[SATLUG] simultaneously burning multiple DVDs

Christopher Chavez chrischavez at gmx.us
Thu Jan 1 11:02:17 CST 2015


This reply seems to have fallen behind to the point others have probably
mentioned all of what I was going for (cf. Craig's links)…

> Maybe the bottleneck is the controller chipset for the DVD writer. This may
> use a limited buffer to spool the data to write from and is not designed for
> multiple devices. The DVD system software maybe the bottleneck because it can
> handle one write at a time.
>
The SATA controller would not have such a buffer for optical drives. A typical
PC only contains buffers in the individual writers; the exceptions would be e.g.
a RAID/SSD cache (for hard drives of course) or standalone disc duplication
units. Each disc burning command also keeps a cache in RAM while burning (wodim
uses 4MB for its "fifo" by default).

Cf. "Process Scheduling Priority", http://linux.die.net/man/1/wodim
I did not think that would be the issue, given how the system has 6-core CPU.

Disc burning is done using a lot of commands to control the burning process
(SCSI ioctl) rather than just block data transfer, so what I suggested earlier
(the read throughput benchmark) might not be sufficient. The issue at the moment
seems to be that these control commands are becoming congested. This is probably
the most interesting thing I've come across:
https://bugzilla.redhat.com/show_bug.cgi?id=877470
Comment 17 in particular: in a way, this issue is a kernel regression, and there
was a patch that never made it into the kernel due to potentially introducing
new issues.

K3B - Cannot burn concurrently to multiple recorders
https://bugs.kde.org/show_bug.cgi?id=179217
Might want to change this to CONFIRMED.

This seems to have become quite a popular thread, and I've learned a lot about
this issue. In my spare time I might want to see if I can help look into this
(I'm an undergrad student on vacation at the moment). I'm inclined for this to
be fixed for everyone rather than have to run out and buy more hardware to solve
the problem, as fun as it sounds.

Christopher Chavez


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