1. A computer system comprising: a plurality of data archiving servers in which a client stores data; a storage subsystem which provides storage extents to the plurality of data archiving servers; and a management computer which include access to the plurality of data archiving servers and the storage subsystem, wherein the plurality of data archiving servers each comprise a first interface coupled to the storage subsystem, a first processor coupled to the first interface, and a first memory coupled to the first processor, and provide the client with data archiving storage extents to which the storage extents are allocated, wherein the storage subsystem comprises a second interface coupled to the plurality of data archiving servers, a second processor coupled to the second interface, and a second memory coupled to the second processor, wherein the management computer comprises a third interface coupled to the plurality of data archiving servers and the storage subsystem, a third processor coupled to the third interface, and a third memory coupled to the third processor, wherein the plurality of data archiving servers each manage data location management information, which holds association between the data and a data archiving server of the plurality of data archiving servers that stores the data, and data archiving storage extent configuration information, which holds association between the data archiving storage extents and the storage extents, wherein the data location management information is managed by the data archiving server that is determined based on the data, wherein the management computer manages data archiving server performance management information, which holds information about performance of the plurality of data archiving servers, and storage utilization information, which holds information about load on the storage extents, wherein the plurality of data archiving servers are each configured to: identify, upon reception of a request from the client to read data, according to the data requested to be read, a data archiving server of the plurality of data archiving servers that manages data location management information of the requested data; obtain the data location management information managed by the identified data archiving server; identify, from the obtained data location management information, a data archiving server of the plurality of data archiving servers that is storing the requested data; and obtain the requested data from the data archiving server that is storing the requested data, wherein the management computer is configured to: select, based on the data archiving server performance management information and the storage utilization information, data archiving storage extents whose association with the storage extents are to be changed; select, based on the storage utilization information, storage extents that are to be newly allocated to the selected data archiving storage extents; and notify the plurality of data archiving servers that provide the selected data archiving storage extents of the storage extents selected to be newly allocated, and wherein the plurality of data archiving servers are each further configured to: allocate the storage extents notified by the management computer to the selected data archiving storage extents; and update the data location management information and the data archiving storage extent configuration information.