Re: write performance difference 3.18.21/4.2.1

[Marc Lehmann <schmorp@schmorp.de>]

On Thu, Sep 24, 2015 at 11:28:36AM -0700, Jaegeuk Kim <jaegeuk@kernel.org> wrote:
> One thing that we can try is to run the latest f2fs source in v3.18.
> This branch supports f2fs for v3.18.

Ok, please bear with me, the last time I built my own kernel was during
the 2.4 timeframe, and this is a ubuntu kernel. What I did is this:

   git clone -b linux-3.18 git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git
   cd f2fs/fs/f2fs
   rsync -avPR include/linux/f2fs_fs.h include/trace/events/f2fs.h /usr/src/linux-headers-3.18.21-031821/.
   make -C /lib/modules/3.18.21-031821-generic/build/ M=$PWD modules modules_install

I then rmmod f2fs/insmod the resulting module, and tried to mount my
existing f2fs fs for a quick test, but got a null ptr exception on "mount":

http://ue.tst.eu/e4628dcee97324e580da1bafad938052.txt

Probably caused me not building a full kernel, but recreating how ubuntu
build their kernels on a debian system isn't something I look forward to.

> For example, if I can represent blocks like:
[number of logs discussion]

Thanks for this explanation - two logs doesn't look so bad, from a
locality viewpoint (not a big issue for flash, but a big issue for
rotational devices - I also realised I can't use dmcache as dmcache, even
in writethrough mode, writes back all data after an unclean shutdown,
which would positively kill the disk).

Since whatever speed difference I saw with two logs wasn't big, you
completely sold me on 6 logs, or 4 (especially if it seepds up the gc,
which I haven't much tested yet). Two logs was merely a test anyway (the
same with no_heap, I don't know what it does, but I thought it is worth
a try, as metadata + data nearer together is better than having them at
opposite ends of the log or so).

-- 
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