Re: [00/17] Large Blocksize Support V3

From: Jens Axboe
Date: Thu Apr 26 2007 - 14:28:52 EST

On Thu, Apr 26 2007, Christoph Hellwig wrote:
> On Thu, Apr 26, 2007 at 08:12:51PM +0200, Jens Axboe wrote:
> > > Exactly. But the only counter-proposal we have so far seems far worse :)
> >
> > Lets look at some numbers. I'll just concentrate on the scatterlist,
> > since the bio_vec is smaller. On x86 32-bit, the scatterlist is 20 bytes
> > long. If we accept that 2^1 allocations are ok (they should be), then we
> > can support ~1.6mb ios just like that.
> >
> > My approach would be to support scatterlist chaining. Essentially you'd
> > have the last element of the sglist pointing to the next array of
> > entries. We can then stick to 128 entry arrays which fit nicely in a
> > single page allocation and easily support >> 2mb ios. The only caveat is
> > that you'd need to update the drivers to get there, since a regular
> > iteration over the array isn't enough. My plan was to add an sglist
> > iterator helper that hides this from the drivers, if they need to loop
> > over the scatterlist. Things like {dma/pci}_map_sg() would of course be
> > updated.
> >
> > The above can be implemented fairly cleanly, and on a need-to-have
> > basis. It's not something that'll break drivers.
> >
> > What do you think?
> Purely for the I/O sizes to external arrays problem that's nice,
> and I think we (well, you :)) should implement it.

I will get it implemented, next week.

> But there's other reasons why larger objects in the page cache make
> sense that are mostly related to keeping overhead for large files
> in the operating system down. So I'd go both for s/g list chaining
> and variable order pagecache.

Oh I definitely agree, I just think we should keep the discussion
focused on the seperate issues and not mix everything up.

> Btw, we should talk a little about the sglist iterators on linux-scsi,
> as a lot of the dma mapping API will need updates for bidirection dmas
> anyway, and we should try to get everything done in one rush.


Jens Axboe

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