linux-ext4 - Re: [36/37] Large blocksize support for ext2

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Date:	Wed, 20 Jun 2007 16:19:44 -0600
From:	Andreas Dilger <adilger@...sterfs.com>
To:	Christoph Lameter <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  14:27 -0700, Christoph Lameter wrote:
> > > 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...
> 
> mke2fs allows to specify a 64kb blocksize and IA64 can run with 64kb 
> PAGE_SIZE. So this is a bug in ext2fs that needs to be fixed regardless.

True.  I had increased the e2fsprogs blocksize to 16kB after testing it,
and after that it seems Ted increased it to 64kB after that.  The 64kB
directory problem only came out recently.

> > 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.
> 
> mke2fs fails for blocksizes > 64k so you are safe there. I'd like to see 
> that limit lifted?

I don't think extN can go to past 64kB blocksize in any case.

> > 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.
> 
> I'd love to see such a patch. That is also useful for arches that have 
> PAGE_SIZE > 4kb without this patchset.

Definitely, which is why we had been working on this originally.

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

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