linux-ext4 - Re: confused on different inode size

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Message-ID: <20070408175218.GB29180@thunk.org>
Date:	Sun, 8 Apr 2007 13:52:18 -0400
From:	Theodore Tso <tytso@....edu>
To:	coly <colyli@...il.com>
Cc:	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: confused on different inode size

On Sun, Apr 08, 2007 at 02:39:03PM +0800, coly wrote:
> Hi, list:
> 
> I find size of struct ext4_inode is 152 bytes, but from the dumpe2fs, it
> tells me the inode size is 128 bytes.
> 
> I am confused that, the ext4_inode is the on-disk inode format, so how
> can dumpe2fs tells the inode size is 128 bytes.
> 
> Further more, when I use sb_bread() to read inode from inode table (with
> 152 bytes inode size), I can not read proper data from the bh->b_data.
> Once I use 128 bytes inode size, I can read what I want from the
> bh->b_data.

The inode size for ext4 filesystems can be multiple sizes; the
traditional ext2/ext3 inode size is 128 bytes.  If so, then you won't
have any of the features that require inode fields starting at
i_extra_isize.  If you use an inode size of 256 bytes, then you will
be able to use nanosecond granularity timestamps, and the extra space
(256-152 bytes) can be used for fast access to extended attributes.
If there is an expectation that the filesystem will need a larger
amount of space for extended attributes, the filesystem can be
formatted with 512, 1024, or even larger sizes (so long as it is a
power of two >= 128 bytes).

					- Ted
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