Re: confused on different inode size

[coly <colyli@gmail.com>]

Theodore:

Thanks for your replying. 

Can I understand this way:
* Though sizeof(struct ext4_inode) is 152, the real inode size on disk
still depends on mount options.
* If use old inode size, the on disk inode will be 128 bytes.
* If use new inode size(e.g. extent option in mount), the on disk inode
will be 256, or more bytes.
* If on disk inode size is 128 bytes, only first 128 bytes of struct
ext4_inode take effects.

Best regards.

Coly

在 2007-04-08日的 13:52 -0400，Theodore Tso写道：
> 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