Re: Linux 2.6.29

[Hugh Dickins]

On Thu, 26 Mar 2009, Jens Axboe wrote:
> On Thu, Mar 26 2009, Hugh Dickins wrote:
> > On Thu, 26 Mar 2009, Jens Axboe wrote:
> > > On Wed, Mar 25 2009, Hugh Dickins wrote:
> > > > 
> > > > Tangential question, but am I right in thinking that BIO_RW_BARRIER
> > > > similarly bars across all partitions, whereas its WRITE_BARRIER and
> > > > DISCARD_BARRIER users would actually prefer it to apply to just one?
> > > 
> > > All the barriers refer to just that range which the barrier itself
> > > references.
> > 
> > Ah, thank you: then I had a fundamental misunderstanding of them,
> > and need to go away and work that out some more.
> > 
> > Though I didn't read it before asking, doesn't the I/O Barriers section
> > of Documentation/block/biodoc.txt give a very different impression?
> 
> I'm sensing a miscommunication here... The ordering constraint is across
> devices, at least that is how it is implemented. For file system
> barriers (like BIO_RW_BARRIER), it could be per-partition instead. Doing
> so would involve some changes at the block layer side, not necessarily
> trivial. So I think you were asking about ordering, I was answering
> about the write guarantee :-)

Ah, thank you again, perhaps I did understand after all.

So, directing a barrier (WRITE_BARRIER or DISCARD_BARRIER) to a range
of sectors in one partition interposes a barrier into the queue of I/O
across (all partitions of) that whole device.

I think that's not how filesystems really want barriers to behave,
and might tend to discourage us from using barriers more freely.
But I have zero appreciation of whether it's a significant issue
worth non-trivial change - just wanted to get it out into the open.

Hugh
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