Question

we are receiving about 10000 messages per hour. We store them as individual files in hourly directories on an ext3 filesystem. The file name includes a sequence number. We use rsync to mirror these files every 20 seconds at another location (via a SAN, but that doesn’t matter).

Sometimes an rsync run picks up files n-3, n-2, n-1, n+1, and then next rsync run continues with n, n+2, n+3, n+4 and so on.

Is it possible that when one process creates files in a certain sequence within a directory, that another process using readdir() sees the files appearing in a different sequence?

Kind regards,
Sebastian

Answer 1

I suppose your question can be restated as:

If process A creates file d/x and
then creates file d/y, is it
possible for process B to peform a
concurrent readdir() on directory d and see
an entry d/y, but not see an entry
d/x?

The answer is Yes. The ordering guarantees for readdir are very weak indeed.

If you want to enforce an ordering, you will need to explicitly fsync() a file descriptor for the directory d itself after creating each file.