Go back using active flag in metadata index header instead of using a… #2267
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… separate 'active' file, which introduced race conditions. A separate file cannot guarantee the order of the following events: removal of active file (file system's metadata server operation), and arrival of content of metadata index file on disk (a storage operation). Index content may be en route to disk but held up to indefinite time while the active file disappears, so a reader may not get the last steps believing the file is not active anymore. In contrast, active flag is written to the same file after the last steps are written, so order is guaranteed as long as the file system guarantees the order of writes to the same file.