rough estimate of kadmin addprinc performance?

[Nico Williams]

Looking at iprop code I've noticed that it does more msync()s than it
probably has to.  I suspect that this is a result of someone thinking
it'd be better to msync() just the bytes written to, but it's not:
each msync() acts as a write() + fsync(), and it's the fsync() that
kills performance, not the write().  OTOH, msync() could actually act
as a write() of all that memory (as opposed to a bunch of pwrite()s of
just the dirty pages), in which case an msync() of the whole ulog
could be very painful.  It'd be useful to know what the behavior of
msync() is on the various OSes.

I'm starting to think that we should use mmap() only for reading the
log, but for writing we should pwrite() and then fsync() when we're
done.  This can probably be done with relatively little violence to
the code.

Or am I missing something?  All writes to the ulog are done under
exclusive lock, so msync()ing parts at a time is not done for
synchronization reasons.

I think what we want is something like a guarantee that several
msync()s with MS_ASYNC followed by an fsync() will cause the msync()ed
regions to be written to disk before the fsync() returns.  But that's
not what the semantics of msync() are.

Nico
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