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

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Date:	Fri, 04 May 2007 07:33:54 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Theodore Tso <tytso@....edu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Chinner <dgc@....com>, clameter@....com,
	linux-kernel@...r.kernel.org, Mel Gorman <mel@...net.ie>,
	William Lee Irwin III <wli@...omorphy.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Badari Pulavarty <pbadari@...il.com>,
	Maxim Levitsky <maximlevitsky@...il.com>
Subject: Re: [00/17] Large Blocksize Support V3

Theodore Tso <tytso@....edu> writes:

> On Fri, Apr 27, 2007 at 01:48:49AM -0700, Andrew Morton wrote:
>> And other filesystems (ie: ext4) _might_ use it.  But ext4 is extent-based,
>> so perhaps it's not work churning the on-disk format to get a bit of a
>> boost in the block allocator.
>
> Well, ext3 could definitely use it; there are people using 8k and 16k
> blocksizes on ia64 systems today.  Those filesystems can't be mounted
> on x86 or x86_64 systems because our pagesize is 4k, though.
>
> And I imagine that ext4 might want to use a large blocksize too ---
> after all, XFS is extent based as well, and not _all_ of the
> advantages of using a larger blocksize are related to brain-damaged
> storage subsystems with short SG list support.  Whether the advantages
> offset the internal fragmentation overhead or the complexity of adding
> fragments support is a different question, of course.
>
> So while the jury is out about how many other filesystems might use
> it, I suspect it's more than you might think.  At the very least,
> there may be some IA64 users who might be trying to transition their
> way to x86_64, and have existing filesystems using a 8k or 16k
> block filesystems.  :-)

How much of a problem would it be if those blocks were not necessarily
contiguous in RAM, but placed in normal 4K pages in the page cache?

I expect meta data operations would have to be modified but that otherwise
you would not care.

Eric
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