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Date: Mon, 27 Sep 2004 16:32:34 +0200
From: Lars Ellenberg <Lars.Ellenberg@linbit.com>
To: drbd-dev@linbit.com
Subject: Re: [Drbd-dev] inter-arch PAGE_SIZE problem
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	<200409271604.18224.philipp.reisner@linbit.com>
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/ 2004-09-27 16:04:18 +0200
\ Philipp Reisner:
> Am Freitag, 24. September 2004 14:37 schrieb Lars Ellenberg:
> > int drbd_make_request_26(request_queue_t *q, struct bio *bio)
> > {
> > ...
> >   /*
> >    * what we "blindly" assume:
> >    */
> >   D_ASSERT(bio->bi_size > 0);
> >   D_ASSERT( (bio->bi_size & 0x1ff) == 0);
> >   D_ASSERT(bio->bi_size <= PAGE_SIZE);
> >   D_ASSERT(bio->bi_vcnt == 1);
> >   D_ASSERT(bio->bi_idx == 0);
> >
> > oopsie.
> > we are going to send PAGE_SIZE requests over the wire,
> > but the other side may have a different PAGE_SIZE...
> >
> > // mirrored write
> > int receive_Data(drbd_dev *mdev,Drbd_Header* h)
> > {
> > ...
> >         /* I expect a block to be a multiple of 512 byte, and
> >          * no more than 4K (PAGE_SIZE). is this too restrictive?
> >          */
> >         ERR_IF(data_size == 0) return FALSE;
> >         ERR_IF(data_size &  0x1ff) return FALSE;
> >         ERR_IF(data_size >  PAGE_SIZE) return FALSE;
> >
> >
> > we need to agree to use fixed 4K, I guess.  optionally negotiate a
> > higher "drbd_page_size" during the initial connection handshake.
> 
> Right. we should consider this. But I think a general high performace
> solution is not neccesary, since cluster of machines with
> different PAGE_SIZE are of academic interest only. (IMHO)
> 
> I think for drbd-0.8 it is sufficient to inform the user about the fact...

at least negotiate the size used as the minimum of the page sizes
of the peers. that should be easy enough to implement.

	lge