Client-based migrating of data from content-addressed storage to file-based storage

A client computer migrates data objects from a first data store employing a flat namespace to a second data store employing a hierarchical directory-based file system. File storing includes (1) generating a data object (for user data file) and an object identifier which is returned to an application for use in retrieving the user data file; (2) mapping the object identifier to originate a subdirectory name; and (3) sending a write request to the file system to create the named subdirectory and store the data object therein. File retrieving includes (4) sending a read request including the object identifier to the first data store; (5) extracting a user data file from a received data object and returning it to the application; and (6) performing the steps of the file storing to store the data object at the second data store, completing the migration of the data object.

BACKGROUND

The invention is related to the field of data storage systems.

Data storage systems of a type referred to as “content addressed storage” or CAS are known. A CAS storage system may be used for bulk storage of a large number of files or similar data objects that may be relatively fixed, i.e., not subject to frequent (or any) change. One aspect of CAS storage is so-called content addressing, which refers to the association of each stored data object with a unique identifier that is generally calculated from the contents of the data object, for example by a hash function applied to the entire contents of a data file. A client of a CAS storage system can submit files for storage without any knowledge of how the storage system actually stores them. The CAS storage calculates the content address (CA) of each data object and stores it in association with the data object, as well as returning it to the client. When the client wishes to read the data object, it provides the CA to the CAS system, which then performs an associative lookup to retrieve the associated data object and return it to the client.

SUMMARY

While CAS storage systems provide very useful functionality for applications needing access to large sets of fixed data objects, there are important drawbacks to the deployment of CAS storage systems. In general, they have tended to be specialized hardware and software platforms optimized for CAS storage in order to provide the best performance. Because of the small demand for CAS systems in relation to more generally usable storage systems, such as so-called network attached storage or NAS for example, they do not benefit from certain economies of scale, most notably the economy of amortizing ongoing development costs across a large number of sold products. It may be difficult to incorporate new advances in storage technology into CAS systems because the cost is not justified by sufficiently increased revenue. Thus, uses of CAS systems obtain the specialized CAS functionality at the cost of keeping pace with other technology advances.

One theoretically possible way to implement a CAS data model using a file-oriented NAS system might be to simply store all the data objects in one single directory, and use the content address as the file name. Such an approach would have several problems, running up against practical limits of real NAS file systems that are designed based on certain assumptions that such an organization might violate.

The present disclosure is directed to client-based methods and apparatus for migrating data objects from a legacy CAS system to a file-based NAS system while maintaining backward compatibility with existing client applications. Migration is performed in parallel with normal operation, avoiding disruption to the operation of existing systems, and can be done in a manner providing for fail-back to the CAS system if necessary or desired. The disclosed technique enables the continued use of CAS-dependent applications with the ability to exploit contemporary and future technology developments that become incorporated into the general-purpose storage systems.

In particular, a method is disclosed of operating a client computer to migrate a set of data objects from a first remote data store employing a flat linear namespace to a second remote data store employing a file system having a hierarchical directory structure. For file storing operations for respective newly created user data files, a series of operations are performed, including (1) generating a data object and an object identifier, the data object containing the user data file being stored and the object identifier uniquely identifying the data object, and returning the object identifier to an application program for subsequent use in retrieving the user data file being stored; (2) performing a mapping function on the object identifier to originate a subdirectory name of a subdirectory of the hierarchical directory structure; and (3) sending a write request to the file system of the second remote data store, the write request including the data object, the subdirectory name, and a command to create a subdirectory having the subdirectory name and store the data object therein. The new user data files reside in the second remote data store and can be retrieved from it instead of from the first remote data store in subsequent read operations. If desired, the writes can be mirrored to the first remote data store to provide for “fail-back”, i.e., the ability to resume operation exclusively using the first remote data store if it becomes necessary or desired.

File retrieving operations are performed for user data files contained in respective data objects stored in the first remote data store. Each file retrieving operation is initiated with a respective object identifier in the flat linear namespace uniquely identifying a data object containing a user data file being retrieved. Each file retrieving operation includes (4) sending a read request including the object identifier to the first remote data store; (5) upon receiving a data object from the first remote data store, extracting a user data file and returning the extracted user data file to the application; (6) performing the mapping function on the object identifier to originate a subdirectory name of a subdirectory of the hierarchical directory structure; and (7) sending a write request to the file system of the second remote data store, the write file request including the received data object, the subdirectory name, and a command to create a subdirectory having the subdirectory name and store the data object therein. The data object now resides in the second remote data store and can subsequently be retrieved therefrom.

The above technique may be augmented by a dedicated migration manager functioning as a background or demon process that forces reads (and hence migrations) of files that might not be read frequently or at all by the application in normal operation. This can speed up the completion of migration to the second remote data store.

DETAILED DESCRIPTION

FIG. 1shows a computing system including a client computer (client)10, a network12, and two different types of storage systems in the form of a legacy content-addressed storage (CAS) system14and a file-based network-attached storage (NAS) system. The system may optionally include another computer functioning as a migration node18. In the simplified system ofFIG. 1, only one of each system component is shown; it will be appreciated that in a real system there may be multiple instances of one or more of the components (e.g., multiple clients10, NAS systems16, etc.).

The client10is typically a server type of computer hosting one or more application programs providing services to users, who may be part of a single organization (such as a company that owns the server) or members of the public (subscribed or not) accessing the services over a public network. In particular, the client10executes one or more applications of a type that benefit from content-addressed storage such as provided by the CAS system14. Specific examples are provided below.

The NAS system16is a storage system/device that includes one or more file systems and presents a file system interface to its external clients such as client10. It may support well-known distributed file system protocols such as NFS or CIFS, for example. As generally known in the art, a file system typically employs a hierarchical “directory” paradigm for organizing the data objects (files) that it contains. The underlying storage media in the NAS system16(such as magnetic disk devices, FLASH memory, etc.) stores user and system data files as collections of fixed-size blocks of data, and also stores directory files including pointers to the user/system data files as well as to other directory files. This structure enables a user or system to organize data in the well-known top-down nested fashion. For example, a top level may have directories for different functions or departments of a company, while subdirectories may have divisions for different users, applications, geographies or date ranges, etc.

In general, NAS systems are well suited for data access patterns that may be relatively random and may be intensive in terms of update frequency. Applications have knowledge of the directory structure and can make pinpoint requests for reading/writing files, with an expectation of fast response time and acceptably high throughput. The hierarchical directory paradigm helps support this workload by its divide-and-conquer aspect—files can be located using only a small number of directory lookups. Techniques such as caching can be used to enhance random-access performance, accommodating the latency limitations of mechanical data access devices such as disk drives. One benefit of the use of a NAS system16is its ubiquity in modern systems. Because of wide deployment, NAS technology and products enjoy benefits such as low cost, excellent performance in a variety of applications, an ongoing advancement through the investments of NAS system manufacturers.

The CAS system14is a specialized storage system or device that is optimized to provide storage and access for large collections of relatively fixed data objects, such as libraries of documents, images, backed-up files, etc. These change infrequently or never, but they must be available for use by application programs (applications) hosted by the client10. An example might be a library of fingerprint images used as part of a fingerprint-based identification system. An application on the client10may provide a service of comparing sample fingerprints with fingerprints stored in the CAS system14to identify an individual. This type of storage may be referred to as “write once read many” or “WORM” storage.

In the present case, an important aspect of the CAS system14is the use of a content-addressing scheme in contrast to the more conventional directory-based organization schemes employed by file systems such as provided by the NAS system16. While more details are provided below, one aspect of content addressing is its use of a “flat” address space, also referred to as a “name space” herein. “Flat” refers to the lack of inherent hierarchy. For example, a content addressing scheme may utilize a hash function or similar high-entropy function that associates each file or data object with a corresponding n-digit number or address. Hash algorithms generating 256-bit hash values, for example, are currently in use. Each hash value can be interpreted as a single point in a linear array of 2256points. Two data objects that may be very similar, such as different versions of the same spreadsheet for example, might hash to widely separated points in this space. Thus the hash value, or more generally content address, generally contains no organization information such as is inherent in a directory-based organization scheme.

The term “legacy” with respect to the CAS system14refers to its preexistence at a time of a migration of its data to the NAS system16, as described in more detail herein. The CAS system14is used with a client-side software library that presents a CAS-specific application programming interface (API), termed a “legacy API” herein, to applications executing on the client10. One feature of the presently disclosed system is a client-based data migration technique that transfers data from the CAS system14to the NAS system16without requiring modification of existing client applications using the legacy API, as also described in more detail below.

A migration manager node18is a specialized client system used primarily or exclusively for migrating data from the CAS system14to the NAS system16. It is shown as optional because the primary migration method described herein is based on functionality in the client10, which performs data migration as part of its normal processing of application data. The migration manager18, if present, may be used in a more dedicated manner, programmed as a background or demon type of process to read data from the CAS14and write it to the NAS16. The data may be accessed in a linear fashion, for example, starting at one end of the CAS address space and proceeding to the other end. It may be desirable to perform the transfers in batches or chunks of successive data objects. It will be appreciated that if a migration manager18is used, migration of a large data set can generally be achieved in substantially less time than if all migration is done by the client10itself.

FIG. 2is a generalized depiction of a computer such as may be used to realize the client10, CAS system14, NAS system16and migration node18. It includes one or more processors20, memory22, local storage24and input/output (I/O) interface circuitry26coupled together by one or more data buses28. The I/O interface circuitry26couples the computer to one or more external networks (such as network12), additional storage devices or systems, and other input/output devices as generally known in the art. System-level functionality of the computer is provided by the hardware executing computer program instructions (software), typically stored in the memory22and retrieved and executed by the processor(s)20. Any description herein of a software component performing a function is to be understood as a shorthand reference to operation of a computer or computerized device when executing the instructions of the software component. Also, the collection of components inFIG. 2may be referred to as “processing circuitry”, and when executing a given software component may be viewed as a function-specialized circuit, for example as a “mapping circuit” when executing a software component implementing a mapping function. It will be appreciated that storage systems such as CAS system14and NAS system16may have more specialized hardware organizations providing desired large storage capacity, high data transfer bandwidth, and other storage-specific features as generally known in the art. Any specialized hardware features for the disclosed operations are described below.

FIG. 3is a schematic depiction of elements of the client10in conjunction with organizations of data stored on the CAS system14and NAS system16. The client10includes software in the form of an application program (also referred to as application or app)30and a library32of routines via which the application30accesses data stored on the CAS system14or NAS system16. In one embodiment, the library32is a so-called “compile time” library that is linked into an executable application image that becomes installed on the client10. In this case, reference to the application30is to be understood as that part of an application image separate from the linked-in components of library32, i.e., the core software components executing higher-level operations that use the library32to carry out lower-level operations. For ease of description only one application30is described, but in general a client10may execute one or more applications that use CAS storage in the manner described herein.

At its interface to the application30, the library32provides the above-discussed legacy API34, as shown. Facing the network12and storage devices14,16ofFIG. 1, the library32supports both a CAS protocol36and a NAS protocol38. The CAS protocol is based on a CAS model of the data storage as shown on the left, which is a collection of elements each having a data object40and an associated content address (CA)42. The data objects40correspond to files as known to the application30, as described more below. The NAS protocol38is based on a file-based model of the data storage as shown on the right, which is a hierarchical, directory-based file system such as discussed above. This file system is not necessarily visible to the application30, and in fact it is assumed herein that the application30is a legacy application having the CAS model view of its stored data. The file system of the NAS system16is used as the underlying storage structure for an emulation of the CAS model which is provided by the library32. The NAS protocol38preferably employs the preferred read/write access method of the NAS system16, such as HTTP, NFS, CIFS, etc.

Referring to the CAS model again, each data object40typically includes a corresponding data file known to the application30, and may include file-related metadata as well (e.g., file name, data of creation/modification, owner, etc.). Each content address42is an address value generally unique to the corresponding data object40. When a file is initially transferred from the application30to the CAS system14for storage, it is provided without any specification of a location or address where it should be stored. The CAS system14performs a hash or similar function using the contents (and perhaps metadata) of the file to generate the file-specific CA42, and this value is returned to the application30for use in subsequent read operations directed to the same file. During a read, the application30provides a CA, and this CA is compared with stored CAs42. If a matching CA42is found, then the associated object40is returned to the application30in satisfaction of the read request.

Referring briefly to the file-based model, as known in the art each application access to a user file is accompanied by a path name or other specification of a directory in which the file resides. The file system uses the directory information in the request to locate a directory file for the directory where the file resides, then uses information in that directory file to locate the user file that is the target of the operation. It will be appreciated that without some kind of intervening translation, a directory-based file system does not lend itself to supporting the CAS model. This partly explains the existence of specialized CAS systems14, which can employ data organizations specifically tailored for efficient storing and retrieving of content-addressed data objects.

As described in more detail below, the library32performs operations involving both the CAS system14and the NAS system16in response to operations of the application30. When interacting with the CAS system14, the library32uses the CAS protocol36and its underlying data model as described above. These operations have a somewhat pass-through nature, meaning that functions and data defined at the legacy API34have direct counterparts in the CAS protocol36. This is not the case when the library32interacts with the NAS system16, which has no understanding of the CAS protocol36or the CAS model of data organization as described above. Thus, in these operations the library32performs a translation between functions and data of the legacy CAS API34and the functions and data of the NAS protocol38and the file system provided by the NAS system16.

FIG. 4provides an illustration of this translation. A file as visible to an application30is associated with a specific directory and file name in the file system of the NAS system16by two mappings. In a first mapping50, an identifier52of the file (such as the file name) is mapped to a corresponding content address (CA)54. The first mapping50may be realized by one or more hashes or similar functions. In a second mapping56, the content address54is mapped to a corresponding directory name58of a directory in the hierarchical file system of the NAS system16where the file is stored. In operation, it is also necessary to have a file name to be used for the file in the NAS system16. It may be convenient as well as useful to use the same file name as is used in the file system of the application30. It should be noted that the CA54is not the same as the above-discussed CA42, and in fact in one embodiment described below a CA used in the same manner as CA54is derived from another CA that functions as a file identifier.

FIG. 4includes a simple example. In this case the file name is used as the identifier52, and it has the value “asdf.txt”. This is the name of the file in the file system of the client10. This file name is hashed to the CA54that is represented by the string ABCDEF . . . . The CA54may be of a fixed size (e.g., 64 bits) as dictated by a hash function used for the first mapping50, although in some embodiments the CA54may not be of a fixed size. Sequential sets of bits are interpreted as characters usable to form directory names in the file system of the NAS system16. A 64-bit CA54may be taken to represent an 8-character string, for example. The string constituting the CA54is then mapped to a path name of a directory, for example by grouping successive characters and treating each group or set as a directory name at a given hierarchical level. In the example, the string ABCDEF . . . is mapped to a three-level path name AB/CD/EF, identifying a third-level subdirectory EF under a top level directory AB and its subdirectory CD. This name is obtained by extracting the first six characters of the CA54, then dividing the extracted characters into three successive 2-character groups. When the file name is appended, the fully qualified file name is /AB/CD/EF/asdf.txt.

The directory depth is not required to be a fixed depth. The depth can vary at run-time. For example, the library30might fill up the /AB/ directory with a certain number of files, then once it has reached the maximum number of desired files, it would fill subdirectories of /AB/with files, and so forth. This adaptive approach can improve search times for small numbers of files, as well as reduce the number of directories created for a small number of files. As the number of files grows, the library30searches down the tree to the maximum expected depth. The directory depth and max-files-per-directory may be configurable parameters of the library30.

FIG. 5illustrates operation of the library32in connection with use of the NAS system16to store and retrieve data on behalf of the application30that is using the legacy CAS API34. Step60shows actions taken as part of a file storing operation for a user data file initiated by the application30. It includes the following:1) Applying a first mapping to a file identifier for the file to originate a corresponding content address in a flat content address space. An example of a file identifier is a file name (as known to the application30), but other types of identifiers may be used. The first mapping may be a hash function as described above, but other mappings may also be used.2) Applying a second mapping to the content address to originate a corresponding multi-level directory name usable to identify a subdirectory in a hierarchical directory structure of a file system of the file-based storage system (e.g., NAS system16). The second mapping may be a function as described above, i.e., dividing some or all of the content address into ordered groups of digits or characters and then interpreting each group as a directory name at a corresponding level of the directory hierarchy.3) Invoking the file system to store the user data file in the subdirectory identified by the multi-level directory name.

Step62shows actions taken as part of a subsequent file retrieving operation for the user data file (as may be initiated by the application30using the same file identifier). It includes the following:4) Applying the first mapping to the file identifier to regenerate the content address.5) Applying the second mapping to the regenerated content address to regenerate the multi-level directory name.6) Invoking the file system using the regenerated multi-level directory name to retrieve the user data file from the subdirectory identified by the directory name.

FIG. 6illustrates a variant of the scheme ofFIG. 4according to a more specific embodiment for emulating CAS storage using a NAS system16. A user data file64along with associated metadata65(e.g., information from file system of client10such as file name, owner name, data of creation, etc.) are packaged together into a data object40that will be the container for storing the user data file64. Additionally, a first hash function66is calculated from the contents of the user data file64. As shown, the result constitutes the content address (CA, referred to as 1stCA)42ofFIG. 3. The 1stCA42is returned to the application30as the identifier52ofFIG. 4. During a read, the application30presents the object identifier52, which is analogous to requesting a file by its file name. The operations up to this point are the same as those normally performed in the CAS system14in its interaction with an application30. Thus, one aspect of the present disclosure is locating these functions in the library32at the client10, to facilitate migration as described in more detail below.

Also shown inFIG. 6is a second hash function50applied to the first CA42to originate the 2ndCA54ofFIG. 4. From this point, the scheme is as discussed above, i.e., the 2ndCA is mapped (2ndmap56ofFIG. 4) to a directory name58which is used to identify a subdirectory in the file system of the NAS system16where the data object40is to be stored and retrieved from. It will be appreciated that when the application30first writes the user data file64, all the operations depicted inFIG. 6are performed. During subsequent reads of the user data file64, the application30provides the object identifier52, and operation is as described above with respect toFIGS. 4 and 5.

FIGS. 7-9are used to describe migration of a CAS data store from the legacy CAS system14to the NAS system16. It is assumed that at the time of initial operation of the NAS system16the legacy CAS system14already contains data objects for user data files previously generated by the application30. These existing data objects have associated object identifiers taken from the flat, linear name space used by the CAS model (seeFIG. 2and CAs42). Migration involves two aspects. First, newly generated user data files are stored on the NAS system16as the new primary store (although they may also be mirrored to the legacy CAS14for certain purposes, as explained below). Second, all user data files already stored on the legacy CAS system14are transferred to the NAS system16. This transfer preferably occurs while normal operation of the application30continues. While migration-related data traffic may cause some minor performance degradation, this may be offset by superior performance of the NAS system for reads on previously migrated data as well as acknowledgement of new writes.

FIG. 7illustrates a manner by which the library32handles file storing operations (writes) for new user data files received from the application30according to the legacy CAS API34. As indicated, new user data files are packaged into corresponding writes70to the NAS16. The writes70are generated using a form of the above-described mapping process, as described more below. In connection with each write70, the library32generates an object identifier (e.g., CA42) that is returned to the application30for later use in retrieving a newly written data object40that includes the user data file being stored.

The application30never knows where the data object is actually stored. The object identifier may have no state information about the location of the data object. The library32may be configured for reading and/or writing to CAS system14and/or NAS16according to an explicit scheme. In general, configuration options could be represented conceptually by a table as follows:

The configuration options may be used in the following manner:

1. Writing New Data:If writing data is enabled for both CAS and NAS, then the library32is configured for mirroring of data between the CAS system14and the NAS system16.If writing data is only enabled on the NAS system16, then the user feels confident about the overall solution and is comfortable having newer data available only on the NAS system16.If writing data is only enabled for the CAS system14, then the user probably went from a mirrored configuration to one where the user wants to revert to a CAS-only solution.

2. Reading Data:If reading data is enabled for both CAS and NAS, then the library32can apply one of several techniques depending on desired behavior and depending on the relative performance of the NAS system16versus the CAS system14. The library32can attempt to read data from one of the systems14or16, and if found then return it to the user, and if not found then try the other system. If the data is found on the second system, then it is returned to the user, and if not found on the second system then a failure-to-locate message may be returned. There may be a configuration of which system to check first, which could be determined based on relative speed, percentage completion of the migration job (affects probability of finding data on one system versus the other), load difference on the storage systems, or other specific criteria. Alternatively, the library32could query both storage systems14,16at the same time, leveraging the parallel execution for faster response time.If reading data is enabled for only the CAS system14, then the user probably went from a mirrored configuration to one where the user wants to revert to a CAS-only solution.If reading data is enabled for only the NAS system16, then the user is probably preparing to switch to a NAS-only solution.

The object identifier (e.g., CA) returned to the application30is opaque to the application30, meaning the application30does not interpret it. The library32, as an intermediate layer between the application30and the storage systems14,16, does understand the format of the object identifier. The library32may place a hint in the object identifier of newly written data which can help the library32determine the location of the data object in a subsequent access. In one embodiment such a hint may always be included, whereas in other embodiments any use of such a hint may be used in some conditional manner depending on any of a variety of criteria known to the library32. Such alternatives also apply to any other state and information that might be stored in the object identifier for later use in an interaction with the application30.

Referring again toFIG. 7, as indicated at72, the writes70may be mirrored or duplicated to the CAS system14so that its contents remain current. This might be done to enable the migration operation to be aborted if necessary or desired without losing any data newly stored during the migration operation. The use of mirroring to the CAS14can also provide increased confidence in case of concerns about the robustness of the migration operation—migration can proceed knowing that it will be possible to revert to operation with the CAS14if any problems develop. This possibility exists until the mirroring is stopped, which may occur when there is sufficient confidence in the integrity and operation of the NAS16to commit to NAS-only operation going forward.

FIG. 8illustrates a manner by which the library32handles file retrieving operations (reads) for user data files that are requested by, and returned to, the application30according to the legacy CAS API34. In particular, the operation depicted assumes that the target data objects are stored on the CAS system14, such as data objects that were generated and stored prior to the beginning of the migration operation. For objects generated and stored after the beginning of the migration operation, they may be obtained from the NAS system16as the new primary store, or perhaps from the CAS system14if mirroring has been used. As indicated above, there may be a variety of ways by which the library32locates target data objects.

As shown at80, the library32performs a retrieve operation (read)80from the CAS system14. The request from the application30includes an object identifier previously returned to the application30when the file was initially given to the library32for storing. The read30includes this object identifier. The CAS system14uses an associative lookup to obtain the corresponding data object (e.g., data object40) and returns it to the library32, which extracts the user data file (e.g., user data file64) from the data object and returns it to the application30in satisfaction of the request. Additionally, the library32generates a write82to store the data object in the NAS16, achieving the migration of that particular data object from the CAS14to the NAS16. Subsequent reads for this data object are satisfied from the NAS16rather than from the CAS14. If the system is not keeping the CAS system14up to date (e.g., by using mirroring), then at this point the data object may be deleted from the CAS14.

FIG. 9is a flowchart describing in more detail the operations of the client computer10, specifically the library32, outlined with respect toFIGS. 7-8. Specifically,FIG. 9describes a method of operating the client10to migrate a set of data objects (e.g., objects40) from a first remote data store employing a flat linear namespace (e.g., CAS system14) to a second remote data store employing a file system having a hierarchical directory structure (e.g., NAS system16).

Step90is performed for each of a plurality of file storing operations for respective user data files. It includes generating a data object and an object identifier, where the data object contains the user data file being stored and the object identifier uniquely identifies the data object. The object identifier is returned to an application program (e.g., application30) for subsequent use in retrieving the user data file being stored. Secondly, a mapping function is performed on the object identifier to originate a subdirectory name of a subdirectory of the hierarchical directory structure of the second remote data store. Thirdly, write request is sent to the file system of the second remote data store. The write request includes the data object, the subdirectory name, and a command to create a subdirectory having the subdirectory name and store the data object in the subdirectory. As mentioned above, this write may also be mirrored to the first remote data store for the above-discussed purposes.

Step92is performed for each of a plurality of file retrieving operations directed to user data files contained in respective data objects stored in the first remote data store, wherein each file retrieving operation is initiated with a respective object identifier in the flat linear namespace uniquely identifying a data object containing a user data file being retrieved. This step includes (4) sending a read request including the object identifier to the first remote data store, (5) upon receiving a data object from the first remote data store in satisfaction of the read request, extracting a user data file and returning the extracted user data file to the application, (6) performing the mapping function on the object identifier to originate a subdirectory name of a subdirectory of the hierarchical directory structure, and (7) sending a write request to the file system of the second remote data store, the write file request including the received data object, the subdirectory name, and a command to create a subdirectory having the subdirectory name and store the data object in the subdirectory. Parts (6) and (7) correspond to the writes82ofFIG. 8.

As mentioned above, the migration performed by the writes82and operations92can be augmented by additional operations generated by a dedicated migration manager18, which may be desirable if it is desired to complete migration quickly and/or when the application30is not guaranteed to read every data object stored on the legacy CAS system14.