Date: Wed, 24 Jun 2009 19:59:43 +0200
From: Pavel Machek <pavel@ucw.cz>
To: Marco <marco.stornelli@gmail.com>
Cc: tim.bird@am.sony.com, jamie@shareable.org,
       Linux Embedded <linux-embedded@vger.kernel.org>,
       Linux Kernel <linux-kernel@vger.kernel.org>,
       Linux FS Devel <linux-fsdevel@vger.kernel.org>,
       Daniel Walker <dwalker@soe.ucsc.edu>
Subject: Re: [PATCH 00/14] Pramfs: Persistent and protected ram filesystem
Message-ID: <20090624175943.GB6618@elf.ucw.cz>
References: <4a4254e2.09c5660a.109d.46f8@mx.google.com> <4A425907.2060105@gmail.com> <4A42649D.6080509@gmail.com>
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On Wed 2009-06-24 19:38:37, Marco wrote:
> >>> Pavel Machek wrote:
> >>>> On Mon 2009-06-22 14:50:01, Tim Bird wrote:
> >>>>> Pavel Machek wrote:
> >>>>>>> block of fast non-volatile RAM that need to access data on it using a
> >>>>>>> standard filesytem interface."
> >>>>>> Turns a block of fast RAM into 13MB/sec disk. Hmm. I believe you are
> >>>>>> better with ext2.
> >>>>> Not if you want the RAM-based filesystem to persist over a kernel
> >>>>> invocation.
> >>>> Yes, you'll need to code Persistent, RAM-based _block_device_. 
> >>> First of all I have to say that I'd like to update the site and make it
> >>> clearer but at the moment it's not possible because I'm not the admin
> >>> and I've already asked to the sourceforge support to have this possibility.
> >>>
> >>> About the comments: sincerely I don't understand the comments. We have
> >>> *already* a fs that takes care to remap a piace of ram (ram, sram,
> >>> nvram, etc.), takes care of caching problems, takes care of write
> >> Well, it looks pramfs design is confused. 13MB/sec shows that caching
> >> _is_ useful for pramfs. So...?
> > 
> > caching problems means to avoid filesystem corruption, so dirty pages in
> > the page cache are not allowed to be written back to the backing-store
> > RAM. It's clear that there is a performance penalty. This penalty should
> > be reduced by the access speed of the RAM, however the performance are
> > not important for this special fs as Tim Bird said, so this question is
> > not relevant for me. If this issue is not clear enough on the web site,
> > I hope I can update the information in the future.
> > 
> >>> You are talked about journaling. This schema works well for a disk, but
> >>> what about a piece of ram? What about a crazy kernel that write in that
> >>> area for a bug? Do you remember for example the e1000e bug? It's not
> >> I believe you need both journaling *and* write protection. How do you
> >> handle power fault while writing data?
> >> 								Pavel
> > 
> > Ah now the write protection is a "needed feature", in your previous
> > comment you talked about why not use ext2/3.......
> > 
> > Marco
> > 
> 
> Just for your information I tried the same test with pc in a virtual machine with 32MB of RAM:
> 
> Version 1.03e       ------Sequential Output------ --Sequential Input- --Random-
>                     -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
> Machine   Size:chnk K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
> hostname     15M:1k 14156  99 128779 100 92240 100 11669 100 166242  99 80058  82
>                     ------Sequential Create------ --------Random Create--------
>                     -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
>               files  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
>                   4  2842  99 133506 104 45088 101  2787  99 79581 101 58212 102
> 
> These data are the proof of the importance of the environment, workload and so on when we talk 
> about benchmark. Your consideration are really superficial.

Unfortunately, your numbers are meaningless.
									Pavel

(PCs should have cca 3GB/sec RAM transfer rates; and you demosstrated
cca 166MB/sec read rate; disk is 80MB/sec, so that's too slow. If you
want to prove your filesystem the filesystem is reasonably fast,
compare it with ext2 on ramdisk.)

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