| What to do with a Terabyte of RAM |
Mar. 21, 2008
Opinion -- OK, so we're not there yet. Well, none of us who isn't running supercomputers or one heck of a cluster is there yet, anyway. But as RAM continues to drop in price, I can see the day coming.
Come the day we get a terabyte of RAM in our systems, we'll find a need for it. After all, recall that great prophet Bill Gates who swore 640K of memory would be all we'd need. Real-time high-definition video editing anyone?
That day, by the way, isn't as far away as you might think. Violin Scalable Memory will be happy to sell you a terabyte-capable memory device to attach to your server, the Violin 1010 Memory Appliance. OK, so it would cost you a few—well more than a few—hundred thousand dollars. Still, you can see it coming.
But while I can dream up applications that could use a terabyte of RAM, that leaves aside the wee technical problem of how you access that much memory. Daniel Phillips, a Linux developer, has an idea: the ramback virtual device.
In the LKML (Linux Kernel Mailing List), Phillips describes the ramback as a "virtual device with the ability to back a ramdisk by a real disk, obtaining the performance level of a ramdisk but with the data durability of a hard disk. To work this magic, ramback needs a little help from a UPS [uninterruptible power supply]. In a typical test, ramback reduced a 25 second file operation to under one second including sync."
Phillips goes on: "The difference between ramback and an ordinary ramdisk is: when the machine powers down the data does not vanish because it is continuously saved to backing store. When line power returns, the backing store repopulates the ramdisk while allowing application io to proceed concurrently. Once fully populated, a little green light winks on and file operations once again run at ramdisk speed."
Of course, if the power goes out, you're in a world of trouble with your huge, hyperfast ramback drive. Phillips has some answers, starting with building the entire configuration with a UPS. "If line power goes out while ramback is running, the UPS kicks in and a power management script switches the driver from writeback to writethrough mode,” Phillips writes. “Ramback proceeds to save all remaining dirty data while forcing each new application write through to backing store immediately."
In short, Phillips is using several cache coherency concepts to make using huge amounts of RAM as a virtual drive a practical approach to speeding up I/O-intensive interactions. For example, with enough RAM, you could run, say, an Oracle database running on Oracle or Red Hat Linux, at real-time speeds.
Of course, Phillips’ idea isn't the same as coming up with an API for applications to address multiple gigabytes of RAM, but still, it is a very useful idea. Indeed, with its potential for vastly increasing database interactions, I can see it being a commercial success and driving the demand for huge memory.
This in turn will bring RAM prices down to normal user prices.
So, what will you do with your first laptop with a terabyte of RAM?
You don't need to worry about it yet. Well, not quite yet, anyway.
—Steven J. Vaughan-Nichols
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