Max blocks per group?

Max blocks per group?

[Phillip Susi]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

I notice that mke2fs refuses to allow more than 8 * block size blocks
per group.  Why is this?  Is it not permissible to have more than one
block for the block allocation bitmap?

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v2.0.17 (MingW32)
Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/

iQEcBAEBAgAGBQJP7HVPAAoJEJrBOlT6nu75X0cIAJCQnJRhmCh4KhkeEzc+zkMw
u1LZ2FodigtJQ5GrhZUrj4X6ZvxL6rXuGYkIenNUYo+V29TGCqUu+infGb/8dkGG
yZkGx3HiuLqLl+fY5sJwz7brykbHYhZMZ6AP/t98ppd+u/3ZiPQpj8tY9rYF6daM
+m+Rkzl8IL7U24JnZ8wAAEUSNkA2b9nEo/rJvh43HdQOtqFGrGui6tBG94xFiHPI
l5XcdRu1rL6ltmpdktYX/6B+peQwhWnVWWi+Nb8zq7l7maIqMpHFPeZ563b/nhR2
xH4OX/jCU/Ys/kbUu+/pvWrjDmwqST48mBG4M/N8UcaacXWoe1HJKBm/kQ302Gc=
=vtLz
-----END PGP SIGNATURE-----

Re: Max blocks per group?

[Theodore Ts'o]

On Thu, Jun 28, 2012 at 11:16:31AM -0400, Phillip Susi wrote:
> 
> I notice that mke2fs refuses to allow more than 8 * block size blocks
> per group.  Why is this?  Is it not permissible to have more than one
> block for the block allocation bitmap?

No, it's not permissible; it's been a fundamental design constraint in
ext2/3/4 since the very beginning.  You can, however, get the moral
equivalent by using flex_bg which moves the metadata blocks to the
beginning of an aggregated of (for example) 16 block groups of a
flex_bg.  The block allocation bitmaps are allocated contiguously, so
between that and locating all of the metadata blocks of the bg's at
the beginning of a flex_bg, that should get most of what you'd want
with a larger number of blocks in a block group.

And BTW, this is already the default in ext4 (using 16 block groups in
each flex_bg).

						- Ted