From: Neil Brown <neilb@cse.unsw.edu.au>
To: Jens Axboe <axboe@suse.de>
Date: Mon, 6 Sep 2004 11:36:50 +1000
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Message-ID: <16699.48946.29579.495180@cse.unsw.edu.au>
Cc: Christoph Hellwig <hch@lst.de>, linux-kernel@vger.kernel.org
Subject: Re: bug in md write barrier support?
In-Reply-To: message from Jens Axboe on Saturday September 4
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On Saturday September 4, axboe@suse.de wrote:
> On Sat, Sep 04 2004, Neil Brown wrote:
> > On Friday September 3, hch@lst.de wrote:
> > > md_flush_mddev just passes on the sector relative to the raid device,
> > > shouldn't it be translated somewhere?
> > 
> > Yes.  md_flush_mddev should simply be removed.  
> > The functionality should be, and largely is, in the individual
> > personalities. 
> 
> Yes, sorry I was a little lazy there even though I followed the plugging
> conversion :(
> 
> > Is there documentation somewhere on exactly what an issue_flush_fn
> > should do (is it  allowed to sleep? what must happen before it is
> > allowed to return, what is the "error_sector" for,  that sort of thing).
> 
> It is allowed to sleep, you should return when the flush is complete.
> error_sector is the failed location, which really should be a dev,sector
> tupple.

Could I get a little more information about this function please.
I've read through the code, and there isn't much in the way of
examples to follow: only reiserfs uses it, only scsi-disk and ide-disk
supports it (I think).

It would seem that this is for write requests where b_end_io has already
been called, indicating that the data is safe, but that maybe the data
isn't really safe after-all, and blk_issue_flush needs to be called.

I would have thought that after b_end_io is called, that data should
be safe anyway.  Not so?

How do you tell a device: it is OK to just leave the data is cache,
I'll call blk_issue_flush when I want it safe.
Is this related to barriers are all??

NeilBrown
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