On Sat, 29 Nov 2008, Ray Clark wrote: > Regarding the cache, right now there is 150MB of free memory not > being used by ANYBODY, so I don't think there is a shortage of > memory for the ZFS cache... and 150MB >> 128K, or even a whole slew
To be more clear, memory which is claimed to be free is often actually still used for caching. Even if the virtual memory system has not mapped a VM page to a process, if a minor page fault occurs (due to an access), the data in that seemingly "unused" page may still be immediately switched in and used because the VM system tracks where the current content of that page came from. This is primarily the case for memory-mapped regions such as ordinary files, shared libraries, executable text, or even a video frame buffer. This is pretty much normal operation since when new processes are started, the VM maps the existing pages that the new process requires into its address space. It is pretty common for Unix systems to lie about free memory and use that free memory for the filesystem cache with the expectation that this "free" memory can be freed up for use fast enough that no one really notices. If the critical "working set" of VM pages is larger than available memory, then the system will become exceedingly slow. This is indicated by a substantial amount of major page fault activity. Since disk is 10,000 times slower than RAM, major page faults can really slow things down dramatically. Imagine what happens if ZFS or an often-accessed part of the kernel is not able to fit in available RAM. Bob ====================================== Bob Friesenhahn [EMAIL PROTECTED], http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/ _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
