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Date: Wed, 18 Jul 2012 23:45:21 +0200
From: Borislav Petkov <bp@alien8.de>
To: =?utf-8?B?SsO2cm4=?= Engel <joern@logfs.org>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org,
        Jeff Moyer <jmoyer@redhat.com>, Steve Hodgson <steve@purestorage.com>
Subject: Re: [PATCH] add blockconsole version 1.1
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On Wed, Jul 18, 2012 at 02:53:35PM -0400, Jörn Engel wrote:

[ … ]

> > $ ./mkblockconsole /dev/sdc
> > 
> > <reboot>
> 
> You can also run hdparm -z <dev> instead.  Or replug the device.  Main
> danger of hdparm is that running the command twice will cause two
> instances of blockconsole to use the same device.  Not sure how to
> solve that problem - or if.

Actually, I meant <reboot> in the sense here that I wanted to test the
case where user has a prepared stick and wants to catch full boot log of
the booting system.

> > So why is that first megabyte full of zeros there?
> 
> It gives you some scratch space to store information in.

How? By me writing something in that empty line in vim? Or something
else storing stuff there?

> How useful that actually is may be a matter of opinion. But
> independent of that, you will find large amounts of zeroes all over.
> Every time you reboot, the new blockconsole will start writing at a
> megabyte-aligned offset and whatever remains of the last megabyte
> should be zero-filled as well.

Ah, those are the tiles you're talking about in the docs, right?

> Vim treats this as a single line, which makes it only mildly annoying
> to me.

Ok, I should try that.

> > Other than that, it works like a charm and I like the idea that no
> > kernel cmdline args are needed.
> > 
> > Also, you might want to add a step-by-step fast howto to the docs with
> > concrete steps like the above so that people can try this out faster.
> 
> I will try to find a quiet moment for that.  If you happened to beat
> me to it, you certainly won't hear any complaints.

Oh, I didn't mean anything involved but rather a quick steps write-up
(steps can always be expanded and made more verbose later):

Blocksonsole in three easy steps
================================

1. Find an unused USB stick and prepare it for blockconsole by writing
the blockconsole signature to it:

$ ./mkblockconsole /dev/sdc

  [ Assuming /dev/sdc is the device node of the USB stick you just mounted. ]

2. USB stick is ready for use, replug it so that the kernel can start
logging to it.

3. After you've done logging, read out the logs from it like this:

$ ./bcon_tail

  [ This creates a file called /var/log/bcon.<random number> which
  contains the logs. Open it with a sane editor like vim which can
  display zeroed gaps as a single line and start staring at the logs. ]

---

Something like the above, just slap it at the beginning of
Documentation/block/blockconsole.txt for impatient people like me and
that's it :-).

Thanks.

-- 
Regards/Gruss,
    Boris.