Re: network storage solutions

[Joe Landman <landman@scalableinformatics.com>]

On Thu, 2003-05-15 at 13:50, Jeff Layton wrote:

>    Since we use our cluster for production work (please, I'm
> not trying to offend anyone), we HAVE to have non-corrupted
> data. This is why we use hard mounts with 'sync' as well as
> a few other options. The URL above to Chuck's paper has
> several examples of "good" mount options.

Hmmm.  I am reasonably sure that when the IO system returns an error, it
does in fact get propagated to the appropriate user-land calling
program.  The program then makes the determination as to whether or not
to continue.  There are quite a few programs that rarely inspect return
code from file operations.  If you really require uncorrupted data, then
you are probably using the synchronous/unbuffered file writes anyway
(the O_SYNC, and possibly O_DIRECT options, though NFS has experimental
support for O_DIRECT from reading the note around Trond's patches).

> > The way I and other who use soft mounts view it, data lossage occurs
> > when the server crashes, as you cannot guarantee (except with sync),
> > that the data was committed to disk.
> >
> 
> However, if I read Chuck's paper correctly, with soft mount
> you can get a soft time-out that can interrupt an operation
> but the client will continue then with corrupted data. Am I
> understanding this correctly? Therefore, the clients may be
> up, but now the data is corrupt and the appliation doesn't
> know it.

I would like to know that as well.  I would like to believe it will not
continue with corrupt data, but return an error code/condition which
should be handled.

[...]

> I'm not sure... If the server crashes, I think this is true.
> But what if you get an interrupt. Soft mounts will allow
> the application to continue with corrupted data while hard
> mounts will produce an error, but not corrupt data (I think).

I hope not.  The programs that I send an INTR to on an NFS system (with
the intr flag allowed) seem to accept the signal and die.  I guess the
question is here, what should be the state of the filesystem upon
acceptance of that signal?  Can you assume it is in a known state?


-- 
Joseph Landman, Ph.D
Scalable Informatics LLC,
email: landman@scalableinformatics.com
web  : http://scalableinformatics.com
phone: +1 734 612 4615



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