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
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
Perhaps it's time for some wild ideas. And there is no better source for
such ideas than Daniel Phillips, 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
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.
FUD or not, those concerns are likely to be a difficult barrier for Ramback
to overcome. Meanwhile, Daniel is looking for people to help test out the
code, but that presents challenges of its own:
This driver is ready to try for a sufficiently brave developer. It
will deadlock and livelock in various ways and you will have to
reboot to remove it. But it can already be coaxed into running
well enough for benchmarks, and when it solidifies it will be
pretty darn amazing.
So far, reports from suitably courageous testers have been, well, scarce.
Your editor fears that this work could suffer the same fate as many of
Daniel's other patches: they can contain brilliant ideas and great coding
but just don't quite survive the encounter with the real, messy world.
But we need people thinking about how our systems will work in the
coming years; one hopes that Daniel won't stop.
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