linux-kernel - Re: [concept & "good taste" review] persistent store

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Message-ID: <4D0BEE1F.7020008@zytor.com>
Date:	Fri, 17 Dec 2010 15:11:27 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Tony Luck <tony.luck@...el.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	tglx@...utronix.de, mingo@...e.hu, greg@...ah.com,
	akpm@...ux-foundation.org, ying.huang@...el.com,
	Borislav Petkov <bp@...en8.de>,
	David Miller <davem@...emloft.net>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Kyungmin Park <kmpark@...radead.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [concept & "good taste" review] persistent store

On 12/17/2010 03:08 PM, Tony Luck wrote:
> 
> Do we have any good models for "journals" ... apart from
> the console log?  Ying & I did talk about just using printk
> to drop all the saved information onto the console (with
> some sort of "previous" prefix on each line to make
> it easy to find, and to make sure that someone glancing
> at the messages wouldn't worry that that OOPs scrolling
> by was happening now).  But this seemed like a really bad
> idea (especially if someone has enough persistent store to
> capture all of __logbuf).
> 
> So the downside of "everything is a file" is that we don't
> have much infrastructure for things that don't look like
> files - and trying to build some results in some special
> custom tools being needed to access the data, which
> just makes things harder to use.
> 
> People trying to write to /dev/pstore will figure out quickly
> that you can't do that. There are no "ops" to make new files,
> directories, symlinks or even to rename existing ones. You
> can overwrite existing files (because I don't trap "open" to
> deny them write access - but the 0444 mode is supposed
> to be a visual clue to not do that).
> 

There are two models I can think of:

1. a file where the head is automatically dropped as space requires.
2. a filesystem where the oldest files are automatically reclaimed.

1 has been implemented in actual systems, 2 is kind of a logical extension.

	-hpa
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