Re: [slightly OT] dvdrecord 0.3.1 -- and yes, dev=/dev/cdrom works ;)

[Rob Sims <lkml-z@robsims.com>]

On Tue, Feb 28, 2006 at 01:58:34PM -0500, Phillip Susi wrote:
> Jan Engelhardt wrote:
> >Yes. A 650 MB *CD*-RW (DVD-RW too?) formatted in packet mode only has like
> >500-something megabytes to allow for the sort of seeks required.
> >On DVD+RW, you get the full 4.3 GB (4.7 gB) AFAICS.
 
> DVD-RAM physically is formatted like a hard disk.  It is broken up into 
> zones that hold different numbers of sectors which are individually and 
> randomly read/writable.  CD/DVD+-RW media is organized as a single long 
> groove that consists of an unbroken series of large blocks composed of 
> small blocks with user and control data interleaved and error corrected. 
>  It is for this reason that historically it could only be recorded from 
> start to finish in one pass.

While DVD-RAM has per-sector embossing of headers, the ECC size is
still 16 sectors, so writing any one sector requires a read-modify-write
pass.  

> There are two modern techniques to allow pseudo random write access for 
> all forms of CD/DVD +/- RW media.  These are packet mode, and mount 
> rainier mode.  MRW mode formats the disk into 32 KB blocks made up of 
> 2048 byte sectors which are individually writable as far as the OS 
> knows, because an MRW compliant drive is required to internally handle 
> any required read/modify/write cycles to update the 32 KB blocks.  MRW 
> mode also reserves some of the disk for sector sparing which the drive 
> firmware also handles.  MRW mode is typically used on dvd+rw media. 
> IIRC, this format typically "wastes" about 10% of the capacity of the 
> medium.

DVD+RW and theoretically DVD-RW support writing of 32K chunks randomly
on the disk.  DVD+RW has a tight tolerance on positioning (+/-16 bits)
and DVD-RW about 150 bytes.  Both rely on ECC to correct those bits,
though DVD+RW obviously eats less of the ECC budget.  Neither format
uses a special packet format.  The drives themselves are supposed to do
read-modify-write as required.

> The other technique is packet mode.  Packet mode formats the media into 
> packets of sectors and each packet can be randomly rewritten.  The 
> current default size is only 32 sectors per packet.  Each packet has 7 
> sectors of linking loss so around 18% of the disk space is wasted.  I 
> recently submitted a patch to pktcdvd and have some patches to the 
> udftools package to support larger packet sizes.  A packet size of 128 
> sectors reduces the waste to only 5.2%.
 
Fixed packet writing is only a CD attribute.

Using 128 sector packets will likely break UDF interchangeability, and
likely even some drive firmware.
-- 
Rob

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