Date: Mon, 29 Jul 2002 11:45:33 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: David Miller <dmiller@sparks.net> Cc: David Gilbert <dgilbert@velocet.ca>, Keith Pitcher <kpitcher@locallink.net>, freebsd-hackers@FreeBSD.ORG Subject: Re: [hackers] Multi CDR burn Message-ID: <200207291845.g6TIjXAo055731@apollo.backplane.com> References: <Pine.BSF.4.21.0207261253431.98442-100000@search.sparks.net>
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:On Tue, 9 Jul 2002, Matthew Dillon wrote:
:>
:> There is no multi-target command that I know of. You are absolutely
:> correct in your bandwidth calculations... a SCSI bus should have no
:> problem at all duping the data 8 times to each of 8 CDR's, and the
:> operating system ought to do a fine job caching the input image file
:> (so the data is only read off the hard disk once). CDRs are really slow
:> compared to what a SCSI bus is designed to handle.
:>
:> I'm somewhat interested in knowing that this concept actually works :-)
:
:The concept works fine, but the numbers are a little off. The actual
:bandwidth calculations are right, but make the assumption that the max
:data transfer rate is in effect, without the overhead of
:connect/disconnect/command queuing.
:
:Several years ago I built a DVD-ram duplicatior with FreeBSD. DVD-ram
:drives, at the time, only came in 10 MB scsi versions, and they took data
:at about 650Kbit. A single 10 MB scsi channel could not, however, stream
:data to more than about 5 of them at once.
:
:As for the general concept, I can say it works fine. I built a system
:with nearly 30 DVD-ram drives on 6 separate scsi channels. At first I
:tried using a utility that would read from the input image (on hard
:disk) and write it out to all the drives. Bad media gave me fits, Ken
:Merry was a huge help with the drivers, and in the end it worked fine to
:just dd the image to all of them. The CPU was an 800 MHz athlon,
:admittedly much faster than a P-100, but was practically idle when copying
:to all drives at once.
:
:--- David
That's very interesting! Effectively you have a 'buffer' which is
nearly all of physical memory as the kernel caches the file data you
are reading (so only the first dd to request a particular sector actually
has to read it from disk). As long as the various DVDs being written
too do not drift apart more then the size of the cache the data would
only have to be read from the hard disk once. So then it just comes down
to PCI and SCSI bus bandwidth / command overhead in regards to getting
the data out to the units.
-Matt
Matthew Dillon
<dillon@backplane.com>
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