Re: [PATCH 04/10] AXFS: axfs_inode.c

Arnd Bergmann
2008-08-22T10:00:40Z

On Friday 22 August 2008, Phillip Lougher wrote:
> >
> > This looks very nice, but could use some comments about how the data is
> > actually stored on disk. It took me some time to figure out that it actually
> > allows to do tail merging into compressed blocks, which I was about to suggest
> > you implement ;-). Cramfs doesn't have them, and I found that they are the
> > main reason why squashfs compresses better than cramfs, besides the default
> > block size, which you can change on either one.
>
> Squashfs has much larger block sizes than cramfs (last time I looked it
> was limited to 4K blocks), and it compresses the metadata which helps to
> get better compression.  But tail merging (fragments in Squashfs
> terminology) is obviously a major reason why Squashfs gets good compression.

The *default* block size in cramfs is smaller than in squashfs, but they both
have user selectable block sizes. I found the impact of compressed metadata
to be almost zero. I hacked up a mksquashfs to avoid tail merging, and found
that the image size for squashfs and cramfs is practically identical if you
use the same block size and no tail merging.

> The AXFS code is rather obscure but it doesn't look to me that it does
> tail merging.  The following code wouldn't work if the block in question
> was a tail contained in a larger block.  It assumes the block extends to
> the end of the compressed block (cblk_size - cnode_offset).

yes, I thought the same thing when I first read that code, and was about
to send a lengthy reply about how it should be changed when I saw that
it already does exactly that ;-).