Date: Thu, 03 Jul 2014 08:07:44 -0600 From: Ian Lepore <ian@FreeBSD.org> To: John Hay <jhay@meraka.org.za> Cc: freebsd-arm@FreeBSD.org Subject: Re: /tmp, /var/log, /var/tmp as /dev/md - why? Message-ID: <1404396464.20883.404.camel@revolution.hippie.lan> In-Reply-To: <20140703105519.GA37593@zibbi.meraka.csir.co.za> References: <201407010925.s619PHeT006679@mech-cluster241.men.bris.ac.uk> <44a6e8a451a.810fa8f@mail.schwarzes.net> <53B3EB29.4030908@gmail.com> <alpine.BSF.2.11.1407030436060.99346@wonkity.com> <20140703105519.GA37593@zibbi.meraka.csir.co.za>
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On Thu, 2014-07-03 at 12:55 +0200, John Hay wrote: > 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@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 I have patches ready to commit that do exactly that, but they weren't exactly enthusiastically received when I posted them on arch@ for review. -- Ian
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