linux-ext4 - Re: Beginner questions about ext4

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Message-ID: <20130715185953.GE26359@thunk.org>
Date:	Mon, 15 Jul 2013 14:59:53 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Felipe Monteiro de Carvalho <felipemonteiro.carvalho@...il.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: Beginner questions about ext4

On Mon, Jul 15, 2013 at 08:18:16PM +0200, Felipe Monteiro de Carvalho wrote:
> wow, that's a pretty amazing tool, didn't know about it.

Are you familiar with debugfs?  I'd also strongly suggest that you try
using that program to try to understand the file system layout.

>But my table of group descriptors read from group nr 0 is all correct
>between group nr 0 and 31, but group nr 32 to 63 are filled with
>zeroes.

> Filesystem features:      has_journal ext_attr resize_inode dir_index filetype needs_recovery extent flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize

The file system does not have the 64-bit feature set.  Hence, the
layout of the block grup descriptor is struct ext2_group_desc, and not
struct ext4_group_desc.

I'm guessing that you were using the ext4_group_desc structure (which
is 64 bytes) instead of the ext2_group_desc structure (which is 32
bytes).

I would suggest that you match up the contents of your in-program data
structures, and compare it to the outputs of debugfs and dumpe2fs.

						- Ted
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