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*******                      A few truths about WBFS                    *******
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This file is part of the documentation of 'WIT' (Wiimms ISO Tools).


 CONTENTS:

    Sector size and maximal number of discs
    Free Blocks Table
    Extension called 'INODE INFO'.


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*******             Sector size and maximal number of discs             *******
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Overall in the internet you can read that the maximal number of discs is that
a WBFS partition can contain is about 500. This is true but not the whole truth.
The value depends of the sector size parameter. It defines the number of bytes
of 1 hd sector. The first sector of a WBFS partition is reserved for the WBFS
header (12 bytes) and for a disc table (all other bytes, 1 byte per disc).

The user/tool can set the sector size while formatting a WBFS partition. The
default sector size is 512. WWT knows the option --hss and may format
a WBFS partition with different sector sizes. This is nice for testing.

The following dump shows the specific parameters of a WBFS partition with about
2 tera bytes (a file with exact 2.000.000.000.000 bytes). WWT creates this as
sparse file so that the WBFS partition only needs 16 KiB disc space after
formatting:

 | DUMP of a.wbfs
 |
 |   WBFS-Header:
 |     MAGIC:                  'WBFS' = 57 42 46 53
 |     number of sectors:   279303400
 |     hd sector size:              9 ->        512
 |     WBFS sector size:           26 ->   67108864
 |
 |   hd:   sector size:           512 = 2^9
 |   hd:   num of sectors: 3906250000
 |   hd:    total size:       1907349 MiB
 |
 |   wii:  sector size:         32768 = 2^15
 |   wii:  num of sectors:   61035008
 |   wii:  sectors/disc:       286864
 |   wii:   total size:       1907344 MiB
 |
 |   wbfs: sector size:      67108864 = 2^26
 |   wbfs: num of sectors:      29802
 |   wbfs: sectors/disc:          140
 |   wbfs:  total size:       1907328 MiB
 |
 |   partition lba:                 0
 |   free blocks lba:          131064
 |   disc info size:             1024
 |
 |   used disk space:             640 MiB =   1%
 |   free disk space:         1906688 MiB =  99%
 |   total disk space:        1907328 MiB = 100%
 |
 |   number of wii discs:           0     =   0%
 |   max disc:                    500
 |   disc open:                     0

With sector size 512 the maximal number of discs is set to 500 (=512-12).
But when a WBFS is formatted with sector size 2048 than it may contain up
to 2036 (2048-12) discs. See the dump:

 | DUMP of a.wbfs
 |
 |   WBFS-Header:
 |     MAGIC:                  'WBFS' = 57 42 46 53
 |     number of sectors:  1143551290
 |     hd sector size:             11 ->       2048
 |     WBFS sector size:           25 ->   33554432
 |
 |   hd:   sector size:          2048 = 2^11
 |   hd:   num of sectors:  976562500
 |   hd:    total size:       1907349 MiB
 |
 |   wii:  sector size:         32768 = 2^15
 |   wii:  num of sectors:   61034496
 |   wii:  sectors/disc:       286864
 |   wii:   total size:       1907328 MiB
 |
 |   wbfs: sector size:      33554432 = 2^25
 |   wbfs: num of sectors:      59604
 |   wbfs: sectors/disc:          280
 |   wbfs:  total size:       1907328 MiB
 |
 |   partition lba:                 0
 |   free blocks lba:           16380
 |   disc info size:             2048
 |
 |   used disk space:             640 MiB =   1%
 |   free disk space:         1906688 MiB =  99%
 |   total disk space:        1907328 MiB = 100%
 |
 |   number of wii discs:           0     =   0%
 |   max disc:                   2036
 |   disc open:                     0

I don't know if other tools and the usb loaders support other sector sizes
(this is subject of later tests) but the solution is very easy: Read the WBFS
sectors before calling the function wbfs_open_partition() and calculate the
sector size:

 | wbfs_head_t whead;
 | stat = ReadAt(file,0,&whead,sizeof(whead));
 | if (stat) { ERROR_HANDLING; }
 | sector_size = 1 << whead.hd_sec_sz_s;
 | wbfs_open_partition( ReadSector, WriteSector, file, sector_size, 0,0,0 );

The functions ReadSector() and WriteSector() should look like this:

 | int ReadSector ( void * handle, u32 lba, u32 count, void * iobuf )
 | {
 |     return ReadAt( handle, (off_t)lba * sector_size,
 |                    iobuf, count * sector_size );
 | }
 |
 | int WriteSector ( void * handle, u32 lba, u32 count, void * iobuf )
 | {
 |     return WriteAt( handle, (off_t)lba * sector_size,
 |                    iobuf, count * sector_size );
 | }

That's all!


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*******                        Free Blocks Table                        *******
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--------------
 Introduction
--------------

A WBFS is divided into blocks with equal size. The block size is a power of 2.
The calculation of of the block size is made when formatting a WBFS:
 1) Use the minimal blocksize so that there are less than 2^16 blocks.
 2) Adjust the block size so that all management data fits into one block.
The block size power is stored into the WBFS header (struct wbfs_head) as
member 'wbfs_sec_sz_s'.

Based on this calculation only whole multiple of the sector size of the WBFS
file or device can be used. The rest is not used. "wwt DUMP" shows the end of
the used are in the memory map.

After formatting we have N blocks, addressed by 0..N-1. Block #0 contains the
managment data. All other blocks (1..N-1) are used to store the discs. A block
is not used or exactly assigned to 1 disc.

The "free block table" (FBT) is located at the end of block #0, beginning at
a hd sector (parameter "sector size"). For each block there is exact one bit
reserved. Is the bit set (1), the block is free (not assigned to a disc).
If the bit is unset (0), the block is used by a disc.

Each byte of the FBT contains the status of 8 blocks. The first byte is to
manage the blocks 1..8, the next byte for blocks 9..15 and so on. As you see
this assignments are NOT zero based! In the beginning of the WBFS there was a
"delete bug": The function free_block() used a zero based calculation of the
FBT and marked the wrong blocks for free (e.g. marked block 2 instead of block
1 for free). But there are some more bugs in libwbfs. And the more bugs are
the reason of this article.

I plan to publish a patch for all discussed bugs later.


------------------
1.) Formatting bug
------------------

While formatting the WBFS only the first N/8 bytes (N=number of WBFS blocks)
of the FBT are set with the value 0xff. Remember: A set bit means "free block".

a) If N is a multiple of 8 (N%8 == 0) then 1 block to much is marked as free.
   But because of the allocation bug (see below) this bug appears only, if N
   is a multiple of 32.

b) If N%8 > 1 then to less blocks are marked as free. But this means only that
   not the whole WBFS is used for discs.

c) If N%8 == 1 then the correct number of blocks is marked as free.

This formatting bugs can be fixed very easy:

a) Just unset the wrong bit when opening the WBFS.

b) When formatting or checking the disc then mark all blocks free. This can
   not be done if there is no space for the additional needed byte in block #0.


------------------
2.) Allocation bug
------------------

libwbfs uses a run time optimization and scans the FBT in u32 (32 bits) steps.
Because of this optimization only the first (N/32)*4 bytes of the FBT are used.
And this means that up to to 4 bytes of the FBT are ignored. And this means
that up to 30 WBFS blocks will not be used for storing discs. On a WBFS with
about 500 GiB the WBFS block size is 2^23 -> up to 240 MiB wasted space.

A patch must be compatible to already existing WBFS. And this means, that only
(N/8)*8 blocks can be used, because they are set correctly by the old
formatting. The last up to 7 blocks should never be used.


------------------
3.) Free-Block bug
------------------

The free block function free_block() doe's not make a check of the block
number. If the block number is =0 or >=N wrong bits anywhere in the memory
are set. No problem is the WBFS is valid.

But because of other errors in the WBFS file/device it is possible that there
are wrong values in the disc memory maps. I have already seen dumps with such
errors.


------------------
4.) Add-Disc bug
------------------

If the WBFS becomes full while adding a disc the disc will not added to the
WBFS, that's ok. But the already allocated blocks are not freed.


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*******                  Extension called 'INODE INFO'                  *******
*******************************************************************************

I have implemented a WBFS extension that I call 'INODE INFO'.

There is a discussion thread about this in GBAtemp:
http://gbatemp.net/index.php?showtopic=210772

The idea:
For each disc slot within WBFS there is place to manage the disc. I call this
area INODE (It's not a real inode, but simmilar). Every inode habe two parts:
 - Copy of the first 256 bytes of the ISO image
 - Memory mapping table (location if the ISO blocks within the WBFS).
To manage the disc only the first 128 bytes of the ISO header copy are needed.
The second half is unused memory. Up from wwt v0.33a I use this area for
additional information.

This are definitons and data structure of the INODE INFO (extract of libwbfs.h):



The first 10 bytes can be used to check the existence of a valid inode info.
This data is also good for recovering. Because of this validation check this
extension is abolut compatible with all other USB loaders and WBFS managers:
 1.) They ignore this indoe information.
 2.) They copy the contents of the ISO image and the INDOE-INFO-area is always
     filled with garbage (zeros).

The time stamps are informative. wit and wwt uses them in the following way:

 All commands:
    If creating a inode info because the previous is invalid, then itime,
    mtime, ctime and atime are set to the current time and dtime is cleared.

 wwt FORMAT:
    Clear all time stamps. dtime is set to current time.

 wwt ADD:
    Set itime, ctime, and atime to the current time. mtime is copied from the
    source mtime. dtime is cleared. Adding can be done conditionally with the
    option --newer.

 wwt PHANTOM:
    Set itime, mtime, ctime, and atime to the current time. dtime is cleared.

 wwt EXTRACT:
    Use mtime to set mtime of the destination file. If extracting to a WBFS
    then the time stamps within the inode is set like the ADD command. The
    atime of the source is set to the current time.

 wwt RENAME+SETTITLE:
    If only ID or title of the inode info is changed, then ctime and atime are
    set to the current time. If ID or title of the ISO are changed, then mtime,
    ctime and atime are set.

 wwt REMOVE:
    The dtime is set to the current time.

 wwt TOUCH:
    itime, mtime, ctime and/or atime can be changed to the current time or
    to a user defined time.

 wwt LIST:
    In the extended view it shows the mtime (default) or any other time, but
    only one, controlled by options. Sorting by time is also possible.

 wwt DUMP:
    In disc view it shows all non zero time stamps of each valid inode.


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*******                              END                                *******
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