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SubjectRe: [PATCH] remove 2TB block device limit
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On Thursday 16 May 2002 00:17, Andreas Dilger wrote:
> On May 10, 2002 17:07 -0700, David Mosberger wrote:
> >On Fri, 10 May 2002 17:46:23 -0600, Andreas Dilger <adilger@clusterfs.com>
said:
> > Andreas> For 64-bit systems like Alpha, it is relatively easy to use
> > Andreas> 8kB blocks for ext3. It has been discouraged because such
> > Andreas> a filesystem is non-portable to other (smaller page-sized)
> > Andreas> filesystems. Maybe this rationale should be re-examined -
> > Andreas> I could probably whip up a configure option for e2fsprogs
> > Andreas> to allow 8kB blocks in a few hours.
> >
> > If you do this, please consider allowing a block size up to 64KB.
> > The ia64 kernel offers a choice of 4, 8, 16, and 64KB page size.
>
> Well, taking a look at the ext2 code, there is a slight problem when
> trying to use block sizes > 8kB. This is in the group descriptors,
> where they only store a 16 bit could of free blocks and inodes for
> the group. Since the maximum number of blocks/inodes is 8*blocksize
> (the number of bits that can fit into a single block) you overflow
> these fields if you have more than 64k (8*8k) blocks in a group.
>
> Even 8kB blocks would theoretically overflow these fields, but you
> can't yet have a group _totally_ empty (there are always two bitmaps
> and at least one inode table block), so it would always have less
> than 65535 blocks free. Now I realize that this isn't true of the
> inode table in theory, but you normally also have less than the maximum
> number of inodes per group - need to check for that.
>
> This could be worked around temporarily by limiting the size of each
> group to at most 65535 free blocks/inodes. The permanent solution is
> to probably add an extra byte for each of these two fields to allow up
> to 16M blocks/inodes per group, which gives us a max block size of 2MB.
>
> This could be a compat ext2 feature, since at worst if we didn't take
> the high byte into account on a block free it could overflow this field
> and we wouldn't be able to allocate from this group until more blocks
> are freed. We couldn't underflow because the allocator would stop when
> the free block/inode count hit zero for that group, even if there were
> really more free blocks available.
>
> So, for now I think I'll stick to a maximum of 8kB blocks, and maybe
> we can slip in support for the high byte of the free blocks/inodes
> count when Ted adds in support for metagroups.

Hi Andreas,

Imposing an absolute upper limit of 2**16 blocks per group makes the most
sense for now, and may always make the most sense. Even with a cap on the
blocks per group group size still scales directly with block size. We
don't want it to scale quadratically. If it did, then a data block could
end up 32 GB away from the inode, still in the same group. This
effectively destroys the utility of block groups as a means of reducing
seek latency.

--
Daniel
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