[Pkg-opt-media-team] Bug#768504: cdparanoia: Very poor read performance when running on two optical drives simultaneously

[Tim Small]

Package: cdparanoia
Version: 3.10.2+debian-11
Severity: normal

In a system with 2 SATA optical drives (have tried on two Jessie
systems so-far), if two copies of cdparanoia are run simultaneously, and
-d /dev/sr0 is specified on one copy, and -d /dev/sr1 on the other, then
read performance drops to approx 5% of its normal speed.  No errors are
reported, and the operation completes successfully.


With 3 or more drives, the problem is exacerbated, and a single audio CD
takes many hours to be read.

In the test setup with just 2 drives, if either one or both of the copies
instead use the corresponding /dev/sgX device (e.g.run lsscsi -g to obtain
device node, or use sysfs), then both copies run at full speed.

I assume this is a kernel bug, but thought I'd ask here first in case
there was any known behavior within cdparanoia which could trigger this.

Initial testing with a 3.2 kernel shows similar behaviour (although
perhaps the slowdown is not so severe).

I've reported this to the linux-scsi mailing list, but no response so
far, it would be good to have a bit more certainty that this was a
kernel bug.  Reading the man page, it looks like cdparanoia uses the
same interface (SG_IO ioctl) - however looking at the debug output,
running against the /dev/sg interfaces seems to suggest that they expose
scatter/gather, whereas the /dev/sr interfaces don't - perhaps this is
sufficient to begin to explain the difference.

I'll try and gather a bit more data, and report here (should be able to
test with some IDE drives, and maybe a USB one to see if the problem
persists.  Incidentally, in the SATA test system, both optical drives
are connected to different SATA host controllers.

-- System Information:
Debian Release: jessie/sid
  APT prefers testing-updates
  APT policy: (500, 'testing-updates'), (500, 'testing')
Architecture: amd64

Kernel: Linux 3.16-2-amd64 (SMP w/2 CPU cores)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash

Versions of packages cdparanoia depends on:
ii  libc6           2.19-12
ii  libcdparanoia0  3.10.2+debian-11

cdparanoia recommends no packages.

cdparanoia suggests no packages.

-- no debconf information