Trond Myklebust wrote:
On Sat, 2008-06-21 at 20:05 +0100, Phil Endecott wrote:
Dear Experts,
I have a program which uses an mmap()ed read-mostly data file. When
not using NFS, each instance of the program can use inotify to detect
when other instances have made changes to the data file. Since inotify
doesn't work with NFS, I have now implemented a scheme using network
broadcasts to announce changes. At present it works like this:
All instances of the program mmap(MAP_SHARED) the data file.
One instance stores some new data at the end of the file and calls
msync(MS_SYNC) on the affected pages. It then "atomically commits" the
new data by write()ing a new header at the start of the file with an
"end of data" field advanced to include the new data. It then calls
fdatasync(). Then it transmits a broadcast packet.
The other instance(s) of the program receive the broadcast packet and
read() the header at the start of the file. My hope was that they
would see the new value, but they don't; they continue to see the old value.
open(O_DIRECT) is your friend.
Thanks Trond, I'll give it a try.
This only affects the write()s and read()s though, doesn't it? So are
you suggesting that the mmap()ed data is correctly propagated already,
and only the write-to-read needs fixing?
BTW the man page is a bit discouraging about the combination of
O_DIRECT and mmap(): "applications should avoid mixing mmap(2) of files
with direct I/O to the same files." Fingers crossed....
Phil.
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