Cdrecord warnings

[James Wilkinson]

Fabio Comolli wrote:
> Burning CDs or blanking CD-RWs as root user gives no warnings; as
> normal user always gives:
> 
> cdrecord: Cannot allocate memory. WARNING: Cannot do mlockall(2).
> cdrecord: WARNING: This causes a high risk for buffer underruns.
> cdrecord: Operation not permitted. WARNING: Cannot set RR-scheduler
> cdrecord: Permission denied. WARNING: Cannot set priority using setpriority().
> cdrecord: WARNING: This causes a high risk for buffer underruns.
> 
> This happens either running cdrecord directly or through k3b. So far I
> got no toasters: should I consider this warnings harmless?

Depends. Do you have a drive with "Burn Free"?

("This may be called: Sanyo BURN-Proof, Ricoh Just-Link, Yamaha
Lossless-Link or similar." -- the cdrecord man page.)

If you do, then the drive should be able to recover from buffer
under-runs anyway.

You can check by running
cdrecord -prcap | grep Buffer-Underrun-Free
and looking for
  Does support Buffer-Underrun-Free recording

This is assuming you still have the word "burnfree" in the default place
in /etc/cdrecord.conf. If you haven't touched that file, then it should
be there.

If you don't have "Burn Free", then it depends on such things as how
fast you're recording, how large an on-board buffer you have (= cache:
dmesg | grep CD-R/RW should tell you), and how much else the system is
doing.

The basic problem is that if the CD recorder runs out of data, then it
needs "burn free" to be able to pause a recording. So one without "burn
free" *needs* to have data spooled to the drive constantly.

For example, my DVD±RW has
hdc: ATAPI 48X DVD-ROM DVD-R-RAM CD-R/RW drive, 2048kB Cache, UDMA(33)
and a maximum write speed of 8467 kB/s, which implies that the cache is
capable of handling delays of nearly a quarter of a second at top speed.

Unless it's *very* heavily laden, of the time, a Linux box will respond
faster than a quarter of a second (if it didn't, you'd notice it in X)
and a recent one should be able to sustain 8 MB/s. So I don't really
need burnfree.  As it is, I certainly don't worry...

The worst case scenario is that you'll get a dud CD, yes.

As for what the error messages mean, mlockall is the application wanting
to "lock" its virtual memory into fast real memory, so it doesn't get
paged to slow disk, and wanting "real-time"-like priority, so it gets to
run before all other processes. Only root is allowed to set such things,
by default.

Hope this helps,

James.

-- 
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