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Re: DIOC{G,S}STRATEGY not implemented for all disk devices

On Sun, Jan 27, 2008 at 09:07:44PM -0500, Gary Thorpe wrote:
> >      Do these devices work using concentric cylinders similar to the
> > way magnetic media works, or do they use the one long spiral track
> > the
> > way most optical media (and old vinyl records) work?
> Optical disks are...disks. The last time I read up on them, they did
> indeed support random access (all have seek times that are far below
> tapes). Disk scheduling would probably make a difference even for
> read-only media as reordering the requests to optimize seek times
> should still apply (mostly for things which do not do such scheduling
> internally [most ATAPI, probably most SATA as well]). Not sure what
> they mean either...

Normally they support random access reading in sectors of 2048 kb and for 
writing, well... that depends solely on the disc type; some are writable in 
2048 kb sectors, others are only writable in 16/32 kb ecc blocks and others 
can only grow.

Physically its basicly a groove, a long spiral track though for DVD-RAM its 
a bit different since its cut into pieces.

Sheduling, even for RO access, is in fact a very big deal for cd/dvd's. The 
normal disc sheduling however works fine with it since its basicly a `scan 
forward' algorithm and thats suited well for cd/dvd's.

My UDF writing code (not comitted yet) has in fact a specific module 
dedicated to sheduling read and write requests to the device; i can't just 
VOP_STRATEGY() them directly to the device for it will be either VERY slow 
or plainly won't work in the ECC blocks case or sequential recording mode 
where one can easily lose logical blocks on write->read transitions.

With regards,

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