[WBEL-users] kernel 2.6 and ext3 improvements

[Ed]

Kirby Bohling wrote:
> On Mon, Aug 02, 2004 at 03:26:26PM -0700, Ed wrote:
> 
>>Roger D. Vargas wrote:
>>
>>>I installed Fedora Core 2 with kernel 2.6. I copied all my home folders
>>>to a new ext3 partition. I read that kernel 2.6 implements some changes
>>>to ext3. The question is, if i install Whitebox now, kernel 2.4 will be
>>>able to read the partition?
>>>
>>
>>Ext2 is generally backwards compatible.  You should be able to read your 
>>partitions.
> 
> 
> 	Careful there.  If I remember correctly they were talking about
> adding H-Tree directories (I might have the name wrong, but
> essentially, a more efficient directory stucture so doing an "ls" in
> a directory with thousands of dirs/files wouldn't take so long).

Yes, you're right.  I can't find any reference to hashed directories in 
the RHEL kernel.

However, I think it will work anyway.  Directory hash indexes are 
invisible to kernels which don't support them, and newer kernels, 
including the RHEL 3 kernel, turn off directory indexes on directories 
which are written to.

When we all get to upgrade to a version of linux with directory index 
support, fsck -Df will allow us to turn back on indexing for directories 
bigger than one block.

According to this post to the LKML [1], the changes are "completely 
backwards and forwards compatible".

One of the design criterea for ext2 directory indexes is backwards and 
forwards compatibility.  [2]

   Ed

[1] http://www.uwsg.iu.edu/hypermail/linux/kernel/0210.0/0479.html
[2] http://tinyurl.com/548q5
(goes to 
http://web.archive.org/web/20030416205529/http://people.nl.linux.org/~phillips/htree/paper/htree.html 
)

> 
> 	When that codes get is checked in, I believe it's not backwards
> compatible.  I know that there are incompatible versions of ext2
> over time.  Read the man page on mke2fs specifically the section on
> "-O feature".  Sparse superblocks can't be read by older kernels.
> So you can create compatible filesystems, but you that might not be
> the defaults of the tools.

I think you can mount a sparse-superblock filesystem read-only on an 
older version of linux, since by default linux only reads the super for 
block group 0.  The extra blocks are used by e2fsck only, and older 
versions should refuse to make changes to sparse-super filesystems.

> 
> 	Sorry, I didn't reply sooner, but I figured someone who knew the
> precise details would respond.  I'm not absolutely sure of my
> details, but I thought one of RedHat's goals in for the next RHEL
> release was to get the H-Tree stuff in.

That would be great if they do.  It makes large directory operations 
much, much faster.

> 
> 	Thanks,
> 		Kirby
> 
> 
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