What is sector N used for? Part of a file opened by several processes?
If two processes open a file, one reading and another writing then
they may execute in parallell (particularly on SMP systems).
If the "reader" reads a sector that the "writer" is busy writing to,
then the reader may end up reading half "old" stuff and half "freshly
written" stuff from that sector.
Not forcing synchronization gives better performance, and it is
consistent with the bigger picture: If the writer writes a million
sectors and the reader reads a million sectors then the reader
may get several old and several new sectors on any os.
Linux merely lets this happen on the byte level too.
This is designed into the page cache, and may happen on
any platform, with any kind of block device.
An application that needs synchronization must handle the issues
itself. fsync may help but not necessarily; the "reader" may read
while the writer is in the middle of writing "B", before the writer
does the fsync. You really need a semaphore.
Helge Hafting
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