Re: [PATCH] ide write barrier support

[Matthias Andree <matthias.andree@gmx.de>]

On Fri, 17 Oct 2003, Jens Axboe wrote:

> On Fri, Oct 17 2003, Manfred Spraul wrote:
> > Jens wrote:
> > 
> > >The problem is that as far as I can see the best way to make fsync
> > >really work is to make the last write a barrier write. That
> > >automagically gets everything right for you - when the last block goes
> > >to disk, you know the previous ones have already. And when the last
> > >block completes, you know the whole lot is on platter.
> > >
> > Are you sure?
> > What prevents the io scheduler from writing the last block before other 
> > blocks?
> 
> Very sure, the io scheduler will never put the barrier write before
> previously comitted writes. So yes, it will work as described.

If the "last block" is special however (containing a "complete" marker
for instance), the order needs to be:

write1 write2 write3 write4 barrier write-last barrier

This will guarantee write-last will happen after write4.

The drive is free to permute the order of write1 ... write4, no?

Manfred, was that your idea?

-- 
Matthias Andree

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