Date:	Wed, 27 Sep 2006 14:59:57 +0200
From:	Alexandre Ratchov <alexandre.ratchov@...l.net>
To:	Theodore Tso <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org,
	Jean-Pierre Dion <jean-pierre.dion@...l.net>
Subject: Re: [patch 04/12] rfc: 2fsprogs update

On Tue, Sep 26, 2006 at 01:32:53PM -0400, Theodore Tso wrote:
> 
> /*
>  * Generic (non-filesystem layout specific) extents structure
>  */
> struct ext2fs_extent {
> 	blk64_t	e_pblk;		/* first physical block */
> 	blk64_t	e_lblk;		/* first logical block extent covers */
> 	int	e_len;		/* number of blocks covered by extent */
> };
> 
> 
> Note the use of blk64_t; yes, this means that blk_t will stay as a
> 32-bit value, and blk64_t will be used for new interfaces and be a
> 64-bit value.  

if blk_t stays 32bit and we want to use e2fsprogs on 64bit file systems then
we will have to duplicate most of the current code. I mean, since à 32bit
on-disk file system is a valid 64bit file system then the 64bit part of
e2fsprogs will have to deal also with the 32bit stuff like 32bit indirect
blocks, 32bit group descriptors etc. Thus we'll have to rewrite/dumplicate
all the blk_t code on a new blk64_t branch. So we will have to maintain 2
branches of code that do the same thing:

	- "blk_t" branch: pure 32bit code

	- "blk64_t" branch: 64bit code with 32bit compatibility

So my question is: do we want to (1) maintains both blk_t and blk64_t APIs
or (2) switch to the new "blk64_t" interface and just fix bugs in the old
interface until it dies.

Any thoughts here?

> This will get used to define an extent iterator function, that will look
> something like this:
> 
> errcode_t ext2fs_extent_iterate(ext2_filsys fs,
> 				ext2_ino_t	ino,
> 				int	flags,
> 				char *block_buf,
> 				int (*func)(ext2_filsys fs,
> 					    struct ext2fs_extent *extent,
> 					    void	*priv_data),
> 				int (*meta_func)(ext2_filsys fs,
> 						 blk64_t blk,
> 						 int blk_type,
> 						 char *buf,
> 						 void	*priv_data),
> 				void *priv_data);
> 
> This interface will work for both extent and non-extent-based
> inodes.... that is, if this interface is called on an inode which is
> using direct and indirect blocks, the function will Do The Right Thing
> and find contiguous blocks runs which it will use to fill extent
> structures that will be passed to the callback function.  This is fine,
> since extent-based interfaces will be easier and more efficient to use
> anyway.
> 
> We will also define two interfaces to manipulate the extents tree (and
> which again, will Do The Right Thing on traditional non-extents based
> inods):
> 
> errcode_t ext2fs_extent_set(ext2_filsys fs,
> 			    ext2_ino_t	ino,
> 			    ext2_ino_t	*block_buf,
> 			    struct_ext2fs_extent *extent);
> 
> errcode_t ext2fs_extent_delete(ext2_filsys fs,
> 			       ext2_ino_t	ino,
> 			       ext2_ino_t	*block_buf,
> 			       struct_ext2fs_extent *extent);
> 
> 
> Both of these interfaces may require splitting an existing extent.  For
> example, if ext2fs_extent_set() is passed an extent which falls in the
> middle of an extent in the inode, it could result in one extent turning
> into three extents (namely the before extent, the new extent, and the
> after extent).  Similarly ext2fs_extent_delete() may be asked to delete
> a sub-extent in the middle of an existing extent in the extent tree.
> This would be logically equivalent to the Windows NT "punch" operation,
> which is a more general version of truncate(), except it can remove
> blocks from the middle of a file.
> 
> 
> The other interface which I've started spec'ing out in my mind is a new
> form interface and implementation for bitmaps().  The new-style bitmaps
> will take a blk64_t type, but their biggest difference is that they will
> allow multiple different types of interfaces, much like the io_manager
> abstractions we have right now abstracts our I/O reoutines.  Some
> implementations may use an extents tree to keep track of used and unused
> bits.  Anothers might use a disk file as a LRU backing store (this will
> be necessary to support really large storage devices on systems with
> limited physical memory).  And of course, at least initially the first
> implementation we will support will be the old-fasheioned, "store the
> whole thing in memory" approach.
> 
> So the basic idea is to implement new library abstractions which will
> work well for 32-bit extents, but which can be easily extensible to
> newer patches, and which can solve other problems as well while we're at
> it (such as the people trying to use a cheap processor with small
> amounts of memory with terabytes of storagte and their having problems
> with fsck running out of memory, for example).
> 

i really like the idea. Since the first time i've looked into the e2fsprogs
i'm wondering why don't we use such an interface for the library since the
beginning. I don't see much reasons to export functions and data structures
that deal with the details of the file system layout.

I see 2 different aspects for the libext2fs:

(1) iterate/read/modify/delete inodes, files and directories; that's what
    programs to access ext{2,3,4} file systems without mounting them may
    want to do. Or programs to defragment, produce statistics etc...

    these tasks don't need to know anything about the layout of the
    file-system;

(2) check and fix: that's what fsck does, that's a more complicated and
    depends more closely on the file system layout.

IMO, interfaces you propose are perfect for (1) and do most of the job for
(2), but i don't know if they are enough for a tool like fsck. For instance
it's not clear for me how to check and repair extent indexes and headers;
how to check that the logical block number matches the block number within
the extent without using "lower level" routines.

Perhaps we can always check data structures "on the fly" in the iterator
function and just return an error code if an anomaly is found; in this case
the caller could delete the inode (or partially copy it in /lost+found,
etc...)

This point isn't clear for me; do you have any idea, here?

The same question holds for a future block allocation and for inode
allocation abstract interfaces.

-- Alexandre
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