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

[Gary Thorpe <gathorpe79%yahoo.com@localhost>]

--- John Nemeth <jnemeth%victoria.tc.ca@localhost> wrote:

> On May 14,  9:15am, Gary Thorpe wrote:
> } > On Jun 19,  3:10pm, Reinoud Zandijk wrote:
> } > } On Wed, Jan 16, 2008 at 10:21:27PM -0800, John Nemeth wrote:
> } > } > } There seem to be several almost-identical implementations,
> in
> } > } > } sys/dev/dksubr.c, sys/dev/ld.c, and sys/dev/ata/wd.c; an
> implementation
> } > } > } in sys/dev/scsipi/cd.c that always returns EIO; and
> implementations in
> } > } > 
> } > } >      I'm not sure it makes sense for cd.c since that is a
> read-only
> } > } > device and isn't truely random access.
> } > } 
> } > } I have code here for cd.c that actually permits setting the
> strategy; not 
> } > } used anymore though but for dvd+rw/dvd-ram its a truely random
> access 
> } > } device allthough it wouldn't be fast.
> } > 
> } >      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
> 
>      Just because they are flat and round does not mean that they
> work
> like every other device that is flat and round.
> 
> } indeed support random access (all have seek times that are far
> below
> 
>      More like quasi-random access.  Magnetic media organises data in
> concentric cylinders with each level of the cylinder being a track
> that
> is divided into sectors.  To seek, you move the heads to the
> appropriate cylinder, select the appropriate head, and wait for the
> appropriate sector to appear under the head.  Optical media such as
> CDs
> and (most?) DVDs use a single spiral groove similar to old vinyl
> records.  To seek, you would have to move the head to position
> shortly
> before where you expect the sector to be, wait until you are properly
> aligned with the track, then follow the track to the desired sector.
> It is not something that is designed for true random access
> (remember,
> it was originally designed to play music and then was adapted to
> data).

In other words, disk scheduling is still appropriate: flat and round
with basically the same seek characteristics. See the rest of your
email: you agree.




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