 120, like one of the slots 102-104 of the cache 88, includes a header 122 and data 124. The header 122 corresponds to overhead information used by the system to manage the slot 120. The data 124 is the corresponding data from the disk that is being (temporarily) stored in the slot 120. Information in the header 122 includes pointers back to the disk, time stamp(s), etc. The header 122 also includes a cache stamp 126 used in connection with the system described herein. In an embodiment herein, the cache stamp 126 is eight bytes. Two of the bytes are a �password� that indicates whether the slot 120 is being used by the system described herein. In other embodiments, the password may be one byte while the following byte is used for a pad. As described elsewhere herein, the two bytes of the password (or one byte, as the case may be) being equal to a particular value indicates that the slot 120 is pointed to by at least one entry of the linked lists 74, 76. The password not being equal to the particular value indicates that the slot 120 is not pointed to by an entry of the linked lists 74, 76. Use of the password is described elsewhere herein. The cache stamp 126 also includes a two byte field indicating the sequence number (e.g., N, N−1, N−2, etc.) of the data 124 of the slot 120. As described elsewhere herein, the sequence number field of the cache stamp 126 may be used to facilitate the processing described herein. The remaining four bytes of the cache stamp 126 may be used for a pointer, as described elsewhere herein. Of course, the two bytes of the sequence number and the four bytes of the pointer are only valid when the password equals the particular value that indicates that the slot 120 is pointed to by at least one entry in one of the lists 74, 76. Referring to FIG. 5, a flow chart 140 illustrates steps performed by the HA 28 in connection with a host 22 performing a write operation. Of course, when the host 22 performs a write, processing occurs for handling the write in a normal fashion irrespective of whether the data is part of an R1/R2 RDF group. For example, when the host 22 writes data for a portion of the disk, the write occurs to a cache slot which is eventually destaged to the disk. The cache slot may either be a new cache slot or may be an already existing cache slot created in connection with a previous read and/or write operation to the same track. Processing begins at a first step 142 where a slot corresponding to the write is locked. In an embodiment herein, each of the slots 102-104 of the cache 88 corresponds to a track of data on the standard logical device 72. Locking the slot at the step 142 prevents additional processes from operating on the relevant slot during the processing performed by the HA 28 corresponding to the steps of the flow chart 140. Following step 142 is a step 144 where a value for N, the sequence number, is set. As discussed elsewhere herein, the value for the sequence number obtained at the step 144 is maintained during the entire write operation performed by the HA 28 while the slot is locked. As discussed elsewhere herein, the sequence number is assigned to each write to set the one of the chunks of data 52, 54 to which the write belongs. Writes performed by the host 22 are assigned the current sequence number. It is useful that a single write operation maintain the same sequence number throughout. Following the step 144 is a test step 146 which determines if the password field of the cache slot is valid. As discussed above, the system described herein sets the password field to a predetermined value to indicate that the cache slot is already in one of the linked lists of pointers 74, 76. If it is determined at the test step 146 that the password field is not valid (indicating that the slot is new and that no pointers from the lists 74, 76 point to the slot), then control passes from the step 146 to a step 148, where the cache stamp of the new slot is set by setting the password to the predetermined value, setting the sequence number field to N, and setting the pointer field to Null. In other embodiments, the pointer field may be set to point to the slot itself. Following the step 148 is a step 152 where a pointer to the new slot is added to the active one of the pointer lists 74, 76. In an embodiment herein, the lists 74, 76 are circular doubly linked lists, and the new pointer is added to the circular doubly linked list in a conventional fashion. Of course, other appropriate data structures could be used to manage the lists 74, 76. Following the step 152 is a step 154 where flags are set. At the step 154, the RDF_WP flag (RDF write pending flag) is set to indicate that the slot needs to be transmitted to the remote storage device 26 using RDF. In addition, at the step 154, the IN_CACHE flag is set to indicate that the slot needs to be destaged to the standard logical device 72. Following the step 154 is a step 156 where the data being written by the host 22 and the HA 28 is written to the slot. Following the step 156 is a step 158 where the slot is unlocked. Following step 158, processing is complete. If it is determined at the test step 146 that the password field of the slot is valid (indicating that the slot is already pointed to by at least one pointer of the lists 74, 76), then control transfers from the step 146 to a test step 162, where it is determined whether the sequence number field of the slot is equal to the current sequence number, N. Note that there are two valid possibilities for the sequence number field of a slot with a valid password. It is possible for the sequence number field to be equal to N, the current sequence number. This occurs when the slot corresponds to a previous write with sequence number N. The other possibility is for the sequence number field to equal N−1. This occurs when the slot corresponds to a previous write with sequence number N−1. Any other value for the sequence number field is invalid. Thus, for some embodiments, it may be possible to include error/validity checking in the step 162 or possibly make error/validity checking a separate step. Such an error may be handled in any appropriate fashion, which may include providing a message to a user. If it is determined at the step 162 that the value in the sequence number field of the slot equals the current sequence number N, then no special processing is required and control transfers from the step 162 to the step 156, discussed above, where the data is written to the slot. Otherwise, if the value of the sequence number field is N−1 (the only other valid value), then control transfers from the step 162 to a step 164 where a new slot is obtained. The new slot obtained at the step 164 may be used to store the data being written. Following the step 164 is a step 166 where the data from the old slot is copied to the new slot that was obtained at the step 164. Note that that the copied data includes the RDF_WP flag, which should have been set at the step 154 on a previous write when the slot was first created. Following the step 166 is a step 168 where the cache stamp for the new slot is set by setting the password field to the appropriate value, setting the sequence number field to the current sequence number, N, and setting the pointer field to point to the old slot. Following the step 168 is a step 172 where a pointer to the new slot is added to the active one of the linked lists 74, 76. Following the step 172 is the step 156, discussed above, where the data is written to the slot which, in this case, is the new slot. Referring to FIG. 6, a flow chart 200 illustrates steps performed in connection with the RA's 30 a-30 c scanning the inactive one of the lists 72, 74 to transmit RDF data from the local storage device 24 to the remote storage device 26. As discussed above, the inactive one of the lists 72, 74 points to slots corresponding to the N−1 cycle for the R1 device when the N cycle is being written to the R1 device by the host using the active one of the lists 72, 74. Processing begins at a first step 202 where it is determined if there are any entries in the inactive one of the lists 72, 74. As data is transmitted, the corresponding entries are removed from the inactive one of the lists 72, 74. In addition, new writes are provided to the active one of the lists 72, 74 and not generally to the inactive one of the lists 72, 74. Thus, it is possible (and desirable, as described elsewhere herein) for the inactive one of the lists 72, 74 to contain no data at certain times. If it is determined at the step 202 that there is no data to be transmitted, then the inactive one of the lists 72, 74 is continuously polled until data becomes available. Data for sending becomes available in connection with a cycle switch (discussed elsewhere herein) where the inactive one of the lists 72, 74 becomes the active one of the lists 72, 74, and vice versa. If it is determined at the step 202 that there is data available for sending, control transfers from the step 202 to a step 204, where the slot is verified as being correct. The processing performed at the step 204 is an optional �sanity check� that may include verifying that the password field is correct and verifying that the sequence number field is correct. If there is incorrect (unexpected) data in the slot, error processing may be performed, which may include notifying a user of the error and possibly error recovery processing. Following the step 204 is a step 212, where the data is sent via RDF in a conventional fashion. In an embodiment herein, the entire slot is not transmitted. Rather, only records within the slot that have the appropriate mirror bits set (indicating the records have changed) are transmitted to the remote storage device 26. However, in other embodiments, it may be possible to transmit the entire slot, provided that the remote storage device 26 only writes data corresponding to records having appropriate mirror bits set and ignores other data for the track, which may or may not be valid. Following the step 212 is a test step 214 where it is determined if the data that was transmitted has been acknowledged by the R2 device. If not, the data is resent, as indicated by the flow from the step 214 back to the step 212. In other embodiments, different and more involved processing may used to send data and acknowledge receipt thereof. Such processing may include error reporting and alternative processing that is performed after a certain number of attempts to send the data have failed. Once it is determined at the test step 214 that the data has been successfully sent, control passes from the step 214 to a step 216 to clear the RDF_WP flag (since the data has been successfully sent via RDF). Following the step 216 is a test step 218 where it is determined if the slot is a duplicate slot created in connection with a write to a slot already having an existing entry in the inactive one of the lists 72, 74. This possibility is discussed above in connection with the steps 162, 164, 166, 168, 172. If it is determined at the step 218 that the slot is a duplicate slot, then control passes from the step 218 to a step 222 where the slot is returned to the pool of available slots (to be reused). In addition, the slot may also be aged (or have some other appropriate mechanism applied thereto) to provide for immediate reuse ahead of other slots since the data provided in the slot is not valid for any other purpose. Following the step 222 or the step 218 if the slot is not a duplicate slot is a step 224 where the password field of the slot header is cleared so that when the slot is reused, the test at the step 146 of FIG. 5 properly classifies the slot as a new slot. Following the step 224 is a step 226 where the entry in the inactive one of the lists 72, 74 is removed. Following the step 226, control transfers back to the step 202, discussed above, where it is determined if there are additional entries on the inactive one of the lists 72, 74 corresponding to data needing to be transferred. Referring to FIG. 7, a diagram 240 illustrates creation and manipulation of the chunks 56, 58 used by the remote storage device 26. Data that is received by the remote storage device 26, via the link 29, is provided to a cache 242 of the remote storage device 26. The cache 242 may be provided, for example, in the memory 38 of the remote storage device 26. The cache 242 includes a plurality of cache slots 244-246, each of which may be mapped to a track of a standard logical storage device 252. The cache 242 is similar to the cache 88 of FIG. 3 and may contain data that can be destaged to the standard logical storage device 252 of the remote storage device 26. The standard logical storage device 252 corresponds to the data element 62 shown in FIG. 2 and the disks 34 a-34 c shown in FIG. 1. The remote storage device 26 also contains a pair of cache only virtual devices 254, 256. The cache only virtual devices 254, 256 corresponded device tables that may be stored, for example, in the memory 38 of the remote storage device 26. Each track entry of the tables of each of the cache only virtual devices 254, 256 point to either a track of the standard logical device 252 or point to a slot of the cache 242. Cache only virtual devices are described in a copending U.S. patent application titled CACHE-ONLY VIRTUAL DEVICES, filed on Mar. 25, 2003 and having Ser. No. 10/396,800, which is incorporated by reference herein. The plurality of cache slots 244-246 may be used in connection to writes to the standard logical device 252 and, at the same time, used in connection with the cache only virtual devices 254, 256. In an embodiment herein, each of track table entry of the cache only virtual devices 254, 256 contain a null to indicate that the data for that track is stored on a corresponding track of the standard logical device 252. Otherwise, an entry in the track table for each of the cache only virtual devices 254, 256 contains a pointer to one of the slots 244-246 in the cache 242. Each of the cache only virtual devices 254, 256 corresponds to one of the data chunks 56, 58. Thus, for example, the cache only virtual device 254 may correspond to the data chunk 56 while the cache only virtual device 256 may correspond to the data chunk 58. In an embodiment herein, one of the cache only virtual devices 254, 256 may be deemed �active� while the other one of the cache only virtual devices 254, 256 may be deemed �inactive�. The inactive one of the cache only virtual devices 254, 256 may correspond to data being received from the local storage device 24 (i.e., the chunk 56) while the active one of the cache only virtual device 254, 256 corresponds to data being restored (written) to the standard logical device 252. Data from the local storage device 24 that is received via the link 29 may be placed in one of the slots 244-246 of the cache 242. A corresponding pointer of the inactive one of the cache only virtual devices 254, 256 may be set to point to the received data. Subsequent data having the same sequence number may be processed in a similar manner. At some point, the local storage device 24 provides a message committing all of the data sent using the same sequence number. Once the data for a particular sequence number has been committed, the inactive one of the cache only virtual devices 254, 256 becomes active and vice versa. At that point, data from the now active one of the cache only virtual devices 254, 256 is copied to the standard logical device 252 while the inactive one of the cache only virtual devices 254, 256 is used to receive new data (having a new sequence number) transmitted from the local storage device 24 to the remote storage device 26. As data is removed from the active one of the cache only virtual devices 254, 256 (discussed elsewhere herein), the corresponding entry in the active one of the cache only virtual devices 254, 256 may be set to null. In addition, the data may also be removed from the cache 244 (i.e., the slot returned to the pool of free slots for later use) provided that the data in the slot is not otherwise needed for another purpose (e.g., to be destaged to the standard logical device 252). A mechanism may be used to ensure that data is not removed from the cache 242 until all mirrors (including the cache only virtual devices 254, 256) are no longer using the data. Such a mechanism is described, for example, in U.S. Pat. No. 5,537,568 issued on Jul. 16, 1996 and in U.S. patent application Ser. No. 09/850,551 filed on Jul. 7, 2001, both of which are incorporated by reference herein. In some embodiments discussed elsewhere herein, the remote storage device 26 may maintain linked lists 258, 262 like the lists 74, 76 used by the local storage device 24. The lists 258, 262 may contain information that identifies the slots of the corresponding cache only virtual devices 254, 256 that have been modified, where one of the lists 258, 262 corresponds to one of the cache only virtual devices 254, 256 and the other one of the lists 258, 262 corresponds to the other one of the cache only virtual devices 254, 256. As discussed elsewhere herein, the lists 258, 262 may be used to facilitate restoring data from the cache only virtual devices 254, 256 to the standard logical device 252. Referring to FIG. 8, a flow chart 270 illustrates steps performed by the remote storage device 26 in connection with processing data for a sequence number commit transmitted by the local storage device 24 to the remote storage device 26. As discussed elsewhere herein, the local storage device 24 periodically increments sequence numbers. When this occurs, the local storage device 24 finishes transmitting all of the data for the previous sequence number and then sends a commit message for the previous sequence number. Processing begins at a first step 272 where the commit is received. Following the step 272 is a test step 274 which determines if the active one of the cache only virtual devices 254, 256 of the remote storage device 26 is empty. As discussed elsewhere herein, the inactive one of the cache only virtual devices 254, 256 of the remote storage device 26 is used to accumulate data from the local storage device 24 sent using RDF while the active one of the cache only virtual devices 254, 256 is restored to the standard logical device 252. If it is determined at the test step 274 that the active one of the cache only virtual devices 254, 256 is not empty, then control transfers from the test step 274 to a step 276 where the restore for the active one of the cache only virtual devices 254, 256 is completed prior to further processing being performed. Restoring data from the active one of the cache only virtual devices 254, 256 is described in more detail elsewhere herein. It is useful that the active one of the cache only virtual devices 254, 256 is empty prior to handling the commit and beginning to restore data for the next sequence number. Following the step 276 or following the step 274 if the active one of the cache only virtual devices 254, 256 is determined to be empty, is a step 278 where the active one of the cache only virtual devices 254, 256 is made inactive. Following the step 278 is a step 282 where the previously inactive one of the cache only virtual devices 254, 256 (i.e., the one that was inactive prior to execution of the step 278) is made active. Swapping the active and inactive cache only virtual devices 254, 256 at the steps 278, 282 prepares the now inactive (and empty) one of the cache only virtual devices 254, 256 to begin to receive data from the local storage device 24 for the next sequence number. Following the step 282 is a step 284 where the active one of the cache only virtual devices 254, 256 is restored to the standard logical device 252 of the remote storage device 26. Restoring the active one of the cache only virtual devices 254, 256 to the standard logical device 252 is described in more detail hereinafter. However, note that, in some embodiments, the restore process is begun, but not necessarily completed, at the step 284. Following the step 284 is a step 286 where the commit that was sent from the local storage device 24 to the remote storage device 26 is acknowledged back to the local storage device 24 so that the local storage device 24 is informed that the commit was successful. Following the step 286, processing is complete. Referring to FIG. 9, a flow chart 300 illustrates in more detail the steps 276, 284 of FIG. 8 where the remote storage device 26 restores the active one of the cache only virtual devices 254, 256. Processing begins at a first step 302 where a pointer is set to point to the first slot of the active one of the cache only virtual devices 254, 256. The pointer is used to iterate through each track table entry of the active one of the cache only virtual devices 254, 256, each of which is processed individually. Following the step 302 is a test step 304 where it is determined if the track of the active one of the cache only virtual devices 254, 256 that is being processed points to the standard logical device 252. If so, then there is nothing to restore. Otherwise, control transfers from the step 304 to a step a 306 where the corresponding slot of the active one of the cache only virtual devices 254, 256 is locked. Following the step 306 is a test step 308 which determines if the corresponding slot of the standard logical device 252 is already in the cache of the remote storage device 26. If so, then control transfers from the test step 308 to a step 312 where the slot of the standard logical device is locked. Following step 312 is a step 314 where the data from the active one of the cache only virtual devices 254, 256 is merged with the data in the cache for the standard logical device 252. Merging the data at the step 314 involves overwriting the data for the standard logical device with the new data of the active one of the cache only virtual devices 254, 256. Note that, in embodiments that provide for record level flags, it may be possible to simply OR the new records from the active one of the cache only virtual devices 254, 256 to the records of the standard logical device 252 in the cache. That is, if the records are interleaved, then it is only necessary to use the records from the active one of the cache only virtual devices 254, 256 that have changed and provide the records to the cache slot of the standard logical device 252. Following step 314 is a step 316 where the slot of the standard logical device 252 is unlocked. Following step 316 is a step 318 where the slot of the active one of the cache only virtual devices 254, 256 that is being processed is also unlocked. If it is determined at the test step 308 that the corresponding slot of the standard logical device 252 is not in cache, then control transfers from the test step 308 to a step 322 where the track entry for the slot of the standard logical device 252 is changed to indicate that the slot of the standard logical device 252 is in cache (e.g., an IN_CACHE flag may be set) and needs to be destaged. As discussed elsewhere herein, in some embodiments, only records of the track having appropriate mirror bits set may need to be destaged. Following the step 322 is a step 324 where a flag for the track may be set to indicate that the data for the track is in the cache. Following the step 324 is a step 326 where the slot pointer for the standard logical device 252 is changed to point to the slot in the cache. Following the step 326 is a test step 328 which determines if the operations performed at the steps 322, 324, 326 have been successful. In some instances, a single operation called a �compare and swap� operation may be used to perform the steps 322, 324, 326. If these operations are not successful for any reason, then control transfers from the step 328 back to the step 308 to reexamine if the corresponding track of the standard logical device 252 is in the cache. Otherwise, if it is determined at the test step 328 that the previous operations have been successful, then control transfers from the test step 328 to the step 318, discussed above. Following the step 318 is a test step 332 which determines if the cache slot of the active one of the cache only virtual devices 254, 256 (which is being restored) is still being used. In some cases, it is possible that the slot for the active one of the cache only virtual devices 254, 256 is still being used by another mirror. If it is determined at the test step 332 that the slot of the cache only virtual device is not being used by another mirror, then control transfers from the test step 332 to a step 334 where the slot is released for use by other processes (e.g., restored to pool of available slots, as discussed elsewhere herein). Following the step 334 is a step 336 to point to the next slot to process the next slot of the active one of the cache only virtual devices 254, 256. Note that the step 336 is also reached from the test step 332 if it is determined at the step 332 that the active one of the cache only virtual devices 254, 256 is still being used by another mirror. Note also that the step 336 is reached from the test step 304 if it is determined at the step 304 that, for the slot being processed, the active one of the cache only virtual devices 254, 256 points to the standard logical device 252. Following the step 336 is a test step 338 which determines if there are more slots of the active one of the cache only virtual devices 254, 256 to be processed. If not, processing is complete. Otherwise, control transfers from the test step 338 back to the step 304. In another embodiment, it is possible to construct lists of modified slots for the received chunk of data 56 corresponding to the N−1 cycle on the remote storage device 26, such as the lists 258, 262 shown in FIG. 7. As the data is received, the remote storage device 26 constructs a linked list of modified slots. The lists that are constructed may be circular, linear (with a NULL termination), or any other appropriate design. The lists may then be used to restore the active one of the cache only virtual devices 254, 256. The flow chart 300 of FIG. 9 shows two alternative paths 342, 344 that illustrate operation of embodiments where a list of modified slots is used. At the step 302, a pointer (used for iterating through the list of modified slots) is made to point to the first element of the list. Following the step 302 is the step 306, which is reached by the alternative path 342. In embodiments that use lists of modified slots, the test step 304 is not needed since no slots on the list should point to the standard logical device 252. Following the step 306, processing continues as discussed above with the previous embodiment, except that the step 336 refers to traversing the list of modified slots rather than pointing to the next slot in the COVD. Similarly, the test at the step 338 determines if the pointer is at the end of the list (or back to the beginning in the case of a circular linked list). Also, if it is determined at the step 338 that there are more slots to process, then control transfers from the step 338 to the step 306, as illustrated by the alternative path 344. As discussed above, for embodiments that use a list of modified slots, the step 304 may be eliminated. Referring to FIG. 10, a flow chart 350 illustrates steps performed in connection with the local storage device 24 increasing the sequence number. Processing begins at a first step 352 where the local storage device 24 waits at least M seconds prior to increasing the sequence number. In an embodiment herein, M is thirty, but of course M could be any number. Larger values for M increase the amount of data that may be lost if communication between the storage devices 24, 26 is disrupted. However, smaller values for M increase the total amount of overhead caused by incrementing the sequence number more frequently. Following the step 352 is a test step 354 which determines if all of the HA's of the local storage device 24 have set a bit indicating that the HA's have completed all of the I/O's for a previous sequence number. When the sequence number changes, each of the HA's notices the change and sets a bit indicating that all I/O's of the previous sequence number are completed. For example, if the sequence number changes from N−1 to N, an HA will set the bit when the HA has completed all I/O's for sequence number N−1. Note that, in some instances, a single I/O for an HA may take a long time and may still be in progress even after the sequence number has changed. Note also that, for some systems, a different mechanism may be used to determine if all of the HA's have completed their N−1 I/O's. The different mechanism may include examining device tables in the memory 37. If it is determined at the test step 354 that I/O's from the previous sequence number have been completed, then control transfers from the step 354 to a test step 356 which determines if the inactive one of the lists 74, 76 is empty. Note that a sequence number switch may not be made unless and until all of the data corresponding to the inactive one of the lists 74, 76 has been completely transmitted from the local storage device 24 to the remote storage device 26 using the RDF protocol. Once the inactive one of the lists 74, 76 is determined to be empty, then control transfers from the step 356 to a step 358 where the commit for the previous sequence number is sent from the local storage device 24 to the remote storage device 26. As discussed above, the remote storage device 26 receiving a commit message for a particular sequence number will cause the remote storage device 26 to begin restoring the data corresponding to the sequence number. Following the step 358 is a step 362 where the copying of data for the inactive one of the lists 74, 76 is suspended. As discussed elsewhere herein, the inactive one of the lists is scanned to send corresponding data from the local storage device 24 to the remote storage device 26. It is useful to suspend copying data until the sequence number switch is completed. In an embodiment herein, the suspension is provided by sending a message to the RA's 30 a-30 c. However, it will be appreciated by one of ordinary skill in the art that for embodiments that use other components to facilitate sending data using the system described herein, suspending copying may be provided by sending appropriate messages/commands to the other components. Following step 362 is a step 364 where the sequence number is incremented. Following step 364 is a step 366 where the bits for the HA's that are used in the test step 354 are all cleared so that the bits may be set again in connection with the increment of the sequence number. Following step 366 is a test step 372 which determines if the remote storage device 26 has acknowledged the commit message sent at the step 358. Acknowledging the commit message is discussed above in connection with FIG. 8. Once it is determined that the remote storage device 26 has acknowledged the commit message sent at the step 358, control transfers from the step 372 to a step 374 where the suspension of copying, which was provided at the step 362, is cleared so that copying may resume. Following step 374, processing is complete. Note that it is possible to go from the step 374 back to the step 352 to begin a new cycle to continuously increment the sequence number. It is also possible to use COVD's on the R1 device to collect slots associated with active data and inactive chunks of data. In that case, just as with the R2 device, one COVD could be associated with the inactive sequence number and another COVD could be associated with the active sequence number. This is described below. Referring to FIG. 11, a diagram 400 illustrates items used to construct and maintain the chunks 52, 54. A standard logical device 402 contains data written by the host 22 and corresponds to the data element 51 of FIG. 2 and the disks 33 a-33 c of FIG. 1. The standard logical device 402 contains data written by the host 22 to the local storage device 24. Two cache only virtual devices 404, 406 are used in connection with the standard logical device 402. The cache only virtual devices 404, 406 corresponded device tables that may be stored, for example, in the memory 37 of the local storage device 24. Each track entry of the tables of each of the cache only virtual devices 404, 406 point to either a track of the standard logical device 402 or point to a slot of a cache 408 used in connection with the local storage device 24. In some embodiments, the cache 408 may be provided in the memory 37 of the local storage device 24. The cache 408 contains a plurality of cache slots 412-414 that may be used in connection to writes to the standard logical device 402 and, at the same time, used in connection with the cache only virtual devices 404, 406. In an embodiment herein, each track table entry of the cache only virtual devices 404, 406 contains a null to point to a corresponding track of the standard logical device 402. Otherwise, an entry in the track table for each of the cache only virtual devices 404, 406 contains a pointer to one of the slots 412-414 in the cache 408. Each of the cache only virtual devices 404, 406 may be used for one of the chunks of data 52, 54 so that, for example, the cache only virtual device 404 may correspond to the chunk of data 52 for sequence number N while the cache only virtual device 406 may correspond to the chunk of data 54 for sequence number N−1. Thus, when data is written by the host 22 to the local storage device 24, the data is provided to the cache 408 and an appropriate pointer of the cache only virtual device 404 is adjusted. Note that the data will not be removed from the cache 408 until the data is destaged to the standard logical device 402 and the data is also released by the cache only virtual device 404, as described elsewhere herein. In an embodiment herein, one of the cache only virtual devices 404, 406 is deemed �active� while the other is deemed �inactive�. Thus, for example, when the sequence number N is even, the cache only virtual device 404 may be active while the cache only virtual device 406 is inactive. The active one of the cache only virtual devices 404, 406 handles writes from the host 22 while the inactive one of the cache only virtual devices 404, 406 corresponds to the data that is being transmitted from the local storage device 24 to the remote storage device 26. While the data that is written by the host 22 is accumulated using the active one of the cache only virtual devices 404, 406 (for the sequence number N), the data corresponding to the inactive one of the cache only virtual devices 404, 406 (for previous sequence number N−1) is transmitted from the local storage device 24 to the remote storage device 26. For this and related embodiments, the DA's 35 a-35 c of the local storage device handle scanning the inactive one of the cache only virtual devices 404, 406 to send copy requests to one or more of the RA's 30 a-30 c to transmit the data from the local storage device 24 to the remote storage device 26. Thus, the steps 362, 374, discussed above in connection with suspending and resuming copying, may include providing messages/commands to the DA's 35 a-35 c. Once the data has been transmitted to the remote storage device 26, the corresponding entry in the inactive one of the cache only virtual devices 404, 406 may be set to null. In addition, the data may also be removed from the cache 408 (i.e., the slot returned to the pool of slots for later use) if the data in the slot is not otherwise needed for another purpose (e.g., to be destaged to the standard logical device 402). A mechanism may be used to ensure that data is not removed from the cache 408 until all mirrors (including the cache only virtual devices 404, 406) are no longer using the data. Such a mechanism is described, for example, in U.S. Pat. No. 5,537,568 issued on Jul. 16, 1996 and in U.S. patent application Ser. No. 09/850,551 filed on Jul. 7, 2001, both of which are incorporated by reference herein. Referring to FIG. 12, a flow chart 440 illustrates steps performed by the HA 28 in connection with a host 22 performing a write operation for embodiments where two COVD's are used by the R1 device to provide the system described herein. Processing begins at a first step 442 where a slot corresponding to the write is locked. In an embodiment herein, each of the slots 412-414 of the cache 408 corresponds to a track of data on the standard logical device 402. Locking the slot at the step 442 prevents additional processes from operating on the relevant slot during the processing performed by the HA 28 corresponding to the steps of the flow chart 440. Following the step 442 is a step 444 where a value for N, the sequence number, is set. Just as with the embodiment that uses lists rather than COVD's on the R1 side, the value for the sequence number obtained at the step 444 is maintained during the entire write operation performed by the HA 28 while the slot is locked. As discussed elsewhere herein, the sequence number is assigned to each write to set the one of the chunks of data 52, 54 to which the write belongs. Writes performed by the host 22 are assigned the current sequence number. It is useful that a single write operation maintain the same sequence number throughout. Following the step 444 is a test step 446, which determines if the inactive one of the cache only virtual devices 404, 406 already points to the slot that was locked at the step 442 (the slot being operated upon). This may occur if a write to the same slot was provided when the sequence number was one less than the current sequence number. The data corresponding to the write for the previous sequence number may not yet have been transmitted to the remote storage device 26. If it is determined at the test step 446 that the inactive one of the cache only virtual devices 404, 406 does not point to the slot, then control transfers from the test step 446 to another test step 448, where it is determined if the active one of the cache only virtual devices 404, 406 points to the slot. It is possible for the active one of the cache only virtual devices 404, 406 to point to the slot if there had been a previous write to the slot while the sequence number was the same as the current sequence number. If it is determined at the test step 448 that the active one of the cache only virtual devices 404, 406 does not point to the slot, then control transfers from the test step 448 to a step 452 where a new slot is obtained for the data. Following the step 452 is a step 454 where the active one of the cache only virtual devices 404, 406 is made to point to the slot. Following the step 454, or following the step 448 if the active one of the cache only virtual devices 404, 406 points to the slot, is a step 456 where flags are set. At the step 456, the RDF_WP flag (RDF write pending flag) is set to indicate that the slot needs to be transmitted to the remote storage device 26 using RDF. In addition, at the step 456, the IN_CACHE flag is set to indicate that the slot needs to be destaged to the standard logical device 402. Note that, in some instances, if the active one of the cache only virtual devices 404, 406 already points to the slot (as determined at the step 448) it is possible that the RDF_WP and IN_CACHE flags were already set prior to execution of the step 456. However, setting the flags at the step 456 ensures that the flags are set properly no matter what the previous state. Following the step 456 is a step 458 where an indirect flag in the track table that points to the slot is cleared, indicating that the relevant data is provided in the slot and not in a different slot indirectly pointed to. Following the step 458 is a step 462 where the data being written by the host 22 and the HA 28 is written to the slot. Following the step 462 is a step 464 where the slot is unlocked. Following step 464, processing is complete. If it is determined at the test step 446 that the inactive one of the cache only virtual devices 404, 406 points to the slot, then control transfers from the step 446 to a step 472, where a new slot is obtained. The new slot obtained at the step 472 may be used for the inactive one of the cache only virtual devices 404, 406 to effect the RDF transfer while the old slot may be associated with the active one of the cache only virtual devices 404, 406, as described below. Following the step 472 is a step 474 where the data from the old slot is copied to the new slot that was obtained at the step 472. Following the step 474 is a step 476 where the indirect flag (discussed above) is set to indicate that the track table entry for the inactive one of the cache only virtual devices 404, 406 points to the old slot but that the data is in the new slot which is pointed to by the old slot. Thus, setting indirect flag at the step 476 affects the track table of the inactive one of the cache only virtual devices 404, 406 to cause the track table entry to indicate that the data is in the new slot. Following the step 476 is a step 478 where the mirror bits for the records in the new slot are adjusted. Any local mirror bits that were copied when the data was copied from the old slot to the new slot at the step 474 are cleared since the purpose of the new slot is to simply effect the RDF transfer for the inactive one of the cache only virtual devices. The old slot will be used to handle any local mirrors. Following the step 478 is the step 462 where the data is written to the slot. Following step 462 is the step 464 where the slot is unlocked. Following the step 464, processing is complete. Referring to FIG. 13, a flow chart 500 illustrates steps performed in connection with the local storage device 24 transmitting the chunk of data 54 to the remote storage device 26. The transmission essentially involves scanning the inactive one of the cache only virtual devices 404, 406 for tracks that have been written thereto during a previous iteration when the inactive one of the cache only virtual devices 404, 406 was active. In this embodiment, the DA's 35 a-35 c of the local storage device 24 scan the inactive one of the cache only virtual devices 404, 406 to copy the data for transmission to the remote storage device 26 by one or more of the RA's 30 a-30 c using the RDF protocol. Processing begins at a first step 502 where the first track of the inactive one of the cache only virtual devices 404, 406 is pointed to in order to begin the process of iterating through all of the tracks. Following the first step 502 is a test step 504 where it is determined if the RDF_WP flag is set. As discussed elsewhere herein, the RDF_WP flag is used to indicate that a slot (track) contains data that needs to be transmitted via the RDF link. The RDF_WP flag being set indicates that at least some data for the slot (track) is to be transmitted using RDF. In an embodiment herein, the entire slot is not transmitted. Rather, only records within the slot that have the appropriate mirror bits set (indicating the records have changed) are transmitted to the remote storage device 26. However, in other embodiments, it may be possible to transmit the entire slot, provided that the remote storage device 26 only writes data corresponding to records having appropriate mirror bits set and ignores other data for the track, which may or may not be valid. If it is determined at the test step 504 that the cache slot being processed has the RDF_WP flag set, then control transfers from the step 504 to a test step 505, where it is determined if the slot contains the data or if the slot is an indirect slot that points to another slot that contains the relevant data. In some instances, a slot may not contain the data for the portion of the disk that corresponds to the slot. Instead, the slot may be an indirect slot that points to another slot that contains the data. If it is determined at the step 505 that the slot is an indirect slot, then control transfers from the step 505 to a step 506, where the data (from the slot pointed to by the indirect slot) is obtained. Thus, if the slot is a direct slot, the data for being sent by RDF is stored in the slot while if the slot is an indirect slot, the data for being sent by RDF is in another slot pointed to by the indirect slot. Following the step 506 or the step 505 if the slot is a direct slot is a step 507 where data being sent (directly or indirectly from the slot) is copied by one of the DA's 35 a-35 c to be sent from the local storage device 24 to the remote storage device 26 using the RDF protocol. Following the step 507 is a test step 508 where it is determined if the remote storage device 26 has acknowledged receipt of the data. If not, then control transfers from the step 508 back to the step 507 to resend the data. In other embodiments, different and more involved processing may used to send data and acknowledge receipt thereof. Such processing may include error reporting and alternative processing that is performed after a certain number of attempts to send the data have failed. Once it is determined at the test step 508 that the data has been successfully sent, control passes from the step 508 to a step 512 to clear the RDF_WP flag (since the data has been successfully sent via RDF). Following the step 512 is a step 514 where appropriate mirror flags are cleared to indicate that at least the RDF mirror (R2) no longer needs the data. In an embodiment herein, each record that is part of a slot (track) has individual mirror flags indicating which mirrors use the particular record. The R2 device is one of the mirrors for each of the records and it is the flags corresponding to the R2 device that are cleared at the step 514. Following the step 514 is a test step 516 which determines if any of the records of the track being processed have any other mirror flags set (for other mirror devices). If not, then control passes from the step 516 to a step 518 where the slot is released (i.e., no longer being used). In some embodiments, unused slots are maintained in a pool of slots available for use. Note that if additional flags are still set for some of the records of the slot, it may mean that the records need to be destaged to the standard logical device 402 or are being used by some other mirror (including another R2 device). Following the step 518, or following the step 516 if more mirror flags are present, is a step 522 where the pointer that is used to iterate through each track entry of the inactive one of the cache only virtual devices 404, 406 is made to point to the next track. Following the step 522 is a test step 524 which determines if there are more tracks of the inactive one of the cache only virtual devices 404, 406 to be processed. If not, then processing is complete. Otherwise, control transfers back to the test step 504, discussed above. Note that the step 522 is also reached from the test step 504 if it is determined that the RDF_WP flag is not set for the track being processed. In some instances, it may be desirable to switch operation of the system from using virtual ordered writes with chunks of data as described herein (virtual ordered writes mode) to using synchronous RDF transfer mode (J0 or Journal zero mode) or semi-synchronous RDF transfer mode (J1 or Journal one mode). The choice between using the virtual ordered writes mode as described herein and using synchronous or semi-synchronous RDF transfer mode depends on a number of factors, including tolerance for delay, desired throughput, and tolerance for loss of data. It is useful if, during the transition, the integrity of the data transfer be maintained so that even if there is, for example, a failure of the local storage device or in the communication channel during the transition from virtual ordered writes mode to J0/J1 RDF transfer mode, the data at the remote storage device will be consistent so that recovery may be performed at the remote storage device. As set forth in more detail elsewhere herein, the transition is provided by emptying the N−1 chunk of data 54 at the local storage device 24 and then sending most subsequent data using J0 (or J1) RDF transfer mode while emptying the other chunks of data 52, 56, 58. Note also that, for the discussion that follows, descriptions involving transitioning to the J0 RDF transfer mode applies also to transition to the J1 RDF transfer mode (or other appropriate RDF modes) and vice versa. Referring to FIG. 14, a flow chart 540 illustrates steps performed in connection with the local storage device 24 transitioning from virtual ordered writes mode to synchronous or semi-synchronous RDF transfer mode. The local storage device 24 may switch in response to a command from the host 22 or from some other entity. In some embodiments, the local storage device 24 may monitor data transfers and use particular metrics to automatically determine that a switch is appropriate. Processing for the flow chart 540 begins at a first step 542 where cycle changes are frozen at the local storage device 24. As discussed elsewhere herein, the cycle number, N, is used in connection with transferring data in the virtual ordered writes mode. After execution of the step 542, there are no more changes in the cycle number N used by the virtual ordered writes system. Following the step 542 is a step 544 where a message is sent to the remote storage device 26 so that the remote storage device 26 may perform appropriate processing for the transition to synchronous or semi-synchronous RDF transfer mode, as described below. Following the step 544 is a test step 546 which determines if the N−1 chunk of data 54 at the local storage device 24 is empty. If not, then control loops back to the test step 546 to continue to poll until the N−1 chunk of data 54 is empty. The local storage device 24 cannot perform any writes in synchronous or semi-synchronous RDF transfer mode until the N−1 chunk of data 54 on the local storage device 24 is empty. Following the step 546, once the N−1 chunk of data 54 on the local storage device 24 is empty, is a step 548 where the local storage device 24 begins transmitting data from the N chunk of data 52 from the local storage device 24 to the remote storage device 26. The data from the N chunk of data 52 is transmitted using the same protocol that is used for sending the inactive data during steady state operation of the virtual ordered writes mode. Following the step 548, processing is complete. At the end of the process illustrated by the flow chart 540, the chunks of data 52, 54 of the local storage device 24 should be empty or should be in the process of being emptied. Referring to FIG. 15, a flow chart 560 illustrates steps performed by the remote storage device 26 in response to receiving the message from the local storage device 24 sent at the step 544 to indicate that a switch to synchronous or semi-synchronous RDF transfer mode is taking place. Processing begins at a first step 562 where the data from the N−2 chunk of data 58 at the remote storage device 26 is written to the disk 62. Note that the step 562 may not be necessary since the N−2 chunk of data 58 may already be in the process of being written to the disk 62 by virtue of operation of the virtual ordered writes mechanism, discussed elsewhere herein. Following the step 562 is a test step 564 which determines if the N−2 chunk of data 58 is empty. If not, control loops back to the test step 564 to continue to poll until the N−2 chunk of data 58 is empty. Otherwise, control transfers from the step 564 to a test step 566 which determines if the remote storage device 26 has received an acknowledge message from the local storage device 24 indicating that the local storage device 24 has transmitted all of the N−1 chunk of data 54 on the local storage device. If the acknowledge message has not been received, then control loops back to poll until the acknowledge message is received. Otherwise, control transfers from the step 566 to a step 568 where the remote storage device 26 begins to write the data from the N−1 chunk of data 56 to the disk 62. At the end of the process illustrated by the flow chart 560, the chunks of data 56, 58 on the remote storage device 26 should be empty or should be in the process of being emptied. Referring to FIG. 16, a flow chart 600 illustrates steps performed in connection with receiving a write from the host 22 (or from another entity) at the local storage device 24 during a transition from virtual ordered writes mode to synchronous or semi-synchronous RDF transfer mode. Processing begins a first step 602 which determines if the N−1 chunk of data 54 at the local storage device 24 is empty. If not, then control transfers from the test step 602 to a step 604 where the data being written is added to the N chunk of data 52 in a manner consistent with virtual ordered writes mechanism, discussed elsewhere herein. As discussed above, synchronous or semi-synchronous RDF transmission of data may not be begin until the N−1 chunk of data 54 is empty. Thus, the step 604 represents adding the data to the N chunk of data 52 when the system is not yet ready to begin synchronous or semi-synchronous RDF transmission. Following the step 604, processing is complete. If it is determined at the step 602 that the N−1 chunk of data 54 is empty, control transfers to a test step 606 which determines if the N chunk of data 52 is empty. If so, then control transfers from the step 606 to a step 608 where the data that is being written is transferred to the remote storage device 26 using synchronous or semi-synchronous RDF transfer mode. Following step 608, processing is complete. If it is determined at the step 606 that the N chunk of data 52 is not empty, then control transfers from the step 606 to a test step 612 which determines if the data being written is for the same slot as data stored in the N chunk of data 52. If not, then control transfers from the test step 612 to the step 608 where, as described above, the data is transferred to the remote storage device 26 using synchronous or semi-synchronous RDF transmission. Thus, if a data write occurs and the data being written is not related to any data that is in the N chunk of data 52, then the data may be written using synchronous or semi-synchronous RDF transmission. If it is determined at the test step 612 that the data that is being written corresponds to data in a slot that is in the N chunk of data 52, then control transfers from the step 612 to a step 614 where the slot in the N chunk of data 52 is locked to prevent other accesses thereto. Following the step 614 is a step 616 where the data in the N chunk of data is merged with the data being written, with the data being written taking precedence. Merging the data at the step 616 is analogous to merging the data at the step 314 in the flow chart 300 of FIG. 9, discussed above. Following the step 616 is a step 618 where the data from the merged slot is transferred to the remote storage device 26 using synchronous or semi-synchronous RDF transfer mode. Following the step 618 is a step 622 where the slot is unlocked and then removed from the N chunk of data 52 (i.e., a pointer to the slot is removed from the N chunk of data 52). Note that once the data has been transferred at the step 618, the slot is no longer needed and thus may be removed from the N chunk of data at the step 622. Following step 622, processing is complete. Referring to FIG. 17, a flow chart 650 illustrates steps performed at the remote storage device 26 in connection with receiving data transferred thereto by the local storage device 24 using the synchronous or semi-synchronous RDF transfer mode. Processing begins at a first step 652 where it is determined if the received synchronous or semi-synchronous data is for a cycle number corresponding to the inactive buffer of the remote storage device 26. If not, then control transfers from the test step 652 to a step 654 where a synchronous or semi-synchronous receive is performed. Following the step 654 is a step 656 where an acknowledgement is provided from the remote storage device 26 to the local storage device 24 to acknowledge the received data. Following the step 656, processing is complete. If it is determined at the test step 652 that the received data corresponds to the inactive buffer of the remote storage device 26, then control transfers from the step 652 to a test step 658 where it is determined if the inactive buffer is currently being restored at the remote storage device 26 (see the step 568 in the flow chart 560 of FIG. 15, discussed above). If not, then control flows from the step 658 to a step 662 where the received data is added to the inactive buffer. Following the step 662 is the step 656, discussed above. If it is determined at the step 658 that the inactive buffer is being restored at the remote storage device 26, then control flows from the step 658 to a test step 664 where it is determined if the inactive buffer contains a slot that is related to the received data (i.e., for the same slot as the received data). If so, then control transfers from the step 664 to a step 666 where the related slot in the inactive buffer is restored to the disk 62 of the remote storage device 26. Following the step 666 or following the step 664 if the inactive buffer does not contain a related slot is a step 668 where the received data is restored to the disk 62 of the remote storage device 26. Following the step 668 is the step 656, discussed above. Other techniques may be used for transitioning from virtual ordered writes mode to synchronous or semi-synchronous RDF transfer mode. These techniques may combine some or all of the specific features of the techniques described above. Referring to FIG. 18, a flow chart 700 illustrates steps performed for an alternative embodiment in connection with the local storage device 24 switching from virtual ordered writes mode to synchronous or semi-synchronous RDF transfer mode. The local storage device 24 may switch in response to a command from the host 22 or from some other entity. In some embodiments, the local storage device 24 may monitor data transfers and use particular metrics to automatically determine that a switch is appropriate. Processing for the flow chart 700 begins at a first step 702 where it is determined if the number of data entries in the N chunk of data 52 (the active chunk) of the local storage device 24 is greater than one half of the maximum number of entries. If so, then control transfers from the test step 702 to a step 704 where waiting for a predetermined period of time (e.g., one second) is performed. Following the step 704, control transfers back to the step 702, discussed above. The steps 702, 704, are performed because it may be desirable to not perform the transition to synchronous or semi-synchronous RDF transfer mode unless the active buffer (the N chunk of data 52) has a significant amount of unused space so that data that is added to the buffer in the course of the transition, as discussed below, does not overflow the buffer. Of course, the amount of space used for the test at the step 702 could be different than the one half illustrated herein. In addition, in some it may be possible to eliminate the step 702 (and associated steps) altogether and perform the transition irrespective of the amount of available space remaining in the active buffer (the N chunk of data 52). Note also that it is possible to simply abort the transition rather than waiting and trying again if it is determined at the step 702 that the active buffer is more than � full. In such a case, the fact that the abort occurred may be reported back to the calling software, which may then decide whether to attempt to transition again. If it is determined at the test step 702 that the active buffer (the N chunk of data 52) has sufficient space, then control transfers from the test step 702 to a step 706 where a SYNC_STATE variable (SS) is set to one. Use of the SYNC_STATE variable is described elsewhere herein. Following the step 706 is a test step 708 which determines if the SYNC_STATE variable is greater than or equal to three. As described elsewhere herein, other processes increment and set the SYNC_STATE variable to different values in response to the SYNC_STATE not being equal to zero. If it is determined at the test step 708 that these other processes have not incremented the SYNC_STATE variable to three or greater, then the test step 708 loops back to continue to poll to wait for the SYNC_STATE variable to be greater than or equal to three. Otherwise, control transfers from the test step 708 to a step 712 where the virtual ordered writes state (and associated processes) are deactivated. Following the step 712 is a test step 714 which determines if the SYNC_STATE variable has been set back to zero (by other processes). If not, control loops back on the test step 714 to continue polling. Otherwise, control transfers from the step 714 to a step 716 where the virtual ordered writes mode (and associated processes) are activated. Following the step 716, processing is complete. In an alternative embodiment, it is possible to eliminate the steps 714, 716 altogether, in which case the virtual ordered writes mode may be reactivated directly by another process. In such a case, part of the initialization performed by the virtual ordered writes code could be to set the SYNC_STATE variable to zero. Alternatively, the other process that reactivates the virtual ordered writes code, rather than the virtual ordered writes code itself, could set the SYNC_STATE variable to zero. Referring to FIG. 19, a flow chart 720 is similar to the flow chart 350 of FIG. 10. Steps of the flow chart 720 that are the same as steps for the flow chart 350 of FIG. 10 have the same reference number. However, note that the flow chart 720 shows a test step 722 that follows the step 364. At the test step 722, it is determined if the SYNC_STATE variable equals zero. The SYNC_STATE variable being zero indicates non-transitioning steady state operation in the virtual ordered writes mode. Thus, if it is determined at the test step 722 that the SYNC_STATE variable equals zero, then control transfers from the step 722 to the step 366 to continue processing as discussed above in connection with the flow chart 350 of FIG. 10. Control flow from the step 722 to the step 366 represents non-transitioning steady state operation in the virtual ordered writes mode. If it is determined at the test step 722 that the SYNC_STATE variable does not equal zero, then control transfers from the step 722 to a step 724 where the SYNC_STATE variable is incremented. Following the step 724, control transfers to the step 366 to continue processing as discussed above. In some embodiments, it is useful to atomically update the sequence number (incremented at the step 366) and the SYNC_STATE variable (incremented at the step 724). This may be provided in a number of ways including, for example, causing the process illustrated by the flow chart 720 to be uninterruptible from a time before the step 366 to a time after the step 724. In addition, it may be useful to retest (like the test at the step 702) whether the active buffer less than some percentage full (e.g., half) before transitioning from SYNC_STATE=one to SYNC_STATE=two. In embodiments that perform this extra test, it is possible to abort the transition or perhaps even wait. Writes to the local storage device 24 are handled in a special way during the transition. Referring to FIG. 20, a flow chart 740 illustrates handling writes to the local storage device 24 during the transition from normal virtual ordered writes mode to the synchronous or semi-synchronous RDF transfer mode. Processing begins at a first test step 742 where it is determined if the SYNC_STATE variable is zero. If so, then control transfers from the step 742 to a step 744 where a virtual ordered write is performed as discussed elsewhere herein (see, for example, the flow chart 140 of FIG. 5). As discussed elsewhere herein, the SYNC_STATE variable being zero indicates being in non-transitioning virtual ordered writes mode. Following the step 744, processing is complete. If it is determined at the test step 742 that the SYNC_STATE variable does not equal zero, than control transfers from the test step 742 to a test step 746 where it is determined if the SYNC_STATE variable is greater than two. If so, then control transfers from the test step 746 to a step 747 where a synchronous or semi-synchronous RDF transfer record is constructed to transfer the data from the local storage device 24 to the remote storage device 26. Following the step 747 is a step 748 where a cycle number is appended to the synchronous or semi-synchronous RDF transfer record constructed at the step 747. Use of the appended cycle number is described elsewhere herein. Following the step 748 is a step 752 where a synchronous or semi-synchronous RDF transfer is performed. Following the step 752, processing is complete. Note that it is possible for different processes and even different processors to perform different parts of the processing illustrated by the flow chart 740 so that, for example, the HA 28 may perform the steps 747, 748 while one of the RA's 30 a-30 c performs the step 752. If it is determined at the test step 746 that the SYNC_STATE variable is not greater than two (i.e., that the SYNC_STATE variable equals one or two), then control transfers from the test step 746 to a test step 754 where it is determined if the data being written is for a slot that is already in the inactive buffer (the N−1 chunk of data 54) of the local storage device 24. If not, then control transfers from the test step 754 to the step 747, discussed above. If it is determined at the test step 754 that the data being written is for a slot already in the inactive buffer (the chunk of data 54) of the local storage device 24, then control transfers from the step 754 to a step 756 where the slot is locked. Following the step 756 is a step 758 where the data being written is merged, overwritten, or partially overwritten with the slot in the inactive buffer (the chunk of data 54) of the local storage device 24 in a manner discussed elsewhere herein. Following the step 758 is step 762 where the slot is unlocked. Following the step 762, processing is complete. Referring to FIG. 21, a flow chart 800 illustrates steps performed in connection with the remote storage device 26 receiving synchronous or semi-synchronous RDF transferred data. Processing begins at a test step 802, where it is determined if the received data corresponds to the N−1 chunk of data 56 at the remote storage device 26. The test at the step 802 is made using the cycle number appended at the step 748, discussed above. Thus, if a cycle switch occurs after the data is sent by the local storage device 24 but before the data is received and processed by the remote storage device 26, use of the appended cycle number is designed to remove the possibility of improperly processing the data. If it is determined at the test step 802 that the received data does not correspond to the N−1 chunk of data 56 at the remote storage device 26, then control transfers to a test step 804 where it is determined if the received data corresponds to the N−2 chunk of data 58 of the remote storage device 26. The step 804 also uses the appended cycle number. If it is determined at the test step 804 that the received data does not correspond to the N−2 chunk of data 58 at the remote storage device 26, then control transfers to a step 806 where a synchronous or semi-synchronous receive is performed (i.e., steps performed in steady state J0/J1 RDF mode). If it is determined at the test step 802 that the received data corresponds to the N−1 chunk of data 56 at the remote storage device 26, then control transfers to a step 808 where the received data is added to the N−1 chunk of data 56 in a manner consistent with the description elsewhere herein. Following the steps 806,808 is a step 812 where a synchronous or semi-synchronous acknowledgement is provided from the remote storage device 26 to the local storage device 24. Following the step 812, processing is complete. If it is determined at the step 804 that the received data corresponds to the N−2 chunk of data 58, then control transfers from the step 804 to a test step 814 where it is determined if the N−2 chunk of data 58 already contains corresponding data (i.e., data for the same slot). If so, control transfers from the step 814 to a step 816 where the corresponding slot in the N−2 chunk of data is restored. Following the step 816, or following the step 814 if there is no corresponding data, is the step 806, discussed above. As for transitioning from synchronous or semi-synchronous RDF transfer mode to virtual ordered writes mode, it is simply a matter of initiating virtual ordered writes and beginning to accumulate data in the initial chunk at the local storage device 24. While the invention has been disclosed in connection with various embodiments, modifications thereon will be readily apparent to those skilled in the art. Accordingly, the spirit and scope of the invention is set forth in the following claims. Referenced byCiting PatentFiling datePublication dateApplicantTitleUS7085788Mar 10, 2004Aug 1, 2006Hitachi, Ltd.Remote copy system configured to receive both a write request including a write time and a write request not including a write time.US7114044Apr 14, 2004Sep 26, 2006Hitachi, Ltd.Storage system, method of controlling storage system, and storage deviceUS7171517May 19, 2004Jan 30, 2007Hitachi, Ltd.Storage apparatusUS7185152Aug 4, 2005Feb 27, 2007Hitachi, Ltd.Storage system, method of controlling storage system, and storage deviceUS7188218Jul 18, 2005Mar 6, 2007Hitachi, Ltd.Storage system for multi-remote copyUS7197615Sep 2, 2004Mar 27, 2007Hitachi, Ltd.Remote copy system maintaining consistencyUS7287132Sep 26, 2006Oct 23, 2007Hitachi, Ltd.Storage system, method of controlling storage system, and storage deviceUS7293050Dec 3, 2004Nov 6, 2007Hitachi, Ltd.Remote copy systemUS7328373Oct 10, 2006Feb 5, 2008Hitachi, Ltd.Data processing systemUS7330861Oct 11, 2006Feb 12, 2008Hitachi, Ltd.Remote copying system and method of controlling remote copyingUS7343461Sep 8, 2006Mar 11, 2008Hitachi, Ltd.Storage system for multi-site remote copyUS7437389May 2, 2005Oct 14, 2008Hitachi, Ltd.Remote copy systemUS7600089Dec 18, 2006Oct 6, 2009Hitachi, Ltd.Storage apparatus for asynchronous remote copyingUS7624229Sep 29, 2006Nov 24, 2009Emc CorporationSpillover slotUS7685387Jan 15, 2008Mar 23, 2010Hitachi, Ltd.Storage system for multi-site remote copyUS7724599Sep 10, 2004May 25, 2010Hitachi, Ltd.Remote copy systemUS7802137Dec 14, 2007Sep 21, 2010Hitachi, Ltd.Journaling system switching to another logical volume to store subsequently received update historyUS7865678Jan 23, 2007Jan 4, 2011Hitachi, Ltd.Remote copy system maintaining consistencyUS8032726Sep 8, 2008Oct 4, 2011Hitachi, LtdRemote copy systemUS8145865Sep 29, 2006Mar 27, 2012Emc CorporationVirtual ordered writes spillover mechanismUS8176010Mar 29, 2007May 8, 2012Hitachi, Ltd.Remote copy systemUS8200928May 17, 2011Jun 12, 2012Hitachi, Ltd.Storage system and remote copy control method for storage systemUS8250240Apr 6, 2010Aug 21, 2012Hitachi, Ltd.Message conversion method and message conversion systemUS8347053Jun 6, 2012Jan 1, 2013Hitachi, Ltd.Storage system and remote copy control method for storage systemUS8375000Apr 3, 2012Feb 12, 2013Hitachi, Ltd.Remote copy systemClassifications U.S. Classification711/162, 711/164International ClassificationG06F12/00Cooperative ClassificationG06F2212/285, G06F2201/835, G06F11/2064, G06F11/2071, G06F12/0866Legal EventsDateCodeEventDescriptionDec 6, 2010FPAYFee paymentYear of fee payment: 4Mar 8, 2004ASAssignmentOwner name: EMC CORPORATION, MASSACHUSETTSFree format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:YODER, BENJAMIN W.;MEIRI, DAVID;LOGINOV, VADIM;REEL/FRAME:015064/0227Effective date: 20040305RotateOriginal ImageGoogle Home - Sitemap - USPTO Bulk Downloads - Privacy Policy - Terms of Service - About Google Patents - Send FeedbackData provided by IFI CLAIMS Patent Services©2012 Google