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    SubjectRe: [Ext2-devel] [PATCH] ext3: Extends blocksize up to pagesize
    On Jan 20, 2006  23:10 -0800, Andrew Morton wrote:
    > Andreas Dilger <adilger@clusterfs.com> 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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