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