linux-kernel - Re: [00/17] Large Blocksize Support V3

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Date:	Thu, 26 Apr 2007 22:05:43 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Christoph Lameter <clameter@....com>,
	Christoph Hellwig <hch@...radead.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	David Chinner <dgc@....com>, linux-kernel@...r.kernel.org,
	Mel Gorman <mel@...net.ie>,
	William Lee Irwin III <wli@...omorphy.com>,
	Badari Pulavarty <pbadari@...il.com>,
	Maxim Levitsky <maximlevitsky@...il.com>
Subject: Re: [00/17] Large Blocksize Support V3

Jens Axboe <jens.axboe@...cle.com> writes:

> On Thu, Apr 26 2007, Eric W. Biederman wrote:
>
> Yep, if you could just have > PAGE_CACHE_SIZE blocks in the filesystem
> easily, the problem would basically be solved for cd and dvd packet
> writing.

Ok.  I'm not in a position to do this work.  But I will keep it in
mind and look at it.

>> Am I correct in assuming that the problem is primarily about getting
>> filesystems (and other upper layers) to submit BIOs that take into
>> consideration the larger block size of the underlying device, so
>> that read/modify write is not needed in the pktcdvd layer?
>
> Yes, that is exactly the problem. Once you have that, pktcdvd is pretty
> much reduced to setup and init code, the actual data handling can be
> done by sr or ide-cd directly. You could merge it into cdrom.c, it would
> not be very different from mt-rainier handling (which basically does RMW
> in firmware, so it works for any write, but performance is of course
> horrible if you don't do it right).

Thanks for the clarification.

So we do have a clear problem that we do not have generic support for
large sector sizes residing in the page cache.

There is one place where this is a direct effect fs/block_dev.c

We have an indirect affect in the filesystems because there a few
bits of generic support missing and there is no linux convention
on how to handle this case.

I expect if we can enhance fs/block_dev.c to handle this case the
other parts will fall out naturally.

Eric



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