[Lustre-discuss] filesystem corruption

[Richard Smith]

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. 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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