linux-kernel - Re: [36/37] Large blocksize support for ext2

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Date:	Wed, 20 Jun 2007 14:56:02 -0600
From:	Andreas Dilger <adilger@...sterfs.com>
To:	clameter@....com
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@....de>, Mel Gorman <mel@...net.ie>,
	William Lee Irwin III <wli@...omorphy.com>,
	David Chinner <dgc@....com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Badari Pulavarty <pbadari@...il.com>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	linux-ext4@...r.kernel.org
Subject: Re: [36/37] Large blocksize support for ext2

On Jun 20, 2007  11:29 -0700, clameter@....com wrote:
> This adds support for a block size of up to 64k on any platform.
> It enables the mounting filesystems that have a larger blocksize
> than the page size.

Might have been good to CC the ext2/3/4 maintainers here?  I definitely
have been waiting for a patch like this for ages (so definitely no
objection from me), but there are a few caveats before this will work
on ext2/3/4.

> Hmmm... Actually there is nothing additional to be done after the earlier
> cleanup of the macros. So just modify copyright.

It is NOT possible to have 64kB blocksize on ext2/3/4 without some small
changes to the directory handling code.  The reason is that an empty 64kB
directory block would have a rec_len == (__u16)2^16 == 0, and this would
cause an error to be hit in the filesystem.  What is needed is to put
2 empty records in such a directory, or to special-case an impossible
value like rec_len = 0xffff to handle this.

There was a patch to fix the 64kB blocksize directory problem, but it
hasn't been merged anywhere yet seeing as there wasn't previously a
patch to allow larger blocksize...

Having 32kB blocksize has no problems that I'm aware of.  Also, I'm not
sure how it happened, but ext2 SHOULD have an explicit check (as
ext3/4 does) limiting it to EXT2_MAX_BLOCK_SIZE.  Otherwise it appears
that there would be no error reported if the superblock reports e.g. 16MB
blocksize, and all kinds of things would break.

There shouldn't be a problem with increasing EXT{2,3,4}_MAX_BLOCK_SIZE to
32kB (AFAIK), but I haven't looked into this in a while.

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.

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