Re: [Ext2-devel] [PATCH] ext3: Extends blocksize up to pagesize

[Andreas Dilger]

On Jan 20, 2006  23:10 -0800, Andrew Morton wrote:
> Andreas Dilger <adilger@xxxxxxxxxxxxx> wrote:
> >
> > On Jan 18, 2006  22:06 +0900, Takashi Sato wrote:
> > > As a disk tends to get large, a disk storage has had a capacity to
> > > supply multi-TB.  But now, ext3 can't support more than 8TB filesystem
> > > when blocksize is 4KB.  That's why I think ext3 needs to be
> > > more than 8TB.
> > > 
> > > Therefore I think filesystem size can increase on architectures
> > > which has more than 4KB pagesize by extending blocksize to pagesize on
> > > ext3.  For example, the following is in case of ia64.  (Blocksize have
> > > already been supported up to pagesize on ext2. Why is the max blocksize
> > > restricted to 4KB on ext3?)
> > > 
> > > Max filesystem size on ia64:
> > > Original                                   :4096(blocksize) * 2^31 =  8TB
> > > After modification [pagesize=16KB(default)]:16384(blocksize) * 2^31 = 32TB
> > > After modification [pagesize=64KB(max)]    :65536(blocksize) * 2^31 = 128TB
> > 
> > Just for others' info - the fill_super change has been tested in the past
> > by Sonny Rao at IBM also.  e2fsprogs has supported this for a long time
> > already.
> 
> I have a vague memory that there's some piece of metadata (per-block-group
> info, I think) which will overflow at 8kb blocksize.  I say this in the
> hope that you'll remmeber what it was ;)

This is the group descriptor per-group blocks and inode free counts.  The
current mke2fs code limits the number of blocks and inodes per group to
EXT2_MAX_BLOCKS_PER_GROUP (2^16 - 8) and (2^16 - EXT2_INODES_PER_BLOCK)
so that this won't overflow.  We still get linear growth of filesystem
limits with blocksize, but not cubic growth that would otherwise be there.

Bull has a patch which enlarges the group descriptor fields to allow
more blocks and inodes per group.

> I've rewritten ext3 directory readahead to use the generic pagecache
> functions, so changes here shouldn't be needed.
> 
> But I haven't yet got around to performance-testing it.
> 
> ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.16-rc1/2.6.16-rc1-mm2/broken-out/ext3_readdir-use-generic-readahead.patch

Hmm, I'll have to take a look at that when I get a chance.

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

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