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 Forum index » Advanced Topics » Cutting edge » Multi-session live-CD/DVD
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Joined: 04 May 2005
Posts: 11277
Location: Arizona USA

PostPosted: Sun 09 Nov 2008, 00:01    Post_subject:  

Perhaps we're talking past each other. I agree that it would be a waste of time to load a large file from an early session, only to find that file was deleted in a later session. However, I've found that most sessions are pretty small (the sessions appear as directories on a mounted multisession Puppy DVD) so it seems to me that for most people a multisession DVD would boot faster if the sessions were read from the DVD in the natural order of first-to-last.

I don't know much about the details of how a DVD works. I know that the data on a DVD is recorded on a single track which begins at the innermost edge of the data area and extends in a continuous spiral to the outer edge of the data area, similar to the track on an old LP record. By design, a DVD drive can only read from or write data to that track in one direction. There is no physical reason I can think of why it couldn't read the track backward. The DVD drive designers probably just couldn't imagine why anyone would want to, if it occurred to them to wonder about it at all.

It takes the read head a while to synchronize (or whatever the technical term is) with the track before it can read the data. Given that a drive is only designed to read data from the spiral track in one direction, the read head must read the last session, then double back to a previous location on the track and re-synchronize before it can read the previous session, and so on until all the sessions are read. This offends my sense of order somehow.. I imagine that the seek-and-synchronize time takes such a long time that it might be faster to simply zip through the sessions on the track starting with the first one, and let the operating system, which is already loaded and operating, deal with straightening everything out in RAM. The drive starts out loading the main Puppy files from the beginning of the track anyway. Might as well just keep going while the drive is on a roll. That's all I'm trying to say.
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PostPosted: Mon 10 Nov 2008, 03:05    Post_subject:  

I have never disassembled one enough that I could watch the inner workings while it is reading, but listening to an optical drive, it sounds to me a lot like it seeks back and forth between the track's table of contents and the actual files while it is reading. Leastwise, I can hear the read-head skipping back and forth, even when doing a straight copy. So the fact that starting at the end would let you read *just* the table for the majority of the files probably adds more speed than reading in reverse order costs.

Now, if you were to tarball and/or compress the data in each folder so that it could be read as a single file, then it might make a difference... Might get you some extra storage space on your disk too... Except that then you *would* have to read all the data in every session just to pick out the bits you wanted, so I bet it would be slower.

But anyway, we're drifting a little off topic. I'm still curious if anyone has an explanation for why my multisession disk is behaving in this odd manner.
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