（Firebird_v1_ReleaseNotes.pdf から抜粋）
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Engine support for 64-bit file I/O

All Firebird/IB versions prior to FB v1.0 (including IB v6.0) used both 32-bit API/OS calls and 32-bit
(integer) file pointer structures. The engine's capability was thus designed to handle files with a
maximum file size of 4Gb. In the Unix code the file pointer structures were defined as signed
integers, thus limiting Unix and Linux implementations to file sizes of 2Gb.

MS Windows

By design, all versions of Win32 (Win9x/ME/NT/2000) provide full support for 64-bit pointer file
operations, provided the right file pointer structure is passed. To add large file support for Firebird, it
was necessary only to change the file pointer structure to LARGE_INTEGER and to modify the I/O calls
themselves slightly.
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NTFS なら 4G を超えるファイルに出来るみたい。試して見れ。