Re: DDF: regression caused by 273989b9 / ce45c819

[NeilBrown <neilb@suse.de>]

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On Tue, 06 Aug 2013 20:14:35 +0200 Martin Wilck <mwilck@arcor.de> wrote:

> Hi Neil,
> 
> these patches break the unit test 10ddf-geometry. I saw the regression
> with both patches applied. The problem occurs when subarrays are
> deleted. With these patches in place, sync_metadata() will not overwrite
> deleted conf records on disk, but lseek() over them instead. When the
> meta data is read back, this will cause errors.
> 
> I would like to ask you to play safe here and revert these patches. It
> might be possible to fix the --kill-subarray problem, but there are
> other possible scenarios where the number of valid conf records on a
> disk decreases - I don't think we have a reliable way to check whether
> it is safe to skip over empty entries. We must also be prepared for
> other DDF implementations to read our meta data, so we must refrain from
> putting any writing anything that might be confusing.
> 
> Regards
> Martin

Yes, you are right.  That wasn't such a good idea, thanks.

I've changed it to allocate a large buffer (which is done for 'read' - and we
use the same buffer), fill that in and write.  That is a lot faster than lots
of individual writes, due to the fact that the fd is O_DIRECT.

Thanks,
NeilBrown

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