Re: Stracing Amanda (was: RSDL for 2.6.21-rc3- 0.29)

[Gene Heskett <gene.heskett@gmail.com>]

On Monday 12 March 2007, Nish Aravamudan wrote:
>On 3/12/07, Gene Heskett <gene.heskett@gmail.com> wrote:
>> On Monday 12 March 2007, Douglas McNaught wrote:
>> >Patrick Mau <mau@oscar.ping.de> writes:
>> >> Why not temporarly replace "/bin/tar" with a shell script that
>> >> does:
>> >>
>> >> #!/bin/sh
>> >> exec strace -f -o output /bin/real.tar $@
>> >
>> >You beat me to it.  :) I've done that before; it's a great
>> > suggestion.
>> >
>> >Except that if you expect 'tar' to be invoked multiple times in a
>> > run, you should probably use 'output.$$' for the output filename so
>> > things don't get clobbered.
>> >
>> >-Doug
>>
>> In my case, Doug, it will get invoked 64 times, amanda does a dummy
>> run to get an estimate, calculates what to do based on that output
>> which is 32 runs, 1 per disklist entry and I have 32, and then reruns
>> tar with the appropriate level options against each individual
>> disklist entry.
>>
>> But I'm puzzled a bit, what does the double $$ do?, or it buried
>> someplace in the bash manpage?  Its not something I've stumbled over
>> yet.
>
>buried indeed:
>
>"Special Parameters:
>  ...
>       $      Expands to the process ID of the shell.  In a  () 
> subshell,  it expands  to  the  process  ID of the current shell, not
> the sub‐ shell.
>"

Well, that's clear enough, but what of the double $$ case?  Would this 
them make a PID unique to each invocation untill it finally wraps a 16 
bit value, or will the kernel re-use them because they won't all be 
running simultainiously, but limited by the number of unique 'spindle' 
numbers on the system, this to prevent as best as it can, the thrashing 
of a drive by having tar working on 2 separate (or more) partitions at 
the same time.  In my case 2 are possible, as /var is on a separate 
drive.

>Thanks,
>Nish



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Cheers, Gene
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