linux-kernel - Re: block_dump

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Date:	Sun, 06 Sep 2009 18:14:57 +0200
From:	devzero@....de
To:	Theodore Tso <tytso@....edu>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: block_dump - full path ?

Hi, 

i just (too blindly) copied a tiny part of the log to this mail and kjournald and pdflush are indeed bad examples - sure there is no path for kernel threads.

> What do you mean by "the full path of the process"?  Do you mean the
> full path of the process' executable? 

yes!
certainly this applies to userspace processes only :)

> What exactly are you trying to do? 

determine which processes did read/write.

> There might be a more efficient
> way of gathering whatever data you are trying to get for your experiment.

i think that depends.
if you have a trace of lots of temporary processes doing reads/writes in dmesg, it could be hard to correctly associate a pid with a process-binary /somewhere/deep/inside.
furthermore you can`t assume that binary names are unique on a system.

regards
roland



> On Sat, Sep 05, 2009 at 11:44:35PM +0200, devzero@....de wrote:
> > Hi, 
> > 
> > i came across the nice debugging feature sysctl vm.block_dump=1 which leaves information on read/write in dmesg for every process.
> > 
> > Sep  5 20:11:15 neoware kernel: kjournald(423): WRITE block 155542512 on sda2
> > Sep  5 20:11:15 neoware kernel: kjournald(423): WRITE block 155542520 on sda2
> > Sep  5 20:11:15 neoware kernel: kjournald(423): WRITE block 155542528 on sda2
> > Sep  5 20:11:19 neoware kernel: pdflush(14): WRITE block 144498288 on sda2
> > Sep  5 20:11:19 neoware kernel: pdflush(14): WRITE block 144916496 on sda2
> > Sep  5 20:11:19 neoware kernel: pdflush(14): WRITE block 144917376 on sda2
> > Sep  5 20:11:19 neoware kernel: pdflush(14): WRITE block 144704600 on sda2
> > 
> > is it possible to log the full path of the process ?
> 
> What do you mean by "the full path of the process"?  Do you mean the
> full path of the process' executable?  (In this case kjournald and
> pdflush are kernel threads so there is no executable, and thus there
> is no "full path". unless you mean pathname to the kernel, i.e., /vmlinux.)
> 
> Or did you mean the full path name of the file associated with the
> block number?  In this case, kjournald is writing to the file system
> journal, which has no pathname.  pdflush is writing dirty pages from
> the page cache, which could be mapped to file names, but it would take
> up a huge amount of space in the log -- but to what effect?
> 
> What exactly are you trying to do?  There might be a more efficient
> way of gathering whatever data you are trying to get for your experiment.
> 
>        		 	       	       - Ted
> 


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