Date: Thu, 22 Mar 2001 12:06:49 -0800 From: Eric M Logan <ericmlogan@mediaone.net> To: Mike Meyer <mwm@mired.org> Cc: questions@freebsd.org Subject: Re: ramdisks and mfs... Message-ID: <3ABA5B59.6A6330C4@mediaone.net> References: <15033.28284.778431.468125@guru.mired.org> <3AB9B07F.E6F9D481@mediaone.net> <15034.1210.849837.67514@guru.mired.org>
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That clears it up, thanks. Wow, 10 MB limit, that's pretty small. Mike Meyer wrote: > Eric M Logan <ericmlogan@mediaone.net> types: > > First, thanks for your quick reply. Just one last thing, actually two. Am I > > correct in assuming that a "pure" ramdisk from /dev/md* is faster than a pseudo > > ramdisk backed by a swap partition? And, what's the point of the former since it > > relies on a slow hd? Shouldn't the latter be the preferred way to do ramdisks? > > A better way to see what's going on is that md allocates memory from > real, where mfs allocates it from virtual. A ramdisk on real memory > will be faster than one on virtual memory if the virtual memory is > actually paged out. When that happens, *something* has be page be > paged out. Allowing that something to be your ramdisk means you've > raised the threshhold before the system starts paging or thrashing, > which is a good thing. > > There are situations where having a small ramdisk that doesn't have > disk preallocated to it is an advantage (systems without swap, or > during installation, for instance). Even for typical workstation > usage, if you restrict the usage of /tmp to small things, it might be > useful. But I use /tmp for pretty much anything I don't plan on > keeping around (extracting tarballs or things sent in the mail, for > instance) and don't really want to limit it to the 10Meg the kernel > allows an md disk to be by default. > > <mike > > > Mike Meyer wrote: > > > > > Eric M Logan <ericmlogan@mediaone.net> types: > > > > Is there a difference between /dev/md* and mounting a partition from > > > > swap. Let me elaborate. I have a swap partition mounted and I have > > > > /tmp mounted using the same address as that swap partition. Anything I > > > > put in /tmp will therefore be gone upon reboot. Is this what's > > > > considered a ramdisk in Freebsd? Or, is using /dev/md* mounted > > > > somewhere what's known as a ramdisk in FreeBSD? In Linux, it's the > > > > latter. Any help would be appreciated, thanks. > > > > > > I assume you're using mfs for /tmp. Yes, that qualifies as a ramdisk, > > > even though it's backed by swap. If you don't need the memory back, > > > it'll act just like a ramdisk. If you do need the memory for something > > > else, your data will be paged out to swap, and have to be read back > > > from disk. md isn't backed by swap, so the data is always in ram, > > > meaning the memory isn't usable by anything else. > > > > > > <mike > > > -- > > > Mike Meyer <mwm@mired.org> http://www.mired.org/home/mwm/ > > > Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information. > > > > > > > > > -- > Mike Meyer <mwm@mired.org> http://www.mired.org/home/mwm/ > Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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