Success, at last!

I burnt a couple of BD-R discs, on Monday, using a USB 2 card, fitted into the problem PC, as the source external HD connection, and the rate was roughly 22 GiB in half an hour.

Here's the tail end of the output:

xorriso : UPDATE : 22408 of 22530 MB written (fifo 98%) [buf  96%]   3.3x.
xorriso : UPDATE : 22421 of 22530 MB written (fifo 99%) [buf  96%]   3.1x.
xorriso : UPDATE : 22435 of 22530 MB written (fifo 99%) [buf  96%]   3.2x.
xorriso : UPDATE : 22449 of 22530 MB written (fifo 99%) [buf  96%]   3.3x.
xorriso : UPDATE : 22463 of 22530 MB written (fifo 98%) [buf  96%]   3.3x.
xorriso : UPDATE : 22477 of 22530 MB written (fifo 83%) [buf  96%]   3.1x.
xorriso : UPDATE : 22490 of 22530 MB written (fifo 62%) [buf  90%]   3.1x.
xorriso : UPDATE : 22505 of 22530 MB written (fifo 39%) [buf  96%]   3.3x.
xorriso : UPDATE : 22519 of 22530 MB written (fifo 17%) [buf  96%]   3.3x.
xorriso : UPDATE : 22530 of 22530 MB written (fifo  0%) [buf 100%]   2.6x.
xorriso : UPDATE : Closing track/session. Working since 1732 seconds
xorriso : UPDATE : Closing track/session. Working since 1733 seconds
Writing to '/dev/sr0' completed successfully.

xorriso : NOTE : Re-assessing -outdev '/dev/sr0'
Drive current: -outdev '/dev/sr0'
Media current: BD-R sequential recording
Media status : is written , is appendable
Media summary: 1 session, 11535712 data blocks, 22.0g data,  567m free


The big difference, of course, is the fifo being loaded to 90-something percent through most of the writing.

So, I have no evidence that this was a libburn bug.  Everything points to my problem's cause being inappropriate hardware (namely, a USB 1.x interface in the path to the ISO file).


Additional detail:

Cdrecord 2.01-Emulation Copyright (C) 2013 see libburnia-project.org xorriso
xorriso 1.3.2
ISO 9660 Rock Ridge filesystem manipulator and CD/DVD/BD burn program
Copyright (C) 2013, Thomas Schmitt <email address hidden>, libburnia project.
xorriso version   :  1.3.2
Version timestamp :  2013.08.07.110001
Build timestamp   :  -none-given-
libisofs   in use :  1.3.8  (min. 1.3.6)
libjte     in use :  1.0.0  (min. 1.0.0)
libburn    in use :  1.3.8  (min. 1.3.4)
libburn OS adapter:  internal GNU/Linux SG_IO adapter sg-linux
libisoburn in use :  1.3.2  (min. 1.3.2)
Provided under GNU GPL version 2 or later.
There is NO WARRANTY, to the extent permitted by law.

$ dvd+rw-mediainfo /dev/sr0

INQUIRY:                [PIONEER ][BD-RW   BDR-209M][1.20]
GET [CURRENT] CONFIGURATION:
 Mounted Media:         41h, BD-R SRM
 Media ID:              CMCMAG/BA5
 Current Write Speed:   8.0x4495=35960KB/s
 Write Speed #0:        8.0x4495=35960KB/s
 Write Speed #1:        6.0x4495=26970KB/s
 Write Speed #2:        4.0x4495=17980KB/s
 Write Speed #3:        2.0x4495=8990KB/s
 Speed Descriptor#0:    00/11826175 R@8.0x4495=35960KB/s W@8.0x4495=35960KB/s
 Speed Descriptor#1:    00/11826175 R@6.0x4495=26970KB/s W@6.0x4495=26970KB/s
 Speed Descriptor#2:    00/11826175 R@4.0x4495=17980KB/s W@4.0x4495=17980KB/s
 Speed Descriptor#3:    00/11826175 R@2.0x4495=8990KB/s W@2.0x4495=8990KB/s
READ DISC INFORMATION:
 Disc status:           appendable
 Number of Sessions:    2
 State of Last Session: empty
 "Next" Track:          2
 Number of Tracks:      2
READ TRACK INFORMATION[#1]:
 Track State:           partial incremental
 Track Start Address:   0*2KB
 Free Blocks:           0*2KB
 Track Size:            11535712*2KB
 Last Recorded Address: 11535711*2KB
READ TRACK INFORMATION[#2]:
 Track State:           invisible incremental
 Track Start Address:   11535712*2KB
 Next Writable Address: 11535712*2KB
 Free Blocks:           290464*2KB
 Track Size:            290464*2KB
FABRICATED TOC:
 Track#1  :             14@0
 Track#AA :             14@11535712
 Multi-session Info:    #1@0
READ CAPACITY:          11535712*2048=23625138176