Date: Thu, 22 Mar 2001 07:57:14 -0600 From: Mike Meyer <mwm@mired.org> To: Eric M Logan <ericmlogan@mediaone.net> Cc: questions@freebsd.org Subject: Re: ramdisks and mfs... Message-ID: <15034.1210.849837.67514@guru.mired.org> In-Reply-To: <3AB9B07F.E6F9D481@mediaone.net> References: <15033.28284.778431.468125@guru.mired.org> <3AB9B07F.E6F9D481@mediaone.net>
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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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