[Lustre-discuss] filesystem corruption

[Peter Kjellstrom]

On Monday 07 September 2009, Richard Smith wrote:
> Kevin Van Maren wrote:
> > This subject has been discussed many times...
> >
> > Not just the controller, but the drives as well.
> >
> > The problem is with write-back caches that _lie_ about the data being in
> > persistent store.  The drive itself, with write-back cache enabled, lies
> > and says data is on disk.  RAID controllers likewise use write-back
> > cache to lie about the data being on disk.
>
> <snip>
>
> I'm not convinced there's any lie involved. SCSI permits data to be
> written back only as far as a cache and have a GOOD status returned at that
> point. If for any reason a guarantee is required that the data really is on
> media, then my understanding is that's what SYNCHRONIZE CACHE command
> and/or FUA (Force Unit Access) control bit is for.

I also feel that "lie" is a bit incorrect as a description. However, while FUA 
does exist it's typically ignored by controllers. If you're lucky you have 
a "ignore FUA: enable/disable" setting.

/Peter

> What's not so clear to 
> me is under what circumstances either technique is triggered, whether an
> fsync is sufficient for example to propagate the request down to the
> low-level device driver. It sounds like it would be device driver-specific.
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