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The File Change Log records file system changes such as creates, links, unlinks, renaming, data appended, data overwritten, data truncated, extended attribute modifications, holes punched, and miscellaneous file property updates.
Note
The FCL is supported only on disk layout Version 6 and 7.
FCL stores changes in a sparse file in the file system namespace. The FCL log file is located in mount_point/lost+found/changelog. The FCL file behaves like a regular file, but some operations are prohibited. The standard system calls open(2), lseek(2), read(2) and close(2) can access the data in the FCL, while the write(2), mmap(2) and rename(2) calls are not allowed.
The FCL log file contains both the information about the FCL (stored in the FCL superblock), and the changes to files and directories in the file system, stored as FCL records.
See File Change Log programmatic interface.
In 4.1, the structure of the file change log was exposed through the /opt/VRTS/include/sys/fs/fcl.h header file. In 5.0, the internal structure of the FCL file is opaque. The recommended mechanism to access the FCL is through the API described by /opt/VRTSfssdk/5.0/include/vxfsutil.h.
The /opt/VRTS/include/sys/fs/fcl.h header file is included in this release to ensure that applications accessing the FCL with the 4.1 header file do not break. Any new applications should use the new FCL API described in /opt/VRTSfssdk/5.0/include/vxfsutil.h. Existing applications should also be modified to use the new FCL API.
With the addition of new record types, the FCL version in 5.0 has been updated to 4. To provide backward compatibility for the existing applications, this release supports multiple FCL versions. Users now have the flexibility of specifying of specifying the FCL version for new FCLs. The default FCL version is 4.