1. A method of storing and retrieving data by a computer system executing an operating system and supporting first and second persistent storage devices, said operating system including a filesystem module coupled through a first device driver to said first persistent storage device to transfer filesystem data blocks, said filesystem module providing support for synchronous write transactions, said method comprising the steps of: a) providing a second device driver selectively coupled between said filesystem module and said first device driver; b) collecting a predetermined set of data blocks provided from said filesystem module as part of a synchronous write transaction into a data segment, storing said data segment on said second persistent storage device, and signaling completion of said synchronous write transaction to said filesystem module, wherein said step of collecting further provides for 1) constructing a map relating said predetermined set of data blocks to said data segment, whereby said second device driver can identify said data segment by reference to any of said predetermined set of data blocks by said filesystem module; and 2) storing said map, including progressively updated versions of said map, on said second persistent storage device, whereby said map can be reconstructed from said second persistent storage device; c) migrating said predetermined set of data blocks from said second persistent storage device to said first persistent storage device by said second device driver through use of said first device driver independent of said filesystem module, wherein said data segment includes a predetermined set of address references for said predetermined set of data blocks and wherein said step of migrating said predetermined set of data blocks to said first persistent storage device is performed by said second device driver dependant on said predetermined set of address references; d) retrieving any of said predetermined set of data blocks from said first persistent storage device; e) selectively bypass writing said predetermined set of data blocks, as provided from said filesystem module, through said first device driver to said first persistent storage device; and f) selectively bypass reading said data segment from said second persistent storage device to transfer any of said predetermined set of data blocks to said filesystem module.