Re: [9fans] corrupted update on 9vx

["EBo" <ebo@sandien.com>]

Sorry for the delay

> On Thu Apr 22 14:45:38 EDT 2010, steve@quintile.net wrote:
> > replica uses fcp which works multithreaded for speed.
> >
> > Perhaps the implementation of fswrite() #Z in 9vx uses a seek()
> > followed by a write() rather than pwrite() which is not thread safe.
>
> replica actually uses replica/applylog.c:^/worker internally, not
> fcp.  but essentially, it's the same thing.  it copies NBUF=8192
> bytes at a time with 16 parallel processes.  #Z uses pwrite on non-sockets.
>
> in theory, since NBUF < SSIZE_MAX, pwrite should be atomic
> to normal files.

Thanks for the pointer.  I'll try playing around with NBUF and also see if I
can play with the number of processes (thwack it to 1 for testing and see if
that helps).

> you're not using a fuse-based fs, are you?

no.  I did have fuse installed, but it was only used by usermode-utilities,
and I have since removed both that and fuse.

> possible ways to debug:
> 1.  reduce NBUF to PIPE_BUF=4096.  guard against someone treating
> the fd as a pipe.
> 2.  put a qlock in the UnixFd structure in devfs-posix.c.  lock for all io.

If I recall correctly, qlock has not been ported to 9vx.  Ahhh... I see that
it has.  This saves me the hassles of porting it myself ;-)

Laters,

  EBo --