linux-kernel - Re: [PATCH 00/14] Pramfs: Persistent and protected ram filesystem

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Message-ID: <20090622173704.GC21299@elf.ucw.cz>
Date:	Mon, 22 Jun 2009 19:37:04 +0200
From:	Pavel Machek <pavel@....cz>
To:	Tim Bird <tim.bird@...sony.com>
Cc:	Marco Stornelli <marco.stornelli@...il.com>,
	Jamie Lokier <jamie@...reable.org>,
	Linux Embedded <linux-embedded@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Daniel Walker <dwalker@....ucsc.edu>
Subject: Re: [PATCH 00/14] Pramfs: Persistent and protected ram filesystem

On Mon 2009-06-22 10:31:28, Tim Bird wrote:
> Pavel Machek wrote:
> >>> How do you handle hard-links, then?
> >> Indeed hard-links are not supported :) Due to the design of this fs
> >> there are some limitations explained in the documentation as not
> >> hard-link, only private memory mapping and so on. However this
> >> limitations don't limit the fs itself because you must consider the
> >> special goal of this fs.
> > 
> > I did not see that in the changelog. If it is not general purpose
> > filesystem, it is lot less interesting.
> 
> PRAMFS is not a general purpose filesystem. Please read
> the introductory post to this thread, or look at
> http://pramfs.sourceforge.net/ for more information.

Yeah, I seen that. It directly contradicts what you say.

> Since the purpose of PRAMFS is to provide a filesystem
> that is persistent across kernel instantions, it is not
> designed for high speed.  Robustness in the face of
> kernel crashes or bugs is the highest priority, so
> PRAMFS has significant overhead to make the window
> of writability to the filesystem RAM as small as possible.

Really? So why don't you use well known, reliable fs like ext3?

> This is not a file system one would do kernel compiles on.
> This is where someone would keep a small amount of sensitive
> data, or crash logs that one needed to preserve over kernel
> invocations.

Really? Web page says:

#2. If the backing-store RAM is comparable in access speed to system
#memory, there's really no point in caching the file I/O data in the
#page cache. Better to move file data directly between the user buffers
#and the backing store RAM, i.e. use direct I/O. This prevents the
#unnecessary 

So you don't cache it "because its fast", and then it is 13MB/sec?

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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