Described ISO 9660 multi-session on overwriteable media

This commit is contained in:
Thomas Schmitt 2008-09-12 10:04:41 +00:00
parent c640c7954b
commit fb3d2de5df
1 changed files with 75 additions and 1 deletions

View File

@ -432,6 +432,10 @@ The recipes described here are depending on formatting state:
- DVD-RAM and BD-RE formatting
- DVD-RAM and BD-RE speed tuning
Slightly off topic are
- ISO 9660 multi-session emulation on overwriteable media
- ISO 9660 based TOC emulation on overwriteable media
Overwriting in general :
@ -522,7 +526,7 @@ with Close Function 010b despite there is no session open in this scenario.)
Unformatted DVD+RW
Unformatted DVD+RW :
This is the state of previously unused DVD+RW media.
@ -762,6 +766,76 @@ Nevertheless it worked on all tested drives is proper alignment was observed.
(mmc5r03c.pdf 5.4.13, 6.45)
ISO 9660 multi-session emulation on overwriteable media :
Overwriteable media provide a single overwriteable track which may grow up to
the full media capacity. There is no builtin table-of-content which records
the history of write sessions.
mount -t iso9660 will use sbsector=0 as default.
The term "superblock" shall depict the first 64 KiB after the sbsector address.
ISO 9660 multi-session depends on typical TOC information in two ways:
It needs the superblock address MSC1 of the most recently recorded session and
it needs the Next Writeable Address NWA for which to prepare the adress offset.
The following is learned from growisofs and from ECMA-119:
ISO 9660 filesystems provide information about the number of sectors which
is also the lowest unused block address and thus a suitable NWA.
This block address is stored in the Primary Volume Descriptor which is supposed
to be stored in block 16 (eventually relative to MSC1).
The bytes 0 to 5 of a PVD block are
0x01 'C' 'D' '0' '0' '1'
The sector count can then be read from byte 80 to 83
sectors= pvd[80] | (pvd[81] << 8) | (pvd[82] << 16) | (pvd[83] << 24);
To support CD, DVD and BD media alike, it is advisable to round the NWA
to the next multiple of 32 (= 64 KiB).
(Ecma-119.pdf 8.4)
So one can use 0 as MSC1 and prepare a new ISO session for the computed NWA.
After writing the session it is necessary to patch the PVD at LBA 16.
The minimal change would be to update the number of image sectors.
It is stored in both notations LSB and MSB:
for(i= 0; i < 4; i++)
pvd[87 - i]= pvd[80 + i]= (sectors >> (8 * i)) & 0xff;
cdrskin --grow_overwriteable not only patches the sector fields of the
PVD block but also the blocks up to LBA 31 which begin with
0xff 'C' 'D' '0' '0' '1'
libisoburn submits 64 KiB data buffer to libisofs before image generation and
afterwards writes these 64 KiB as new superblock to LBA 0.
ISO 9660 based TOC emulation on overwriteable media :
Above method of multi-session emulation yields a single session image after
each add-on session. No reliable session history can be read because the
sector size of the existing session gets overwritten by the new one.
A TOC with session history is nevertheless desirable with incremental backups
in order to access older backup states by mounting older superblocks at the
start addresses of older sessions.
All usual ISO 9660 formatter programs write a complete superblock to the
start of each session.
With a uniform NWA rounding rule it is possible to compute the address of
superblock N+1 as the NWA after session N. The only problem is N=1
because it gets overwritten by later sessions.
libisoburn preserves the information of session 1 by writing the first session
to LBA 32 rather than LBA 0. Afterwards it writes the overall superblock to
LBA 0 (up to 31).
So with all further add-on sessions the superblock at LBA 0 will enclose the
overall image, while the superblocks of the sessions form a chain beginning
at LBA 32. Each session superblock points to the next one by its sector count
rounded up to 32. The chain end is marked by the overall image size.
This chain gives the start addresses of sessions. The sector count minus start
address gives the size of a particular session. ECMA-119 explains how to
retrieve more info from the PVD (e.g. the volume id).