If you are planning on having a few minutes' worth of UPS backup then why would you need to write to the hard drive continuously? Keep the hard drive spun down (saving power). If the system is being shut down, or AC power fails, then spin up the drive and make a backup of your ramdisk, thus being ready to restore when the power comes back up. The UPS might not last long enough to copy all the data out. The Violin 1010 in the example is a 504GB RAM disk, which would take over an hour to copy to the fastest SATA disks (516096 MB / 100MB/s / 60s/min = 86 minutes). Sure, you could use an array of disks but that is only going to chew through the UPS faster, and the RAM disks will only get bigger. Either way "a few minutes" of UPS isn't enough.
posix_fadvise() technically does allow you to do what you want. You can use posix_fadvise(POSIX_FADV_DONTNEED) to evict the buffer cache in the IO loop of the program. See http://insights.oetiker.ch/linux/fadvise.html for the ugly details. Unfortunately you can't just make one system call and have it effect an entire file or process. POSIX_FADV_NOREUSE is supposed to be the default in the kernel buffer cache management so it is implemented as a no-op.
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posix_fadvise() technically does allow you to do what you want. You can use posix_fadvise(POSIX_FADV_DONTNEED) to evict the buffer cache in the IO loop of the program. See http://insights.oetiker.ch/linux/fadvise.html for the ugly details. Unfortunately you can't just make one system call and have it effect an entire file or process. POSIX_FADV_NOREUSE is supposed to be the default in the kernel buffer cache management so it is implemented as a no-op.