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

Pavel Machek
2009-06-24T18:00:17Z

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.)

--
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