Kernel space: How to use a terabyte of RAM

An experimental new design for Linux's virtual memory system would turn a large amount of system RAM into a fast RAM disk with automatic sync to magnetic media.

By Jonathan Corbet, LinuxWorld.com, 03/18/08

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We have not yet reached a point where systems - even high-end boxes - come with a terabyte of installed memory. But products like those from Violin Memory make it clear that the day is coming; one can buy a Violin box with 500GB in it now. So it seems worth asking the question: once one has spent the not inconsiderable sum to buy a box like that, what does one do with all that memory - especially now that the Firefox developers have gotten serious about fixing memory leaks? Perhaps it's time for some wild ideas. And there is no better source for such ideas than Daniel Philips, whose Ramback patch has stirred up a bit of discussion this week. The core idea behind Ramback is that all of that memory is turned into a ramdisk, but with a persistent device attached to it. In normal conditions, all application I/O involves only the ramdisk, and is, thus, quite fast ("Every little factor of 25 performance increase really helps."). In the background, the kernel worries about synchronizing data from the ramdisk onto permanent storage. But the synchronization process is mostly concerned with I/O performance, rather than providing guarantees about just when any given block will make it onto the disk platters.

Ramback thus differs from the normal block I/O caching done by the kernel in a number of ways. It keeps the entire device in memory, so that, in steady-state operation, applications need never encounter a disk I/O delay. Should an application call fsync(), the expected result (blocking until the data is written to physical media) will not happen. Filesystems take great care to order operations in a way that minimizes the risk of data loss in a crash; Ramback ignores all of that and writes data to physical media in whatever order it decides is best. As Daniel put it, the "most basic principle" of Ramback's design is:

 

[T]he backing store is not expected to represent a consistent filesystem state during normal operation. Only the ramdisk needs to maintain a consistent state, which I have taken care to ensure. You just need to believe in your battery, Linux and the hardware it runs on. Which of these do you mistrust?

Ramback does include an emergency mode which will endeavor to bring the disk up to date in a hurry should the UPS indicate that power has been lost. But that does not seem to be enough for everybody. In the resulting discussion, nobody complained about the sort of performance benefits that a tool like Ramback could provide. But there was a lot of concern about data integrity; it seems that many people distrust their battery, their hardware, and Linux. And that has led to a sort of impasse, with several developers claiming that Ramback would be too risky to use and Daniel dismissing their concerns as FUD.

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