[Gluster-devel] relative ordering of writes to same file from two different fds

[Raghavendra Gowdappa]

----- Original Message -----
> From: "Ric Wheeler" <ricwheeler at gmail.com>
> To: "Raghavendra Gowdappa" <rgowdapp at redhat.com>, "Gluster Devel" <gluster-devel at gluster.org>
> Cc: "ryan ding" <ryan.ding at open-fs.com>
> Sent: Wednesday, September 21, 2016 6:32:29 PM
> Subject: Re: [Gluster-devel] relative ordering of writes to same file from two different fds
> 
> On 09/21/2016 08:06 AM, Raghavendra Gowdappa wrote:
> > Hi all,
> >
> > This mail is to figure out the behavior of write to same file from two
> > different fds. As Ryan quotes in one of comments,
> >
> > <comment>
> >
> > I think it’s not safe. in this case:
> > 1. P1 write to F1 use FD1
> > 2. after P1 write finish, P2 write to the same place use FD2
> > since they are not conflict with each other now, the order the 2 writes
> > send to underlying fs is not determined. so the final data may be P1’s or
> > P2’s.
> > this semantics is not the same with linux buffer io. linux buffer io will
> > make the second write cover the first one, this is to say the final data
> > is P2’s.
> > you can see it from linux NFS (as we are all network filesystem)
> > fs/nfs/file.c:nfs_write_begin(), nfs will flush ‘incompatible’ request
> > first before another write begin. the way 2 request is determine to be
> > ‘incompatible’ is that they are from 2 different open fds.
> > I think write-behind behaviour should keep the same with linux page cache.
> >
> > </comment>
> 
> I think that how this actually would work is that both would be written to
> the
> same page in the page cache (if not using buffered IO), so as long as they do
> not happen at the same time, you would get the second P2 copy of data each
> time.
> 
> Same story for using O_DIRECT - that write bypasses the page cache and will
> update the data directly.
> 
> What might happen in practice though is that your applications might use
> higher
> level IO routines and they might buffer data internally. If that happens,
> there
> is no ordering that is predictable.

Thanks Ric.

1. Are filesytems required to maintain that order?
2. Even if there is no such requirement, would there be any benefit in filesystems enforcing that order (probably at the cost of increased latency).

regards,
Raghavendra

> 
> Regards,
> 
> Ric
> 
> >
> > However, my understanding is that filesystems need not maintain the
> > relative order of writes (as it received from vfs/kernel) on two different
> > fds. Also, if we have to maintain the order it might come with increased
> > latency. The increased latency can be because of having "newer" writes to
> > wait on "older" ones. This wait can fill up write-behind buffer and can
> > eventually result in a full write-behind cache and hence not able to
> > "write-back" newer writes.
> >
> > * What does POSIX say about it?
> > * How do other filesystems behave in this scenario?
> >
> >
> > Also, the current write-behind implementation has the concept of
> > "generation numbers". To quote from comment:
> >
> > <write-behind src>
> >
> >          uint64_t     gen;    /* Liability generation number. Represents
> >                                  the current 'state' of liability. Every
> >                                  new addition to the liability list bumps
> >                                  the generation number.
> >                                                                                                                                                                          
> >                                  a newly arrived request is only required
> >                                  to perform causal checks against the
> >                                  entries
> >                                  in the liability list which were present
> >                                  at the time of its addition. the
> >                                  generation
> >                                  number at the time of its addition is
> >                                  stored
> >                                  in the request and used during checks.
> >                                                                                                                                                                          
> >                                  the liability list can grow while the
> >                                  request
> >                                  waits in the todo list waiting for its
> >                                  dependent operations to complete. however
> >                                  it is not of the request's concern to
> >                                  depend
> >                                  itself on those new entries which arrived
> >                                  after it arrived (i.e, those that have a
> >                                  liability generation higher than itself)
> >                               */
> > </src>
> >
> > So, if a single thread is doing writes on two different fds, generation
> > numbers are sufficient to enforce the relative ordering. If writes are
> > from two different threads/processes, I think write-behind is not
> > obligated to maintain their order. Comments?
> >
> > [1] http://review.gluster.org/#/c/15380/
> >
> > regards,
> > Raghavendra
> > _______________________________________________
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> > Gluster-devel at gluster.org
> > http://www.gluster.org/mailman/listinfo/gluster-devel
> 
> 
>