/tmp, /var/log, /var/tmp as /dev/md - why?

[John Hay]

On Thu, Jul 03, 2014 at 04:47:24AM -0600, Warren Block wrote:
> On Wed, 2 Jul 2014, Mattia Rossi wrote:
> 
> >
> > Am 01.07.2014 21:27, schrieb Andreas Schwarz:
> >> Speed and speed, but I can't understand why using md here, there is already 
> >> tmpfs,
> >> which optimzed for such cases (dynamic allocation, etc.).
> >> 
> >> root at pizelot:~ # df
> >> Filesystem     1K-blocks    Used   Avail Capacity  Mounted on
> >> /dev/mmcsd0s2a    983680   57252  847736     6%    /
> >> devfs                  1       1       0   100%    /dev
> >> /dev/mmcsd0s2d   8106716 3068708 4389472    41%    /usr
> >> /dev/mmcsd0s2e   8106716  155976 7302204     2%    /var
> >> /dev/mmcsd0s2f   8106716236 7457944  <sip:2367457944>      0%    /home
> >> tmpfs            1097160       4 1097156     0%    /tmp
> >> tmpfs            1097160       4 1097156     0%    /var/tmp
> >> 
> >
> > On an embedded systems with little memory I prefer to limit the partitions to 
> > a certain size, like 32M, so dynamic allocation is no advantage. What other 
> > differences are there between tmpfs and a simple md device?
> > I'd be interested in knowing any tricks, that can make the system faster :-)
> 
> The white paper on tmpfs (wiki.deimos.fr/images/1/1e/Solaris_tmpfs.pdf) 
> says:
> 
>    "RAM disks use memory inefficiently; file data exists twice in both
>     RAM disk memory and kernel memory, and RAM disk memory that is not
>     being used by the file system is wasted.  RAM disk memory is
>     maintained separately from kernel memory, so that multiple
>     memory-to-memory copies are needed to update file system data."
> 
> So a limited-size tmpfs will be faster and use less memory overall.  A
> benchmark comparison would be interesting.

Last time I looked the rc scripts that create /etc, /var and /tmp
ramdisks only did it using md devices. It would be great if it was
easily tunable from say rc.conf or if could detect which one is
available and use that.

John
-- 
John Hay -- jhay at meraka.csir.co.za / jhay at meraka.org.za